Pulsars and Transients

Various cosmological models predict the presence of an isotropic stochastic gravitational wave (GW) background that was created in the early phase of the universe (e.g., Carr 1980). It has been proposed that a set of well-timed MSPs (referred to as a pulsar timing array, or PTA) provides an excellent opportunity to identify the influence of such GW background on the time of arrivals (ToAs) of signals from MSPs. The number of well-timed MSPs included in the PTAs is the most important factor in accelerating the detection of the GW background. The discovery and timing follow-up of millisecond pulsars (MSPs) are necessary not just for their usefulness in the PTAs but also for investigating their own intriguing properties. Sharma et al. (2024) provided the findings of the decade-long timing of four MSPs discovered by the Giant Meterwave Radio Telescope (GMRT), including their timing precision, model parameters including newly detected parameters like proper motions. The authors compared the timing results for these MSPs before and after the GMRT upgrade in 2017 and characterized the improvement in timing precision due to the bandwidth upgrade. Sharma et al. (2024) discussed the suitability of these four GMRT MSPs as well as the usefulness of the decade-long timing data for PTA experiments. The figure compares the timing precision obtained for the four GMRT-discovered MSPs to that for the 65 MSPs reported in the International PTA's second data release (Perera et al. 2019). In addition, it presents a comparison between the timing precision of the four GMRT MSPs and the 14 PTA MSPs reported in the Indian PTA's first data release (Tarafdar et al. 2022). It illustrates that these data sets may aid in the global effort to improve the signal-to-noise ratios of recently detected signatures of gravitational waves in cross- correlation statistics of residuals of MSPs.

Black widow (BW) millisecond pulsars (MSPs) are compact binaries in which the energetic wind from the pulsar ablates material off the companion. The ablated material of the companion is assumed to cause eclipses in these systems, where approximately 10% of the binary orbit is obscured. The observed eclipses are frequency-dependent, with the pulsed signal disappearing below a certain frequency, generally denoted as the eclipse cut-off frequency. Kumari et al. (2024) conducted the first systematic monitoring of the temporal changes of the eclipse cut-off frequency in the Fermi BW MSP J1544+4937, which was originally discovered by the GMRT (Bhattacharyya et al. 2013), with a spin period of 2.16 ms. Kumari et al. find drastic changes in the eclipse cut-off frequency of PSR J1544+4937: such strong variations in the cut-off frequency have not been reported for this or any other spider MSP. The authors found significant changes in the eclipse cut-off frequency on timescales of a few days, as shown in the figure, with a maximum change of more than 315 MHz between observations separated by 22 days. In addition, Kumari et al. (2024) observed a change of about 47 MHz in the eclipse cut-off frequency between adjacent orbits, i.e. on timescales of about 2.9 hours. The authors inferred that such changes in the eclipse cut-off frequency are likely to arise from a dynamically evolving eclipse environment, where, along with changes in the electron density, the magnetic field could also be varying. They also reported a significant correlation between the eclipse cut-off frequency and the mass loss rate of the companion. This study provides the first direct evidence of the mass loss rate affecting the frequency-dependent eclipsing in a spider MSP.

The pulsed radio emission from pulsars and their rotational properties (e.g., period, slow-down rate, etc) are the primary observables to understand the pulsar radio emission. The radio signal from pulsars is known to be significantly polarized and this polarization varies from pulse to pulse, but a stable polarization profile can be obtained after folding a few hundred pulses. The polarization properties of single pulses from pulsars reveal various interesting properties such as depolarization and orthogonal mode jumps, potentially carrying major clues about the physical processes responsible for pulsar radio emission. Similarly, fine structures in the single pulses, also known as microstructures, are thought to be fundamental units of pulsar radio emission. To better understand the single pulse properties of pulsars and the origin of microstructures, Singh et al. performed a high-time-resolution polarization study of two bright pulsars, B0950+08 and B1642-03, with the GMRT. They find that pulsar B0950+08 occasionally shows microstructures without significant underlying subpulse emission. These micropulses were labeled as `intrinsic' micropulses and were utilized to study the true nature of micropulse emission. These `intrinsic' micropulses show common trends in their polarization properties, including high linear polarization (~80%), the same sign of circular polarization, and position angle strictly following the position angle track of the folded profile. Using the circular polarization of these `intrinsic' micropulses, the authors argue against the vacuum curvature radiation by a point charge as the origin of micropulse emission. The paper also reports the micropulse width statistics from pulsars B1642-03 and B0950+08. The authors notice many cases of position angle mode changes caused by the presence of overlap between two subpulses or subpulse and micropulses (the figure shows the cases of subpulse and micropulse overlap from pulsar B1642-03). The authors propose simple superposition models of the two possible orthogonal modes to explain these position angle transitions.

Continuous gravitational wave emissions are predicted in colliding galaxies from supermassive black hole binaries (SMBHB) that revolve around each other for millions of years before the ultimate merger. Superposition of continuous gravitational wave emissions from a large number of SMBHBs is expected to create a persistent stochastic gravitational wave background with wavelengths of the order of light years (in the nanohertz frequency range). Detection of such waves would require detectors with light-year arm lengths, and hence cannot be achieved by ground-based or even the upcoming advanced space-based gravitational wave detectors like LISA. However, nature has endowed us with ultra-precise galactic clocks named 'millisecond pulsars' placed light years apart. Fine delay in the super-stable arrival time of radio pulses from these 'clocks' has the potential to detect nanohertz gravitational waves causing the delay. In a recent series of papers published jointly by the Indian Pulsar Timing Array (InPTA) and the European Pulsar Timing Array (EPTA) collaborations, the first direct evidence of such a cosmic gravitational wave background is unraveled. InPTA data acquired using the upgraded Giant Metrewave Radio Telescope (uGMRT) plays a critical role in obtaining these results. In one of the papers (Paper-I), the low-frequency uGMRT data of very high quality was combined with mid and high-frequency data from five different radio telescopes of the EPTA leading to precise measurements of tiny stochastic noise caused by the ISM. Such noise in the intervening space masks the effects of nanohertz gravitational waves and needs to be carefully subtracted for the imprints of a gravitational wave background to emerge. This feat was achieved to a significant level of confidence, and early evidence of a gravitational wave background signal was reported in another paper (Paper-II) in the series. The accompanying figure shows the posterior of the correlation coefficients averaged at ten bins of angular separations with 30 pulsar pairs each (orange: InPTA+EPTA, blue:EPTA), along with the Hellings and Downs (HD) curve (black line) based on theoretical expectation of a GWB signal. [Paper-I: https://doi.org/10.1051/0004-6361/202346842 ; Paper-II: https://doi.org/10.1051/0004-6361/202346844]

Pulsar timing array (PTA) experiments aim to detect ultra-low frequency (∼ 1-100 nHz) gravitational waves (GWs) by monitoring an ensemble of MSPs distributed across the galaxy. GW signals in the PTA frequency range are typically expected to originate from orbiting supermassive black hole binaries (SMBHBs) in the inspiral phase, both as a stochastic GW background (GWB) formed by the incoherent addition of GWs from a large number of SMBHBs, and as strong individual sources standing out above this background. The intrinsic wander of the rotation rate of the constituent pulsars, the variations in dispersion measure (DM) and scatter-broadening caused by the interstellar medium, as well as the instrumental noise of radio telescopes are often covariant with the slowly varying GW signature in the data and act as sources of chromatic and achromatic noise. The detection and characterization of GWs are strongly affected by the faithfulness of noise models and can be highly dependent on custom noise modelling for each pulsar. This work presents the results of single-pulsar noise analysis for each of the 14 pulsars in the Indian Pulsar Timing Array (InPTA) first data release (DR1), which was made possible using the wideband and multi-frequency observations with the upgraded GMRT. This work considers white noise, achromatic red noise, DM variations, and scattering variations in the analysis, and applies Bayesian model selection to obtain the preferred noise models among these for each pulsar. Properties vary dramatically among pulsars. For example, For PSR J1600−3053, no evidence of DM and scattering variations is found, while for PSR J1909−3744, no significant scattering variations are found. A strong chromatic noise with chromatic index ∼ 2.9 is seen for PSR J1939+2134, indicating the possibility of a scattering index that doesn’t agree with that expected for a Kolmogorov scattering medium consistent with similar results for millisecond pulsars in past studies. Despite the relatively short time baseline, the noise models broadly agree with the other PTAs and provide, at the same time, well-constrained DM and scattering variations. The accompanying image shows the posterior distributions for pulsar J1939+2134, with 68%,90%,99% credible intervals for achromatic red noise, DM and Scattering variations.

The Indian Pulsar Timing Array (InPTA) announced its first official Data Release or the 'InPTA DR1', published in October, 2022. The InPTA is an Indo-Japanese collaboration of about forty radio-astronomers working together with the International Pulsar Timing Array (IPTA) towards the detection of a low-frequency gravitational wave background. The InPTA data release stems from three and a half years of observation using the upgraded Giant Metrewave Radio Telescope (uGMRT) operated by the NCRA-TIFR. The uGMRT is capable of conducting simultaneous observations in multiple radio-frequency bands from the same source. The 30 dishes of the uGMRT are sub-divided into groups or 'sub-arrays', each with receivers recording radio signals arriving in different frequency bands at the same time. This feature grants InPTA the unique strength to measure the density of electrons in the interstellar medium (ISM) along our line of sight with some of the highest precisions obtained so far. Fluctuations in the ISM are known to act as notorious sources of noise that mimic the effects of low-frequency background gravitational waves in the pulsar signal arrival times. Hence, precise estimation of variations in the density of the ISM is crucial for identifying and filtering this noise. Such effects, being most prominent in low radio-frequencies, can most ideally be investigated by the Indian PTA using the distinctive low-frequency coverage of the uGMRT. Thus, the InPTA DR1 is a critical addition to the combined pool of data from the global PTA consortiums for a unified search for the elusive stochastic gravitational wave background. The accompanying figure shows the dispersion-measure time-series of 14 pulsars included in InPTA DR1.

Many pulsars in the known population exhibit nulling, which is characterized by a sudden cessation and subsequent restoration of radio emission. Singh et al. present the localization, timing, and emission properties of a pulsar discovered by the GMRT High Resolution Southern Sky survey: J1244-4708. The authors find that the pulsar shows clear nulling, with a nulling fraction close to 60%. The nulling is found to be quasiperiodic, with two timescales. The authors demonstrate the broadband nature of the nulling in this pulsar via simultaneous GMRT observations in Band-3 (300-500 MHz) and Band-4 (550–750 MHz) (see the adjacent figure). The fact that this pulsar shows quasiperiodicity in nulling and the nulling is seen simultaneously in two widely separated frequency bands, favors the cessation of coherent radio emission as the origin of nulling in this pulsar. The authors also present a comparison of the efficiency of various search approaches such as single pulse search, fast folding algorithm (FFA)-based search, and fast Fourier transform (FFT)-based search, to search for nulling pulsars. They conclude that the FFA search is advantageous for detecting extreme nulling pulsars (nulling fraction > 80%); this is also confirmed with multiple epochs of observations of two nulling pulsars using the GMRT.

The timing follow-up of newly discovered millisecond pulsars (MSPs) is usually hindered by the large positional uncertainty (a few tens of arc-minutes) associated with the discovery. The ON-OFF gated imaging approach, which subtracts a pulsar’s OFF pulse visibilities from its ON pulse visibilities, can be used to accurately localize the object. This approach efficiently removes the background sky in the image domain, leaving only the pulsar in the field. The other technique for pulsar localization is by forming multiple phased array (PA) beams on point sources taken from the continuum image of the field containing a pulsar, followed by a periodicity search to detect the events with expected increase in the signal-to-noise ratio. The two techniques were previously implemented for the 16 MHz legacy GMRT baseband data. Sharma et al. (2023) report a two-fold increase in the bandwidth of the coherently dedispersed gating correlator (i.e., 16 MHz to 33 MHz). This new advancement with a factor of two increase in observing bandwidth provides improved sensitivity in the image domain, enabling precise localization of fainter MSPs. The authors demonstrate precise localisation of two MSPs discovered in the GMRT High Resolution Southern Sky (GHRSS) survey using a 33 MHz offline gating correlator. Given the precise location, Sharma et al. also reported results from follow-up studies of these two MSPs with sensitive PA beams of the upgraded GMRT from 300 to 1460 MHz. The figure shows the PA beam (pulse phase as a function of frequency and phase bins) of the newly localized MSPs in uGMRT Band-3 observations. More sensitive observations in the PA mode for these two MSPs produce precise (sub-microsecond) times of arrivals, with very low uncertainties in the dispersion measure. Finally, the authors discuss the use of these MSPs for pulsar timing array (PTA) experiments aiming to detect low-frequency gravitational wave signals. The achieved timing and DM precisions for these two MSPs are well within the ranges of the corresponding values for the 50 MSPs that are regularly observed with the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), one of the leading PTA experiments.

Reliable neutron star mass measurements are key to determining the equation of the state of cold nuclear matter, but such measurements are rare. Black widows and redbacks are compact binaries consisting of millisecond pulsars and semi-degenerate companion stars. Using data from the Fermi Large Area Telescope, gamma-ray eclipses were searched for from 49 spider systems, resulting in the discovery of significant eclipses in 7 systems, including the prototypical black widow, PSR B1957+20. Gamma-ray eclipses require direct occultation of the pulsar by the companion, and so the detection, or significant exclusion, of a gamma-ray eclipse strictly limits the binary inclination angle, providing new robust, model-independent pulsar mass constraints. The figure shows gamma-ray orbital light curves of seven eclipsing spider pulsars. The red dashed lines show the estimated background level. Phase 0 corresponds to the pulsar’s ascending node.

Pulsar timing is the regular monitoring of the rotation of a neutron star by measuring the time of arrival of its pulses. Timing studies of a special class of millisecond pulsars (MSPs) called black widow (BW) MSPs, with an orbital period of less than 1 day, are important because, in these systems, the pulsar and the companion stars are in very compact binary orbits and the highly energetic wind from the pulsar ablates the companion. Complete evaporation of the companion is assumed to be one way to form isolated MSPs. Until now, no BW MSP has been found where it is possible to ablate the companion within the Hubble timescale and the quest to find such pulsars is still on. Long-term timing studies of these systems also allow one to explore the possibility of the inclusion of such systems in pulsar timing arrays. The decade-long timing of PSR J1544+4937 reported here by Kumari et al. (2022) has aided in the studies of proper motion, dispersion measure (DM), and orbital period variation. It is the longest-duration timing study of any galactic field MSP with the Giant Metrewave Radio Telescope (GMRT) and a timing residual of 5.5 µs is achieved for this pulsar using the multi-frequency observations with the GMRT and the Green Bank Telescope (GBT). The authors have obtained a significant detection of the proper motion of 10.14 mas/yr for this pulsar. Studies of proper motion done by them for a sample of BW MSPs and isolated MSPs indicate that BW MSPs may not be the progenitors of the isolated MSPs. The authors report long-term temporal variation of the DM of the order of 0.001 pc per cm^3 along the line of sight to the pulsar. Such variations could arise due to the proper motion of the pulsar or the dynamical evolution of the interstellar medium. The authors also observed frequency-dependent variation in the DM of the order of 0.001 pc per cm^3, using GMRT Band-3 and Band-4 observations. Based on this, they conclude that spatial electron density variations are a possible cause of the frequency-dependent DM values. The authors also used this study to observe long-term orbital period variations in PSR J1544+4937 for the first time. They investigated possible causes and propose that changes in the gravitational quadrupole moment of the pulsar companion could be responsible for the observed temporal changes in the orbital period. The ephemeris from their timing study also provide an improved detection significance in gamma-rays, enabling high-energy studies of this system. The figure shows the timing residual plot obtained from a decade of timing of PSR J1544+4937 using the GMRT and the GBT, with the different colors corresponding to data from different observing frequencies.

Pulsar timing array (PTA) experiments seek to detect the nanoHertz region of the gravitational wave (GW) spectrum, which is thought to be produced by an ensemble of supermassive blackholes. A PTA is composed of an array of millisecond pulsars (MSPs) distributed over the sky that have exceptional rotation stability. The angular correlation between the residuals of the arrival times of pairs of MSPs is used to search for stochastic GW signals. However, the timing data can be heavily contaminated by frequency-dependent effects caused by the interstellar medium or inherent in pulsars (profile evolution). Modeling of frequency-dependent effects is required to mitigate the timing noise to increase sensitivity towards the expected GW background imprints in pulsar timing data. Sharma et al. describe a timing study for a group of eight millisecond pulsars with the upgraded GMRT, aided by the large fractional bandwidth, at observing frequencies ranging from 300 to 1460 MHz. The authors used PulsePortraiture-based wide-band timing analysis, rather than traditional narrow-band analysis with a frequency invariant template profile, to account for the frequency evolution of the profile of pulsar. The wide-band timing method yielded a time of arrival (ToA) precision in Band-3 (300-500 MHz) of order of a micro-sec and a dispersion measure (DM) precision of 10^-4 pc cm^-3 for GMRT-discovered pulsars, and of sub-micro-sec (ToA) and 10^-5 pc cm^-3 (DM) for PTA pulsars. Sharma et al. demonstrate the significance of allocating the entire GMRT array to a single low-frequency band for precise intra-band DM measurements. The effectiveness of profile-modeling at low frequencies is demonstrated by this wide-band timing study over a broad frequency range. This study investigates the possibility of using newly-discovered GMRT pulsars in the PTA experiment and the achievable long-term timing precision for them. The figure shows median ToA (top panel) and DM (bottom panel) uncertainties obtained in the two (narrow-band and wide-band) analyses for eight pulsars. Error bars represent the range of precision obtained for the individual pulsar data sets. Pulsars are arranged on the x-axis in increasing order of their Band-3 DM uncertainty. Green, blue, and black colours are used to represent the values obtained from wide-band analysis in Bands 3, 4, and 5, respectively. Similarly, light-green, sky-blue, and dark-gray colours are used for narrow-band analysis values in Bands 3, 4, and 5, respectively. In general, ToAs are more precise in wide-band analyses than in narrow-band analyses; however, the DM precisions are similar for both the techniques.

Millisecond pulsars are extremely stable rotators allowing to use these as celestial clocks. An ensemble of these can be used for efficient detectors of gravitational wave signals. This paper presents a pulsar timing array implementation by the Fermi-LAT collaboration using 12.5 years of Fermi-LAT data from 35 bright gamma-ray millisecond pulsars file that achieved a sensitivity close to that aimed by the approaches of radio timing of the millisecond pulsars. The sample of millisecond pulsars includes Fermi-directed discoveries using the GMRT, where the gamma-ray pulsation was discovered after folding LAT photons with the timing model derived using the GMRT observations. This paper place a 95% credible limit on the GWB characteristic strain of 1.0 x 10^-14 at a frequency of 1/year. This result provides an independent upper limit on the gravitational wave background signal. This is important since this method as well as the radio timing methods are subject to different noise sources. The observed sensitivity of detection is expected to improve with the observing time span. The figure shows the constraints on the GW background from radio and gamma-ray PTAs. The inferred constraints on the GWB amplitude at 1/year (Agwb) are plotted as a function of the publication date. Colored symbols correspond to each of the PTAs indicated in the key. Upper limits at 95% confidence are shown as downward arrows, and amplitude ranges indicate detections of a common noise process, which could be the GWB or have other origins. The Fermi-LAT 95% upper limit, 1.0 x 10^-14, uses data obtained up to January 2021 and is plotted at a publication date of April 2022. The dashed red line indicates the expected scaling of the Fermi-LAT limit as a function of time.

Kaur et al. report the first HI 21 cm mapping study of the neutral atomic hydrogen (HI) in the host galaxy of a fast radio burst (FRB). They used the Giant Metrewave Radio Telescope (GMRT) to carry out a deep observation of a nearby fast radio burst, FRB 20180916B, and find that the FRB host is a gas-rich galaxy but with low star-formation activity. The combination of gas-richness and near-quiescent star-formation indicates that the galaxy is likely to have acquired a significant mass of HI in the recent past. The GMRT images show that the HI spatial distribution is disturbed, with extended HI 21 cm emission detected in a northeastern tail, a counter-tail toward the south, an HI hole between the galaxy center and the FRB location, and a high HI column density measured close to the FRB position. The FRB host galaxy is part of a group with four companions detected in their HI 21 cm emission, the nearest of which is only 22 kpc from the FRB location. The gas richness and disturbed HI distribution indicate that the FRB host has recently undergone a minor merger with a smaller galaxy or a gas cloud, which increased its HI mass, disturbed the HI in the galaxy disk, and compressed the HI near the FRB location to increase its surface density. Kaur et al. propose that this merger caused the burst of star formation in the outskirts of the galaxy that gave rise to the FRB progenitor. The evidence for a minor merger is consistent with scenarios in which the FRB progenitor is a massive star, formed due to the merger event. The two panels of the figure show the GMRT HI 21cm images (in contours) of the FRB galaxy environment, at angular resolutions of (left panel) 3.5 arcseconds and (right panel) 9 arcseconds; the GMRT images are overlaid on an HST image of the galaxy, shown in colour. The high-resolution GMRT image of the left panel, which is sensitive to only the highest HI column densities, shows that the strongest HI 21cm emission arises from (1) the galaxy centre, (2) close to the FRB location, and (3) the gas cloud called G1. The intermediate-resolution GMRT image of the right panel, which is sensitive to lower HI column densities, shows the tail of HI 21cm emission to the north-east, the counter-tail towards the south, and the hole in the HI distribution close to the FRB location (indicated by the magenta star in both panels).

Nature’s best clocks, millisecond pulsars (MSPs), are ultra-dense dead stars that act like celestial lighthouses, with their radio light beams sweeping the Earth as fast as a few hundred times in a second. Being extremely stable rotators, MSPs act as laboratories for the study of matter in extreme conditions. MSPs often have orbital companions. In some MSP systems, the pulsar and the companion star have separations comparable to the Earth-Moon distance and interact strongly with each other in very compact orbits (<10 hrs); these are called spider MSPs. Energetic radiation from the pulsar can ablate material from the companion and blow it away; this diffuse material could eclipse the radio pulses emitted by the pulsar. Interestingly, the eclipse properties depend on the frequency of the radio pulse, with low radio frequencies being eclipsed, while high radio frequencies are not. The exact mechanism by which this occurs has not been established until now. After the first discovery of spider MSPs by Fruchter et al. (1988), only a few such systems have been studied to understand the eclipse mechanism. Most of these studies used narrow-bandwidth observations and could not probe the transition between the optically-thick and optically-thin regimes. Thus, these studies could only probe the eclipse boundary and could not determine the eclipse mechanism distinctively. For the first time, Kansabanik et al. (2021) used wide-bandwidth observations with the upgraded Giant Metrewave Radio Telescope (uGMRT) to observe a spider MSP, J1544+4937, in order to understand the frequency-dependent eclipse mechanism. They modelled the observed spectrum of the pulsar near superior conjunction at frequencies where the eclipse medium is transitioning from the optically-thick to the optically-thin regime. Simultaneous observations covering the frequency ranges 300-500 MHz and 650-850 MHz allowed them to determine the onset frequency of the eclipse as (345 +/- 5) MHz, 20 times more accurate than earlier estimates. Using this accurate eclipse onset frequency and the broadband spectrum during the full eclipse phase (FEP), Kansabanik et al. determined that the eclipse arises due to synchrotron absorption by relativistic electrons in the eclipse medium; they ruled out other possible eclipse mechanisms like scattering, scintillation, free-free absorption, and induced Compton scattering. Spectral modelling also allowed them to determine the line of sight-averaged magnetic field strength of the eclipse medium. They found that the average magnetic field strength of the eclipse medium is roughly 13 G, similar to the magnetic field strength obtained from assuming energy equipartition. The top panel of the figure shows the observed and the modelled spectra, for free-free absorption (brown line) and induced Compton scattering (blue line) as the eclipse mechanisms, respectively. It is clear that these mechanisms cannot reproduce the observed spectrum (green points) during the eclipse phase. The bottom panel shows the modelled spectrum considering synchrotron absorption (magenta line) as the eclipse mechanism; this is seen to be in good agreement with the observed spectrum (green circles).

Main-sequence Radio Pulse emitters (MRPs) are main-sequence stars that emit coherent radio pulses periodically by the process of electron cyclotron maser emission (ECME). The phenomenon was first discovered from the magnetic late B-type star CU Virginis by Trigilio et al. (2000). Since then, this star has been observed several times at radio bands. While these studies have firmly established that the star always produces two right circularly polarized (RCP) pulses per rotation cycle at frequencies less than 5 GHz, all but one of the observations were carried out at frequencies above 1 GHz. The lone sub-GHz observation was carried out with the GMRT at 610 MHz, but covered only a narrow range of rotational phases. Das and Chandra, for the first time, carried out extensive observation of the star for one full rotation cycle, over the frequency range 400 MHz to 4.0 GHz, using the upgraded GMRT (uGMRT) and the Karl G. Jansky Very Large Array (VLA). Contrary to the notion that the star produces only RCP pulses, Das and Chandra discovered that the star actually produces both left (LCP) and right circularly polarized pulses. In fact, at sub-GHz frequencies, the LCP pulses are much stronger than the RCP pulses. The authors found that the star is much more active at sub-GHz frequencies in terms of the number of pulses that it emits per rotation cycle, as well as the intensity of the pulses. This kind of behavior is entirely inconsistent with current ideas about the ideal MRP behavior. Das and Chandra proposed that such an anomaly could be a signature of very strong plasma density gradients in the stellar magnetosphere, a manifestation of a complex magnetic field, or could arise due to multiple 'engines' (e.g. satellite-induced emission) for production of ECME similar to the case of Jupiter. Apart from these peculiarities, Das and Chandra also witnessed flares from the star at sub-GHz frequencies, a phenomenon totally unexpected from a CU Virginis-like star with an extremely stable global magnetic field; they also observed a giant pulse which was nearly 10 times stronger than the typical pulses observed from this star. Das and Chandra speculate that these could be the signatures of episodic ejection of plasma from the stellar magnetosphere induced by by centrifugal force overpowering the magnetic field tension. If confirmed, the new results will open up a vast potential for this emission to become a magnetospheric probe to yield information regarding dynamical events in the apparently-stable magnetospheres of hot magnetic stars. The figure shows the light curves of the star at different frequencies (red and blue represent RCP and LCP, respectively) along with the stellar longitudinal magnetic field (top panel).

Marthi et al. have measured the speed of the scintillation pattern of PSR B1508+55 on a 10000-km baseline between the GMRT and the Algonquin Radio Observatory (ARO) 46-m telescope. The low cross-correlation coefficient of the scintillation pattern measured at the two telescopes points to the presence of atleast two screens along the line of sight to the pulsar. They use the 45-second delay in the arrival of the scintillation pattern between the telescopes to measure the speed and infer that this scintillation arises from a screen different than seen at the GMRT. The scintillation timescale of 135 second, attributed to the primary scintillation arc seen at the GMRT, is three times longer than the scintillation pattern delay measured on the 10000-km baseline, ruling out both fully isotropic as well as one dimensional scattering, but suggestive of highly anisotropic two dimensional scattering. They hypothesize that the screen causing the primary scintillation arc seen at the GMRT is likely partially resolving the scattering on the screen located further beyond, and that the combined scintillation is responsible for the low cross-correlation seen on the GMRT-ARO baseline. Left: The cross secondary spectrum showing the amplitude and phase gradient across differential Doppler frequency. The amplitude of the cross spectrum normalized by the product of the secondary spectra gives the cross-correlation coefficient of 0.22. Right: The measured phase gradient corresponds to a scintillation delay of ~45 seconds.

Effects like dispersion and scattering are more influential at lower observing frequencies, with the variation of these quantities over week-month timescales requiring high-cadence multi-frequency observations for pulsar timing projects. The mitigation of such interstellar effects is crucial to achieve the necessary precision for detecting the stochastic Gravitational Waves (GWs) background using a large set of high-timing precision millisecond pulsars (MSPs) distributed across the sky. The primary goal of the Pulsar Timing Array (PTA) is to detect and characterise the low-frequency gravitational waves through high-precision timing.  Jones et al. used the low-frequency observing capability of the GMRT and evaluated the potential decrease in dispersion measure (DM) uncertainties when combined with existing pulsar timing array data from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). They observed four PTA MSPs with the GMRT simultaneously at 322 and 607 MHz, and compared the DM measurements with those obtained through NANOGrav observations with the Green Bank Telescope and Arecibo Observatory at 1400–2300 MHz frequencies. It was shown that incorporation of these low-frequency GMRT data into the NANOGrav data set provides improved DM measurements. Comparison of single-epoch DMs for GMRT and NANOGrav 11-year measurements for the four MSPs, PSRs J1640+2224, J1713+0747, J1909−3744, and J2145−0750 showed the presence of frequency-dependent biases in DM measurements, which could be caused by unmodeled pulse profile evolution. The paper also described the effect of pulse profile baseline ripple on high precision timing of MSPs. Being one of the first attempts to utilize the GMRT for International Pulsar Timing Array (IPTA) work, Jones et al. discussed the challenges of incorporating GMRT data into NANOGrav and IPTA data sets.

Maity & Chandra carried out the lowest-frequency measurements of gamma-ray burst (GRB) 171205A with the upgraded Giant Metrewave Radio Telescope (uGMRT), covering a frequency range of 250-1450 MHz and a period of upto 1000 days. This is the first GRB afterglow detected in the 250-500 MHz frequency range and the second brightest GRB detected with the uGMRT. Even though the GRB was observed for nearly 1000 days, there is no evidence of a transition to the non-relativistic regime. The data are fit with a synchrotron afterglow emission arising from a relativistic, isotropic, self-similar deceleration as well as from a shock breakout of a wide-angle cocoon. The authors were able to discern the nature and the density of the circumburst medium, finding that the GRB is likely to have exploded in a stratified wind-like medium. Their analysis suggests that the radio afterglow has a contribution from two components: a weak, possibly slightly off-axis jet and a surrounding wider cocoon, consistent with earlier results. The cocoon emission is likely to dominate at early epochs, whereas the jet starts to dominate at later epochs, resulting in flatter radio light curves. The figure shows the uGMRT Band-5, Band-4 and Band-33 radio light curves, with the Band-4 and Band-5 values scaled by factors of 10 and 100, respectively. The data are best fit with pre- and post peak spectral indices of 1.37 +/- 0.20 and -0.72 +/- 0.06.

Nayana & Chandra report low-frequency radio observations of the fast-rising blue optical transient, AT 2018cow, with the upgraded Giant Metrewave Radio Telescope (uGMRT). They covered epochs from ~10-600 days post-explosion and a frequency range of 250-1450 MHz. The modeling of the radio data reveals an inhomogeneous radio-emitting region expanding into an ionized medium. They constrained various physical parameters of the explosion, such as the evolution of shock radius, shock velocity (v > 0.2c) and the mass-loss rate of the progenitor. The upper limit to the mass loss rate of the progenitor star, 50 years before the explosion, was a millionth of a solar mass per year. This is a hundred times smaller than the previously reported mass-loss rate 2 years before the explosion, indicating an enhanced phase of the mass-loss event close to the end of the life of the progenitor. The results are in line with the speculation of the presence of a dense circumstellar shell in the vicinity of AT 2018cow from previous radio, ultra-violet, and optical observations, and have important implications for these explosions. The figure shows the uGMRT light curves of AT 2018cow at 0.40, 0.75 and 1.25 GHz frequencies. The green and red solid lines denote the best fit SSA and FFA models respectively. The green and red dotted lines denote the best fit inhomogeneous SSA and FFA models, respectively.

Spider pulsars are fast spinning millisecond pulsars (MSPs) in compact binaries with a low-mass companion. Polzin et al. present a study of the low-frequency eclipses of spider pulsars PSR B1957+20 and PSR J1816+4510 with the Low Frequency Array (LOFAR), the upgraded Giant Metrewave Radio Telescope (uGMRT), the Westerbork Synthesis Radio Telescope (WSRT) and the Parkes telescope. This dedicated campaign to simultaneously observe the pulsed and imaged continuum flux densities throughout the eclipses reveals many similarities between the excess material within the two binaries, independent of the companion star properties. Measurements of eclipse durations over a wide range of radio frequencies show a significant dependence of eclipse duration on frequency for both pulsars, with wider eclipses at lower frequencies. The results of the paper provide a marked improvement in the observational constraints available for theoretical studies of the eclipse mechanisms. The observations show that the pulsar fluxes are entirely removed throughout the main body of the eclipses. For PSR J1816 + 4510, Polzin et al. present the first direct evidence of an eclipse mechanism that transitions from one that removes the pulsar flux to one that merely smears out pulsations. The authors argue that this is a consequence of scattering in a tail of material flowing behind the companion. Contrary to the belief that evolution of such systems can ultimately explain formation of the isolated MSPs, the inferred mass-loss rates from the companion stars estimated in this study are found to be too low to evaporate the stars within a Hubble time. The figure presents measurements of the radio emission of PSR B1957+20 throughout the eclipse region.

Millisecond pulsars (MSPs) are rapidly rotating neutron stars, from which we observe pulses having extremely stable rotational periodicity as the beams of radiation sweep across our line of sight. This makes MSPs the most accurate celestial clocks. Searching for pulsations of unknown MSPs in the gamma-ray band  is extraordinarily computationally expensive due to the scarcity of photons, particularly in the case of binaries where the MSP revolves around its companion. While gamma-ray searches have been possible in a few cases, it is generally far more efficient to first search for radio pulsations in the direction of the gamma-ray sources, to identify the pulsar period. Bhattacharyya et al. used the Giant Metrewave Radio Telescope (GMRT) at 322 MHz and 607 MHz to search for radio pulsations in the directions of 375 unassociated Fermi Large Area Telescope (Fermi-LAT) gamma-ray sources. They identified three new MSPs, PSR J0248+4230, PSR J1207-5050 and PSR J1536-4948, named after their locations in the sky. After the discovery, the authors conducted regular timing follow-up observations for about 5 years with the GMRT to pin down the pulsar periods, period derivatives, sky positions, and parameters related to the pulsars' orbits. They then folded the gamma-ray photons from the three MSPs from the Fermi-LAT data with the parameters derived from the GMRT observations, resulting in the detection of gamma-ray pulsations as well. They find that PSR J0248+4230 and PSR J1207-5050 are isolated MSPs, with periods of 2.60 milliseconds and 4.84 milliseconds. PSR J1536-4948 has a period of 3.07 milliseconds, and is in a binary system with an orbital period of about 62 days about a star whose mass is approximately 1/3rd that of the Sun.  PSR J1536-4948 is an unusual MSP with an extremely wide pulse profile in both radio and gamma-rays, a pattern not generally seen in such pulsars. Bhattacharyya et al. examined the pulsar emission models and emission geometries that could account for the observed radio and gamma-ray pulsed emission. PSR J1536-4948 is very bright in gamma-ray, allowing the authors to count every photon emitted from the source from the lowest to the highest energy band of the gamma-ray spectrum, at an accuracy of 1 part in a million. In addition, PSR J1536-4948 shows evidence for very high energy emission (at energies higher than 25 GeV), which is very rare for millisecond pulsars. The figure shows the pulse profiles of the 3 MSPs from the GMRT and Fermi-LAT observations.

Chandra et al. carried out  a comprehensive analysis of supernova SN 2001em covering a period of 19 years since its discovery. SN 2001em is the oldest supernova known to have undergone a metamorphosis from a stripped envelope, with no hydrogen or helium, to an interacting supernova (with late time presence of hydrogen). An early spectrum indicates that it exploded as a Type Ib supernova. Later, the ejecta caught up with a dense circumstellar hydrogen-shell, ejected a few thousand years before the explosion, triggering interaction between the supernova ejecta and the dense shell, producing radio, X-ray, and hydrogen-alpha emission. Chandra et al. used data from the Very Large Array in radio bands and from Chandra, XMM-Newton, and Swift-XRT in the X-ray bands, along with the hydrogen-alpha measurements. They combined these data with their low radio frequency measurements with the Giant Metrewave Radio Telescope at two epochs covering three frequencies. While the observations missed the phase when the shock entered the dense shell, the X-rays indicate that the shock came out of the dense shell at around 1750 days. One of the most interesting features is revealed in the radio data, which show a spectral inversion at late epochs (more than 5000 days after the explosion) at around 3 GHz, which mimics the properties of the central absorbed component seen in SN 1986J. A possible explanation for this component is that the progenitor of SN 2001em was a massive binary system that underwent a period of common-envelope evolution. The hydrogen envelope from the progenitor of SN 2001em may have been lost as a result of binary interaction. SN 2001em is the only other supernova after SN 1986J in which this kind of spectral inversion is seen. The figure shows a comparison of the late time radio spectrum of SN 2001em at approximately 19 years after the explosion with that of SN 1986J at approximately 30 yrs; the latter shows the presence of a central component at late times. This is one of the most direct pieces of evidence of common-envelope evolution causing asphericity in the explosion environment.

Superluminous supernovae (SLSNe) are a type of supernova that have an optical absolute magnitude <−21 and are more than 10 times brighter than typical supernovae. Of the SLSNe, the most mysterious ones are the Type I SLSNe, which do not show any hydrogen line in their optical spectra. Little observational evidence exists to test the various theories proposed to explain the high luminosity of these objects. Additionally, at least some of the Type I SLSNe are hypothesised to emit Fast Radio Bursts (FRBs). However, this association was made based on the properties of the host galaxy of a very well studied FRB, FRB121102 and the host galaxies of Type I SLSNe. Until the present work, there had been no quantitative study of the relationship between the radio emission from a Type I SLSN and that from an FRB. Mondal et al. observed the first radio-detected Type I SLSN, PTF10hgi, over a wide frequency range spanning 0.6-18 GHz using the upgraded Giant Metrewave Radio Telescope (uGMRT) and the Karl G. Jansky Very Large Array (JVLA), and quantitatively estimated the various physical properties of the radio-emitting region. The spectral nature of the source was found to be very similar to that of the persistent radio source associated with FRB121102. Their analysis revealed that the radio emission of PTF10hgi originates from a magnetar wind nebula, confirming the hypothesis of Inserra et al. (2013). They also demonstrated that the nebula is powered by the rotational energy of the magnetar. Additionally, Mondal and collaborators analysed archival uGMRT data and extended the available spectrum of FRB121102 to 0.3 GHz. These new measurements put strong constraints on some of the models of FRB121102, ruling out some models. Wang et al. (2020) have already demonstrated that the persistent emission of FRB121102 might be powered by the same mechanism that powers the radio emission of SLSNe, demonstrating for the first time a relationship between a Type I SLSN and a FRB. Based on their calculations, Mondal et al. (2020) also hypothesised that if PTF10hgi is emitting FRBs, their energies will be much lower than that observed from FRB121102. The spectra of the two sources are shown in the adjoining image, where beta is the power-law index of the radio spectrum.

Hot magnetic stars are unique objects that harbour highly stable, kGauss-strength global magnetic fields. The interaction between their magnetic fields and the radiatively driven stellar wind leads to the formation of corotating magnetospheres around them.  A number of these stars have recently been discovered to produce electron cyclotron maser emission (ECME). ECME is a type of coherent radio emission which is seen as periodic pulses giving the host star the name of ‘main-sequence pulsar’.  This emission has several interesting properties like very high directivity and circular polarization, which can be exploited to probe the stellar magnetosphere. Das et al. (2020) developed a 3D framework to predict how the ECME light curve will vary for different plasma distributions in the stellar magnetosphere due to refraction. Note that this is the first and, currently, the only framework to quantitatively study the propagation effect experienced by ECME while passing through the dense magnetosphere of the hot magnetic star. Before that, it was thought that the effect of propagation is limited to introducing a frequency dependence of the rotational phases of arrival of the ECME pulses. In contrast, Das et al. found that for stars which have highly misaligned rotation and magnetic axes, the propagation effect can alter not only the rotational phases of arrival, but also the profile, relative height, and, in the extreme case, the visibility of the pulses as well. This information will be highly useful to constrain the plasma distribution in the stellar magnetosphere based on the observed shape of the ECME lightcurves. In the figure, simulated ECME lightcurves are shown for a case in which the host star has a highly azimuthally asymmetric plasma distribution in its magnetosphere.

Marthi et al. report the GMRT detection of 15 bursts from the fast radio burst (FRB) FRB180916.J0158+65, better known as R3. They used the GMRT Band-4 receiver to observe R3 at 550-750 MHz, with three sessions, each of ~2 hour duration. Each session was scheduled around the peak of the active window of the 16.35-day period of R3. They detected 0, 12 and 3 bursts respectively, suggesting a highly variable burst rate. These 15 beamformer-detected bursts include the largest number detected in a single session (12) as well as the lowest fluence bursts (0.1 Jy ms) of R3 till date. The low fluence bursts have an important implication - they are only ~10-25 times more energetic than the bursts from the Galactic source SGR1935+2154, strengthening the case for a possible astrophysical connection between the brightest Galactic bursts and the faintest FRB events. The bursts show rich structure in time and frequency. Marthi et al. devise a technique based on singular value decomposition to determine the dispersion measure (DM) that maximizes energy in the temporal substructure, but find that the DMs so determined for the brightest bursts are consistent with the median value of 348.82 pc per cubic cm. Although R3 has been localized to a nearby spiral galaxy (at a distance of ~150 Mpc and a redshift of ~0.0337), they image the two brightest bursts using the interferometric visibilities, providing proof-of-concept for future GMRT experiments to localize unlocalized CHIME repeaters. They also report a candidate short timescale periodicity of 15.6 ms. Confirmation of a short timescale periodicity is evidence for a neutron star progenitor, whereas orbital motion or precession could explain the known 16.35-day periodicity. Further GMRT observations of R3 are underway to confirm or rule out the short timescale periodicity, as well as investigate the origin of the 16.35-day periodicity.

Kudale et al. present a multifrequency study of eclipse properties of a transitional redback millisecond pulsar J1227-4853 discovered with the GMRT. Emission from this pulsar is eclipsed at 607 MHz for about 37% of its orbit (shown in panel-B) around the superior conjunction. The authors observed eclipse ingress and egress transitions (identified by excess dispersive delays up to 0.079(3) pc cm^-3) that last for 12% and 15% of the pulsar orbit, respectively, resulting in only 36% of the orbit being unaffected by eclipsing material. Simultaneous dual-frequency GMRT observations (300-500 MHz and 550-750 MHz) allowed a probe of the frequency-dependent eclipse geometry (shown in panel-A) with a power-law index for the frequency dependent eclipse duration as n = -0.44. The simultaneous timing and imaging studies (shown in panel C & D) suggest that the eclipses in J1227-4853 are not caused by temporal smearing due to excess dispersion and scattering, but could be caused by removal of pulsar flux due to cyclotron absorption of the pulsed signal by intra-binary material; this can be used to constrain the magnetic field of the companion. In order to check if cyclotron-synchrotron absorption of pulsar emission by nonrelativistic or relativistic electrons is the cause of the eclipse, the authors estimated the magnetic field of the eclipsing plasma in the vicinity of the companion to be 27 G, and the cyclotron fundamental frequency to be 77 MHz. Observed eclipses reported in the paper for PSR J1227-4853 are centered at 322 and 607 MHz, which are the fourth and eighth harmonics of the above cyclotron fundamental frequency. For PSR J1227-4853, cyclotron absorption at the fundamental cyclotron frequency and its lower harmonics could be the cause of eclipse. Additionally, near the inferior conjunction at orbital phases 0.71 and 0.82 the pulsed emission is significantly delayed (corresponding to a DM change of 0.035(3) pc cm^-3), which is associated with a fading of the pulsed and continuum flux densities (shown in panel-A, by light purple color). The minima in continuum flux densities (up to ~30% of the peak flux density) around the inferior conjunction coincide with the maxima in excess dispersion (shown in panel-D). Such flux fading around a fixed orbital phase near the inferior conjunction is not reported for other eclipsing binaries. This event around the inferior conjunction could be caused by absorption of the pulsed signal by fragmented blobs of plasma generated from mass loss through the L2 Lagrangian point.

With their high magnetic fields, young ages, persistent but highly variable X-rays, and transient radio emission, magnetars comprise one of the exotic parts of the pulsar population. Besides understanding the radio emission mechanism itself, observational probes of several magnetar-based models of the fast radio bursts (FRBs) also highly motivate for magnetar studies at low radio frequencies. However, such studies remain challenging due to the scarcity of radio detections of magnetars as well as the transient nature of their emission. XTE J1810-197 was the first-ever magnetar which was found to emit transient radio emission. It has recently transitioned into the second known radio outburst phase. Maan et al. observed the magnetar at low radio frequencies using the GMRT, soon after the onset of its recent outburst, and reported the first detection of the source at frequencies as low as 300 MHz. The magnetar exhibits radio emission in the form of strong, narrow bursts, with a characteristic intrinsic width of the order of 0.5-0.7 ms. Maan et al. also found that the bursts exhibit spectral structures which cannot be explained by interstellar propagation effects. These structures might indicate a phenomenological link with the repeating FRBs, which also show interesting, more detailed, frequency structures. A sample of the magnetar bursts demonstrating the spectral as well as temporal structures is shown in the accompanied figure. To probe any physical link between the bursts of the magnetar and the repeating FRBs, as well as to understand the underlying emission mechanism, the magnetar is currently being monitored using the GMRT.

Polzin et al. report on ~ 10 yr of observations of PSR J2051-0827, a millisecond pulsar in special evolutionary state, at radio frequencies in the range 110 - 4032 MHz. They investigate the eclipse phenomena of this black widow pulsar using model fits of increased dispersion and scattering of the pulsed radio emission as it traverses the eclipse medium. No clear patterns are found between the low-frequency eclipse widths, orbital period variations, and trends in the intra-binary material density. Using polarization calibrated observations Polzin et al. present the first available limits on the strength of magnetic fields within the eclipse region of this system; the average line of sight field is constrained to lie in the range 0.0001 - 100 G, while for the case of a field directed near-perpendicular to the line of sight we find the perpendicular component of the field to be <~ 0.3 G. The results are considered in the context of eclipse mechanisms, and Polzin et al. find scattering and/or cyclotron absorption provide the most promising explanation, while dispersion smearing is conclusively ruled out. Finally, Polzin et al. estimate the mass-loss rate from the companion to be ~ 10^{-12} solar masses per year suggesting that the companion will not be fully evaporated on any reasonable time-scale. The top panel of the figure shows measured flux densities for all 345 MHz observations covering the eclipse region, with each normalised so that the out-of eclipse mean flux density is unity. The horizontal dashed line corresponds to the detection limit of the telescope. The bottom panel of the figure shows the deviation from mean out-of-eclipse dispersion measures for the same set of observations.

The young pulsar residing inside the Crab nebula is not only one of the handful of pulsars known to emit giant radio pulses, but also the most frequent giant pulse emitter among them. The giant pulses emitted by the Crab pulsar reach pulse energies (flux density integrated over the pulse duration) as large as a few million times higher than the energies of the regular pulses. Statistical properties of the Crab giant pulses have been studied by a number of researchers in the past and it has been found that the energy distribution of the giant pulses is well described by a power law in contrast with that of the regular pulses which follows a log-normal or an exponential distribution. This indicates that the regular and giant pulses are likely to arise from different physical mechanisms. Although giant pulses have been observed to have pulse-energies up to ~1000 Jy ms at frequencies ~1.4 GHz, the relative rarity of the brighter pulses (in typically a few hours of observation) have restricted statistical studies to energies up to around 30 Jy ms. The brightest giant pulses, sometimes termed as the super-giant pulses, have recently become more interesting after the discovery of fast radio bursts (FRBs), which are short intense bursts of radio emission from unknown extragalactic sources. While a few FRBs have been observed to repeat, most of them are still not known to be repeating. Super-giant pulses from Crab-like pulsars in other galaxies have been suggested as a possible origin of the FRBs. Bera and Chengalur observed the Crab pulsar with the NCRA-15m telescope for ~260 hours in 31 observing sessions spanning ~45 days, and detected 1799 super-giant pulses with pulse-energies > 100 Jy ms at an observing frequency of 1.3 GHz. This is the largest sample of giant pulses with pulse-energies > 100 Jy ms at these frequencies, facilitating a statistical study of giant pulses up to pulse energies of ~3000 Jy ms, two orders of magnitude larger than energy ranges probed in similar earlier studies. The sample also contains one of the brightest giant pulses ever observed from the Crab pulsar, with peak flux density > 4 MJy and pulse energy ~4700 Jy ms. Bera and Chengalur studied the statistical properties of a sub-sample of 1153 super-giant pulses complete down to a pulse energy of 130 Jy ms and compared the distribution to that of the currently known FRBs. They find that the pulse-energy distribution (shown in the top panel of the figure) of giant pulses of the Crab pulsar follows a single power law, with power-law index approximately -3, over at least three orders of magnitude in pulse energy, from ~3 Jy ms to ~3000 Jy ms. The power-law index is in excellent agreement with that found for one of the repeating FRBs (FRB 121102). Bera and Chengalur also find that the rate of occurrence of super-giant pulses varies by a factor of approximately 5 on time scales of a few days (shown in the bottom panel of the figure), although the pulse-energy distribution remains the same within the uncertainties in both the "active" and "passive" phases (with relatively high and low rates of occurrence). This varying rate of pulse emission has also been seen for the repeating FRB, FRB 121102. Although the pulse energy of the brightest known super-giant pulse in this sample is still smaller than the inferred pulse energies of the FRBs by a few orders of magnitude, the similarities in the statistical properties suggest that super-giant pulses could be a viable model for repeating FRBs, requiring extremely young pulsars or magnetars (pulsars with extremely high magnetic fields) to explain the observed high pulse energies.

Roychowdhury et al. report Giant Metrewave Radio Telescope (GMRT) HI 21cm imaging of CGCG137-068, the host galaxy of the fast-evolving luminous transient (FELT) AT2018cow, the first study of the gas properties of a FELT host galaxy. They obtain a total HI mass of 660 million solar masses for the host galaxy, and an atomic gas depletion time of 3 Gyr and a gas-to-stellar mass ratio of 0.47, consistent with values in normal star-forming dwarf galaxies. At spatial resolutions of > 6 kpc, the neutral hydrogen of CGCG137-068 appears to be distributed in a disk, in mostly regular rotation. However, at spatial resolutions of 2 kpc, the highest column density neutral hydrogen is found to lie in an asymmetric ring around the central regions; AT2018cow lies within this high column density ring. This HI ring is suggestive of an interaction between CGCG~137-068 and a companion galaxy. Such a ring is ideal for the formation of compact regions of star formation hosting massive stars which are likely progenitors of FELTs. The figure shows the integrated HI 21cm intensity (top panels) and HI 21cm velocity field (bottom panels), at three different resolutions; the green circle in the top panels indicates the position of the FELT AT2018cow. The low-resolution images of the left and middle panels show that the large-scale HI 21cm emission is in the form of a regularly-rotating disk. The high resolution image of the right panel shows that the HI 21cm emission is distributed in a high-column density ring, with AT2018cow arising from the gas in this ring.

Radio emission from hot magnetic stars usually arises from the gyrosynchrotron process. However, a small number of these stars have been found to produce coherent radio emission generated by the Electron Cyclotron Maser Emission (ECME). This emission is observed in the form of highly circularly polarized pulses that arise close to rotational phases where the longitudinal magnetic field of the star is zero (i.e. the magnetic null phase). In the present work, Das et al.  used upgraded Giant Metrewave Radio Telescope (uGMRT) observations to confirm the presence of ECME from another star, HD 142990, at frequencies ~550-850 MHz (speculated to be a possibility by Lenc et al. (2018), based on their detection of highly circularly polarized emission from the star with the Murchison Widefield Array). Das et al. observed the star around both the magnetic null phases, and found significant flux density enhancement in both circular polarizations near both magnetic nulls, consistent with the hypothesis that the detected emissions arise from the ECME mechanism. The ECME pulses are, however, peculiar in the sequence of arrival of the two circulate polarizations, with the observed pattern matching that from neither the extra-ordinary mode (X-mode) nor the ordinary mode (O-mode). Das et al. found that both circular polarizations at 550-850 MHz appear to originate near the same magnetic pole, which has not been observed earlier. To explain this unique observation, the authors propose a scenario involving a transition between magnetic-ionic modes. This observation of mode transition, if confirmed, will be the first of its kind in hot magnetic stars. Further observations at frequencies both above and below the range 550-850 MHz will be needed to test the validity of this hypothesis. The upper panel of the figure shows the variation of the flux density of the star at different GMRT frequency bands. Band 4 corresponds to the frequency range 550-850 MHz and L-band, to 1420 MHz. RCP and LCP stand for right and left circular polarization, respectively. The lower panel shows the variation of the star's longitudinal magnetic field; the latter data were obtained from Shultz et al. (2018). The enhancements in flux density occur close to the magnetic null phases, which is expected for ECME.

Radio emission from supernovae is considered to be synchrotron emission which is absorbed at early epochs. Since the absorption optical depth scales approximately proportional to the square of the wavelength, low frequencies are ideal to probe the optically thick phase. Chandra et al. used low-frequency Giant Metrewave Radio Telescope (GMRT) observations of a core-collapse (Type Ib) supernova, Master OT J120451.50+265946.6, to find that the radio-emitting shock is inhomogeneous, with the inhomogeneities confined within the magnetic field distribution. Because of these inhomogeneities, the absorption is due to the superposition of various optical depths caused by varying magnetic fields. The inhomogeneities are primarily visible at low frequencies, and the high-cadence, high-sensitivity GMRT observations were critical in unraveling the nature of the inhomogeneities, which has important implications for the size of radio emitting regions. The left panel of the figure shows a single-component synchrotron self-absorption model fit to the GMRT 610 MHz data on the supernova, in the optically thick phase; this reveals a very steep electron energy spectrum, which is highly unphysical. The right panel of the figure shows the best-fit model that incorporates inhomogeneous synchrotron self-absorption, again fitting to the supernova light curves. The data at 0.61 GHz and 1.4 GHz data are from the GMRT (with three 1.4 GHz data points from the Karl G. Jansky Very large Array (VLA)), while the 7.1 GHz and 19.1 GHz data are from the VLA. The data before day 87 have been excluded from the fit as the radio emitting shock was crossing a dense shell at this epoch.

Radio emission from core-collapse supernovae carries information about the progenitor stellar system and immediate circumstellar environment. Nayana et al. used the Giant Metrewave Radio Telescope (GMRT) and the Very Large Array (VLA) to carry out a radio study of a Type IIP supernova, SN 2004dj, observing the source over both a wide range of frequencies (0.24 - 43 GHz) and a long time interval (covering ages from around 1 day to around 12 years after the discovery of the supernova). The wide frequency and temporal coverage allowed the authors to perform detailed modelling of local conditions in the supernova environment. Assuming a progenitor stellar wind velocity of 10 km/s, they infer the mass-loss rate of the progenitor star to be approximately a millionth of a solar mass per year. The derived value of the shock deceleration parameter is suggestive of a mildly decelerating blast wave. They studied temporal variation of the radio spectral indices between multiple frequency pairs (the figure shows the evolution of the spectral indices measured between frequencies of 1.06 and 1.4 GHz, 1.4 and 4.9 GHz, and 4.9 and 8.46 GHz), finding that the spectral indices steepen to values of -1 for an extended period from around day 50 to around day 125 after the explosion, especially at higher frequencies (between 4.86 and 8.46 GHz). This is indicative of electron cooling at the supernova shock. They calculate the cooling time scales and break frequencies for both synchrotron cooling and inverse-Compton cooling, and suggest that the steepening in spectral indices is due to inverse-Compton cooling of relativistic electrons at the supernova shock.

Occasional flashes of dispersed radio emission of typically a few milliseconds duration are detected from Rotating Radio Transients (RRATs). The nature of these RRATs, and their association with the rest of the neutron star population is still an open question. Bhattacharyya et al. present the longest-term timing study so far of three RRATs -- J1819-1458, J1840-1419 and J1913+1330 -- performed with the Lovell, Parkes and Green Bank telescopes over the past decade. Investigation of long-term variations of the pulse emission rate from these RRATs brings out a marginal indication of a long-term increase in the pulse detection rate with time for PSR J1819-1458 and J1913+1330. Conversely, a variation of the pulse detection rate of two orders of magnitude is observed between different epochs for PSR J1913+1330. The study also detected, for the first time, a weak persistent mode in PSR J1913+1330, in addition to the RRAT pulses, suggesting a possible connection between RRATs and the normal pulsar population. Although frequency-glitches are commonly seen for pulsars, PSR J1819-1458 is the only RRAT to exhibit glitches; a selection of its pulses is shown in the figure. The authors study the post-glitch timing properties of PSR J1819-1458 in detail and discuss implications of this study for glitch models. Its post-glitch over-recovery of the frequency derivative is magnetar-like; similar behaviour is only observed for two other pulsars, both of which have relatively high magnetic field strengths. Following the over-recovery, the authors find that some fraction of the pre-glitch frequency derivative is gradually recovered. It will be interesting to know if these glitches are representative of RRATs. This can only be verified with regular monitoring to detect possible glitches in other RRATs. 

Roy, Chengalur & Pen have demonstrated that a new way of beam-forming called post-correlation beam-forming (i.e. beam-forming which involves only phased sums of the correlation of the voltages of different antennas in an array) significantly improves the capabilities and sensitivity of the upgraded GMRT for discovering new pulsars and fast radio bursts (FRBs). Compared with the traditionally used incoherent (IA) and phased (PA) beam-forming techniques in radio telescopes for time-domain astronomy, this new technique dramatically reduces the effect of red-noise and radio frequency interference, yielding more than factor of 2 improvements in the  GMRT time-domain survey sensitivity. The eye-catching improvements in the signal-to-noise of the pulses from PSR J2144-3933 can be seen in the single-pulse time-series from the post-correlation beam-former. The extremely well-cleaned post-correlation beam also has an order of magnitude reduction in red-noise, as is clear in the power spectra plot. The post-correlation beam formation beautifully brings out the hitherto unexplored capability of interferometric arrays (the future of radio astronomy) over single dish telescopes.  We describe a time-domain survey with the GMRT using this post-correlation beam formation,  which will be one of the most sensitive surveys for pulsars and FRBs at low and mid-range radio frequencies.

HD133880 is a B-type rapidly-rotating star, with a period less than 1 day, on the main sequence. It is characterised by the presence of an asymmetric dipolar magnetic field of kiloGauss strength. Gyro-synchrotron radio emission has earlier been detected from this star. In 2015, Chandra et al. reported strong enhancement in the star's radio flux (at 610 MHz and 1420 MHz) at certain rotational phases, but the phase coverage was too limited for a detailed study. In the present work, Das, Chandra & Wade aimed to understand the origin of the radio pulses, by using the Giant Metrewave Radio Telescope (GMRT) 610 and 1420 MHz receivers to observe the star over a complete rotation. The GMRT 610 MHz data revealed a dramatic increase (by an order of magnitude) in the star's radio emission at a narrow epoch (phase 0.73) during its rotation, and in the right circular polarization; this can be seen in the upper panel of the attached figure. The observed enhancement is confined to a narrow range of phases and is approximately 100% polarised. Further, the enhancement occurs when the line of sight magnetic field is nearly zero, as can be seen in the lower panel of the figure. Das et al. find that the GMRT data single out Electron Cyclotron Maser Emission as the likely cause of the observed enhanced radiation. This maser process arises, under suitable conditions, due to the interaction of electromagnetic waves with a population of mildly relativistic electrons in a magnetised plasma. Previously, only one magnetic star (CU Vir) was known to host this mechanism, and it was unclear if this is a specific property of CU Vir or a common property of magnetic stars. The discovery of the maser mechanism in a second star rules out the first possibility and, since the maser process is more favourable at low frequencies, emphasizes the importance of more low frequency studies of magnetic stars to further understand the physical conditions that give rise to the maser.

Post-discovery timing studies with the GMRT of the 3rd transitional millisecond pulsar, J1227-4853, have resulted in detection of gamma-ray pulsations after the transition, using data from the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. The gamma-ray light curve of PSR J1227-4853 can be fitted by one broad peak, which occurs at nearly the same phase as the main peak in the 1.4 GHz radio profile. The partial alignment of light-curve peaks in different wavebands suggests that at least some of the radio emission may originate at high altitudes in the pulsar magnetosphere, in extended regions co-located with the gamma-ray emission site. Analysis of the gamma-ray flux over the mission suggests an approximate transition time of 2012 November 30. Continued study of the pulsed emission and monitoring of PSR J1227-4853, and other known redback systems, for subsequent flux changes will increase our knowledge of the pulsar emission mechanism and transitioning systems. The figure shows the phase-aligned gamma-ray (black line) and 1.4 GHz radio (red line with Parkes) light curves of PSR J1227−4853, with two rotations shown for clarity. The low-level peak at phase ∼0.5 in the radio light curve is an inter-pulse, which becomes dominant at lower frequencies.

Low-mass X-ray binaries (LMXB's) and radio millisecond pulsars (MSP's) are linked through stellar and binary evolution, where MSP's are the end products of an episode of accretion of matter and angular momentum from the binary companion during the LMXB state. Over the last decade, the discovery of three transitional millisecond pulsars (tMSP's) has allowed a detailed study of the recycling process. Recent studies of PSR J1824−2452I and PSR J1023+0038 have observationally demonstrated the LMXB – MSP evolutionary link. These systems show direct evidence of back-and-forth state switching between radio MSP and accreting X-ray millisecond pulsar regimes and opened a new avenue of research in pulsar astrophysics. The third such tMSP system, J1227-4853, was discovered by us using the GMRT. PSR J1227-4853 is a 1.69 millisecond pulsar at a dispersion measure of 43.4 pc/cm^3. It transited into the active radio-MSP phase associated with a sudden drop of its X-ray and optical luminosity in 2012 December. Extreme orbital perturbations as well as the signature of proper motion are revealed from our GMRT timing campaign. This pulsar, an ``eclipsing redback'', is the only transitioning system currently in an active rotation-powered state. Simultaneous imaging and timing observations with the GMRT were used to directly show that eclipses are caused by absorption rather than dispersion smearing or scattering. A long-term timing study of PSR J1227-4853 is currently under way, which will help to determine whether these transitional systems will eventually be canonical radio MSP's or whether they form a new sub-class of MSP's that continue to transition between the two states. Also, such studies will result in better understanding of the spin evolution of the systems and the dynamics of accretion during the accretion-powered, propeller stage and the rotation-powered stage. The figure shows the pulsar search output for PSR J1227-4853 showing rapid evolution of period and period-derivative in a compact binary system.

The discovery of millisecond pulsars (MSP's) and their precise localisation using existing methods is hindered by their intrinsic fainter nature. This leads to significant delays between the discovery of MSP's and their further identification using conventional imaging methods. Motivated by the need for rapid localization of newly-discovered faint MSP's, we have developed a coherently dedispersed gating correlator for the GMRT. This gating correlator accounts for the orbital motions of MSP's in binary systems, while folding the visibilities with a best-fit topocentric rotational model derived from a periodicity search using simultaneously generated beamformer output. With this technique, the signal-to-noise ratio of the detection of an MSP in the image domain can be dramatically improved (by a factor of as much as 5). We have also incorporated a superior approach of dispersion correction, called coherent dedispersion, in our imaging technique to reconstruct the intrinsic pulse shape of such MSP's. We could unambiguously localize newly discovered Fermi MSP's in the on–off gated image plane with an accuracy of ±1”. Immediate knowledge of such a precise position enables the use of sensitive coherent beams of array telescopes for follow-up timing observations, which substantially reduces the use of telescope time (by a factor of 20 for the GMRT!). In addition, a precise a priori astrometric position reduces the effect of large covariance in timing fit, which in turn accelerates the convergence to an initial timing model. Moreover, such accurate positions allow for rapid identification of pulsar counterparts in optical and X-ray wavelengths. Figure caption: On–off gated images for newly discovered Fermi MSP's. All the MSP's are marked in the respective 10’ × 10’ facet images.

Bhattacharyya et al. used the outstanding GMRT potential for low-frequency pulsar surveys in the GMRT High Resolution Southern Sky (GHRSS) survey, a low-frequency survey for pulsars and transients away from the Milky Way's plane. The GHRSS survey covers Galactic latitudes |b|>5 degrees, scanning the southern sky, with declination -40 degrees to -54 degrees. This declination coverage is complementary to the coverage of other ongoing low-frequency sky surveys around the world. The first phase of the GHRSS survey was carried out using the narrow bandwidths of the GMRT Software Backend, at 322 MHz, and has already resulted in the discovery of bunch of new pulsars with exciting properties. Bhattacharyya et al. discovered 13 pulsars in the GHRSS survey in a surveyed area of 1800 square degrees, i.e. 0.007 pulsars per square degree, which is one of the highest among pulsar surveys away from the Milky Way’s plane. GHRSS survey discoveries include a millisecond pulsar (in a ~10 hour orbit around a ~0.18 solar mass companion star), a pulsar for which gamma-ray pulsations have been discovered using the Fermi Large Area Telescope, and two mildly recycled pulsars. The second phase, using the GMRT Wideband Backend and the 250-500 MHz receivers of the upgraded GMRT is now under way. Deatils: http://www.ncra.tifr.res.in/ncra/research/research-at-ncra-tifr/research-areas/pulsarSurveys/GHRSS

Bhattacharyya et al. used the GMRT to perform deep observations to search for radio pulsations in the directions of unidentified Fermi Large Area Telescope (LAT) gamma-ray sources, resulting in the discovery of a new milli-second pulsar (MSP), PSR J1544+4947, an eclipsing MSP in a special evolutionary state. PSR J1544+4937 is a 2.16 ms pulsar in a 2.9-hour compact circular orbit with a very low-mass companion star (mass > 0.017 solar masses). At 322 MHz, the pulsar is found to be eclipsing for 13% of its orbit, whereas at 607 MHz the pulsar is detected throughout the low-frequency eclipse phase. Variations in the eclipse ingress phase are observed, indicating a clumpy and variable eclipsing medium. Moreover, additional short-duration absorption events are observed around the eclipse boundaries. The authors used the radio timing solutions to detect gamma-ray pulsation from the pulsar, confirming it as the source powering the gamma-ray emission. The figure shows the frequency-dependent eclipsing detected with the GMRT in PSR J1544+4937. The pulsar radiation is seen to be eclipsed by the companion star at 322 MHz, but not at 607 MHz. The figure plots the variation of the timing residuals and the electron column density around the eclipse phase (which is indicated by the shaded region) at 322 MHz (top) and 607 MHz (bottom).