Pulsars and Transients

(Bhaswati Bhattacharyya, Poonam Chandra, Yashwant Gupta, Bhal Chandra Joshi, Dipanjan Mitra, Jayanta Roy, Sk. Minhajur Rahaman, Biplab Bijay, Prakash Arumugasamy, Avishek K. Basu)

Pulsars are rapidly rotating neutron stars that emit beams of radio radiation from their magnetic poles, at low frequencies, ideally suited for the ORT and the GMRT. NCRA-TIFR members are involved in blind and targeted searches that have already resulted in a number of discoveries of new and interesting pulsars. Other research areas, aimed at understanding the origin of pulsar radio emission, include pulsar timing studies, studies of their emission properties such as the evolution of pulse profiles, nulling and mode changing phenomena, as well as scattering and dispersion of pulsar signals by the ISM. Theoretical attempts are also being made to find evolutionary pathways linking different classes of neutron stars.

 

Transients are astronomical objects that show sudden, dramatic changes in their intensity on short timescales, ranging from seconds, days, weeks, months to several years. Transient phenomenon usually represents extremes of gravity, magnetic fields, velocity, temperature, pressure, and density. In terms of duration, transients phenomenon can be classified into two classes, transients with long time variability (min-days) and short term (sub-second variability) transients. Transients in radio bands are mainly dominated by three kinds of emission mechanisms. Slow transients majorly emit via incoherent synchrotron mechanism and are limited by the brightness temperature. These events are mainly associated with explosive events, such as Gamma Ray Bursts, supernovae, X-ray binaries, tidal disruption events etc. In addition, novae and symbiotic stars show slow variability and are dominated by thermal emission. Fast transients are usually associated with coherent emission and show relatively fast variability, high brightness temperature and often show high polarization associated with them, such as Fast Radio bursts, flare stars etc.


Radio wavelengths, especially in commensal survey mode, are particularly well suited for uncovering the complex transient phenomena. This is because observations at radio wavelengths may suffer less obscuration than in other bands. At the same time, multi waveband information often provides critical source classification rapidly than possible with only radio band data. Therefore, multi waveband observational efforts are the key to the progress of transients astronomy. The new capabilities of the GMRT correlator are being used to search for new types of transients. Multi-waveband studies of supernovae and gamma ray bursts are also being carried out. In particular, radio and X-ray observations of supernovae and gamma ray bursts have been used to trace the density and temperature of the surrounding medium, along with the shock conditions that accompany such events.

Recent Results
Long-term behaviour of a Type IIP Supernova, SN 2004dj, at radio frequencies
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.
A Long-term study of three rotating radio transients
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. 
A Post-correlation Beamformer for Time-domain Studies of Pulsars and Transients
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.
Electron Cyclotron Maser Emission from a radio star
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.
Discovery of Gamma-ray pulsations from the transitional redback PSR J1227-4853
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.
Discovery of PSR J1227-4853: Transitioning from Low-mass X-Ray Binary to Redback Millisecond Pulsar
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.
Coherently dedispersed gated imaging of millisecond pulsars
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.
The GMRT High Resolution Southern Sky (GHRSS) survey
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
GMRT Discovery of PSR J1544+4937: An Eclipsing Black-widow Pulsar Identified with a Fermi-LAT Source
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).

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