GMRT Science Highlights

Supernova Interaction with a Dense Detached Shell in SN 2001em
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.
HI 21-centimetre emission from an ensemble of galaxies at an average redshift of one
Baryonic processes in galaxy evolution include the infall of gas onto galaxies to form neutral atomic hydrogen (HI), which is then converted to the molecular state (H2), and, finally, the conversion of H2 to stars. Understanding galaxy evolution thus requires an understanding of the evolution of stars and of neutral atomic and molecular hydrogen. For the stars, the cosmic star-formation rate density is known to peak at redshifts between 1 and 3, and to decline by an order of magnitude over the subsequent 8 billion years; the causes of this decline are not known. For the gas, the weakness of the hyperfine transition of HI at 21 cm wavelength — the main tracer of the HI content of galaxies—means that it has not hitherto been possible to measure the atomic gas mass of galaxies at redshifts higher than about 0.4; this is a critical gap in our understanding of galaxy evolution. Chowdhury et. al. report a measurement of the average HI mass of star-forming galaxies at a redshift of about one, obtained by stacking the HI 21 cm emission signals from 7,653 galaxies over a 1.2 square degree region of the sky. The figure shows [A] the stacked HI 21 cm emission map and [B] the stacked HI 21 cm emission spectrum; the detection of the average 21cm emission signal can be clearly seen in both panels. The measured average HI mass of the sample of galaxies at z~1 is similar to the average stellar mass of the sample but the HI mass can fuel the observed star-formation rates for only 1 to 2 billion years in the absence of fresh gas infall. This suggests that gas accretion onto galaxies at redshifts of less than one may have been insufficient to sustain high star-formation rates in star-forming galaxies. This is likely to be the cause of the decline in the cosmic star-formation rate density at redshifts below one.
The radio emission from a decade old Type I superluminous supernova, PTF10hgi, and the fast radio burst FRB121102
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.
Detection of 15 bursts from FRB180916.J0158+65 with the upgraded GMRT
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.
High-opacity associated HI 21cm absorbers at z~1.2
HI 21cm absorption arising from neutral hydrogen in the environments of Active Galactic Nuclei (AGNs) can be used to probe physical conditions in the AGN’s vicinity and how these conditions change over time. However, despite a large number of searches spanning many decades, only 7 such ``associated'' HI 21cm absorbers have been detected at redshifts greater than one. Chowdhury et al. used the new wide-band GMRT Band-4 receivers to discover two remarkable HI 21cm absorbers at a high redshift, z~1.2, against faint AGNs. The two absorbers were found in an unbiased search for HI 21cm absorption against all radio-continuum sources over a 1.2 square degree region of the sky, the first time that such a search has been carried out with a high sensitivity at high redshifts. The velocity-integrated HI 21cm optical depths of the two systems, shown in the figure, are greater than those of any known associated HI 21cm absorbers, and the two AGNs are very faint in both the radio and the ultraviolet wavebands. The discovery of these two systems is consistent with an earlier hypothesis that the dearth of associated HI 21cm absorbers at high redshifts, z>1, may be due to an observational bias wherein high-redshift AGNs targeted in surveys for associated HI 21cm absorbers are bright in the ultraviolet and radio wavelengths. The high AGN radio and/or ultraviolet luminosity may ionize or excite the HI in its vicinity, reducing the strength of the HI 21cm absorption. The two new HI 21cm absorbers emphasize the need to carry out unbiased HI 21cm absorption surveys and to extend future searches to low luminosity AGNs.
A study of eclipses for the redback pulsar J1227-4853
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.
Discovery of a large HI ring around the quiescent galaxy AGC 203001
Bait et al. report the discovery using the Giant Metrewave Radio Telescope of an extremely large (diameter approximately 115 kpc) neutral hydrogen (HI) ring, off-centred from a massive quenched galaxy, AGC 203001. The ring does not have a bright extended optical counterpart unlike several other known ring galaxies. Bait et al. present deep g-, r-, and i-band optical imaging of the HI ring, using the MegaCam instrument on the Canada-France-Hawaii Telescope, which shows several regions with faint optical emission at a surface brightness level of approximately 28 mag. per square arcsec. Such extended HI structures are rare, with only one other case known so far -- the Leo ring. Conventionally, off-centred rings have been explained by a collision with an ``intruder'' galaxy, leading to expanding density waves of gas and stars in the form of a ring. However, in such a scenario the impact also leads to large amounts of star formation in the ring which is not observed in the ring presented in this paper. Alternatively, such a ring could also form due to tidal interactions with a neighbouring galaxy or even major mergers. The exact physical mechanism for the formation of such rings is still under debate.
Distinct Properties of the Radio Burst Emission from the Magnetar XTE J1810-197
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.
The GMRT High Resolution Southern Sky Survey for pulsars and transients -II. New discoveries, timing and polarization properties
Bhattacharyya et al. have been carrying out the GMRT High Resolution Southern Sky (GHRSS) Survey for pulsars and transients since 2014. In this paper, they report the discovery of three pulsars, PSRs J1239−48, J1516−43 and J1726−52. They also present long-term timing solutions for three pulsars previously discovered with the GHRSS survey: (1) PSR J2144−5237, a millisecond pulsar with a period of 5 milli-seconds in a 10-day orbit around a companion of mass 0.18 solar masses, (2) PSR J1516−43, a mildly recycled 36 milli-second pulsar in a 228 day orbit with a companion of mass ∼0.4 solar masses, and (3) the 320 millisecond pulsar PSR J0514−4408. For PSR J0514−4408, Bhattacharyya et al. discover pulsed gamma-ray emission. In addition, they report radio polarimetric observations with the Parkes telescope for three of the GHRSS discoveries, PSRs J0418−4154, J0514−4408 and J2144−5237. The top panel of the figure shows a 322 MHz radio profile (red) of PSR J0514−4408 (discovered in the GHRSS survey) plotted with the Fermi Large Area Telescope (LAT) gamma−ray profile (using ∼9.2 years of LAT data above 100 MeV). The bottom panel of the figure shows the gamma-ray pulsation from PSR J0514−4408.
Long Term Variability of a Black Widow's Eclipses - A Decade of PSR J2051-0827
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.
Radio Continuum Emission from Local Analogs of High-redshift Lyman-alpha emitters
Blueberry galaxies are the low-redshift faint counterparts of the recently discovered class of Green Pea galaxies. These galaxies are often considered to be the local analogs of the high-redshift Ly-alpha emitters, which are thought to have contributed to the reionization of the Universe. Sebastian and Bait observed ten of the brightest blueberry galaxies from the sample of Yang et al. (2017), using the upgraded Giant Metrewave Radio Telescope (uGMRT) at 1.25 GHz. Nine of the blueberries were detected in the uGMRT continuum images. However, the 1.25 GHz continuum flux densities were lower by a factor of approximately 3.4 compared to the values expected from scaling relations obtained from normal star-forming galaxies. Possible explanations for the lower radio flux densities in blueberries include a deficit of cosmic ray electrons (CREs) or low values of magnetic fields due to the young ages of these galaxies and the escape of the CREs via diffusion or outflows; it is not possible to distinguish between these models with the current data. Sebastian and Bait also calculated the value of magnetic fields in the blueberries, and found that, despite their young ages, the blueberries show magnetic fields that are larger than those seen in galaxies with large-scale ordered rotation. They hence suggest that small-scale dynamo mechanisms play an important role in the magnetic field amplification in blueberry galaxies. The left panel of the figure shows the uGMRT 1.25 GHz image (in contours) of one of the blueberry galaxies, overlaid on an optical grz-band colour composite image. The right panel shows the star formation rates (SFRs) derived from the uGMRT radio continuum flux densities for the 9 blueberries plotted against the SFRs derived from H-alpha emission; it is clear that the radio SFRs are significantly lower than the H-alpha SFRs, by a factor of around 3.
Atomic hydrogen in star-forming galaxies at intermediate redshifts
Bera et al. used the upgraded Giant Metrewave Radio Telescope to carry out a deep observation of one of the well-known optical deep fields, the Extended Groth Strip, (EGS) covering the frequency range 1000-1370 MHz. This enabled a sensitive search for the hyperfine HI 21cm line from neutral atomic hydrogen (HI) in galaxies in the EGS, in the redshift range z~0.05-0.4. Bera et al. stacked (i.e. averaged) the HI 21cm emission signals from 445 blue star-forming galaxies in the EGS at 0.2<z<0.4 to infer their average HI gas mass, obtaining an average HI mass of (4.93 +/- 0.70) × 10^9 solar masses at a mean redshift of <z>=0.34. This implies a ratio of average gas mass to average stellar mass of ~1.2 for star-forming galaxies at these redshifts, higher than the corresponding value in the local Universe. The author also stacked the rest-frame 1.4 GHz radio continuum emission of the same galaxies, and then used a relation between the 1.4 GHz radio luminosity and the star formation rate (SFR) to obtain a median SFR of (0.54 +/- 0.06) solar masses per year for the galaxies of the sample. If the galaxies continue to form stars at the same rate, their average HI content would be exhausted on a timescale of ~9 Gyr, consistent with values in star-forming galaxies in the local Universe. This suggests that the star-formation efficiency in blue star-forming galaxies has not changed significantly over the last ~4 Gyr. Finally, Bera et al. used the stacked HI 21 cm emission signal to infer the cosmic HI mass density in star-forming galaxies at z=0.2-0.4, obtaining a normalized cosmic HI density of (4.81 +/- 0.75) x 10^−4 at <z>=0.34. This is the first accurate measurement of the cosmic HI density at intermediate redshifts z~0.2-1.8, and indicates no significant evolution in the cosmic HI density from z~0.4 to the present epoch. The top panel of the figure shows the average HI 21cm emission profile of the 445 blue star-forming galaxies whose spectra were stacked together. The bottom panel shows the evolution of cosmic HI density from z~5 to today, with the blue star showing the measurement from the present study.
GMRT polarisation and brightness temperature observations of Venus
Mohan et al. present results from carefully designed Giant Metrewave Radio Telescope (GMRT) low-frequency observations of Venus during its inferior conjunction. This ensured that the apparent angular size and flux density of Venus would be the largest observable from the Earth, making these the most detailed and sensitive observations of Venus that are possible with the GMRT. Mohan et al. used this opportunity to observe Venus at 234 MHz, 608 MHz and 1298 MHz. The figure shows the degree of polarisation maps for Venus at 607.67 MHz (top panel) and 1297.67 MHz (bottom panel), with the contour levels at 8, 12, 16, 20, 24, 28, 32, 36 and 40 percent; these are the lowest frequencies at which polarimetric maps have been made of Venus. Such polarimetric observations are essential for determining the sub-surface dielectric constant. As the penetration depth is substantially larger at low frequencies, metrewave observations allow us to probe the deeper sub-surface layers of Venus. This, in turn, is a very useful input for modeling the planetary surface dielectric properties. Using these observations, Mohan et al. determined the sub-surface dielectric constant to be ~4.5. At 234 MHz, they placed an upper limit of 321 K on the brightness temperature of Venus, firmly establishing that the brightness temperature of Venus begins to falls by about 1.4 GHz; the 234 MHz upper limit implies that the rate at which the temperature falls is even steeper than estimated earlier. This drop in the observed brightness temperature continues to pose a puzzle for present-day thermal emission models, which predict the brightness temperature to remain constant at low frequencies. However, the existing models do not take sub-surface properties into account, while emission at lower frequencies arises from deeper subsurface layers. These results suggest that sub-surface properties (dielectric properties through density and mineral content) can significantly impact the observed brightness temperature at low radio frequencies.
A radio halo surrounding the Brightest Cluster Galaxy in RXCJ0232.2-4420: a mini-halo in transition?
Kale et al. report the discovery of a ``radio halo'', a diffuse radio source, in the galaxy cluster RXCJ0232.2-4420 (SPT-CL J0232-4421, z = 0.2836) using observations with the Giant Metrewave Radio Telescope. Diffuse radio sources associated with the intra-cluster medium - the medium that pervades the space between galaxies in a galaxy cluster- are direct probes of cosmic ray electrons and magnetic fields in the cluster. Although magnetic fields are believed to be ubiquitous in galaxy clusters, such radio sources are rare. The known sample of such sources has been broadly classified into radio halos that are >700 kpc-sized sources that occur in merging clusters and mini-halos that are only a couple of hundred kpc in size and occur in relaxed clusters. It has been proposed that mini-halos transition into radio halos when a relaxed system undergoes a merger; however, this transition has not been observed clearly. The newly-discovered source has an extent of 550 kpc x 800 kpc - a size in the radio halo category. However, it surrounds the Brightest Cluster Galaxy like a typical mini-halo. Kale et al. have compared the radio power of this source with that of known radio halos and mini-halos and found it to be consistent with both populations. In the X-ray bands, this cluster has been classified as a complex system - indicating a state that is neither a merger nor a completely relaxed state. Kale et al. hence propose that this system is among the rare class of transition systems between mini-halos and radio halos. The 3-color image shows the image of the galaxy cluster in radio waves (blue), X-rays (green) and visible light (red).
The host galaxy of the fast-evolving luminous transient AT2018cow
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.
The peculiar radio pulses from the magnetic Bp star HD 142990
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.
Inhomogeneities revealed in a supernova via low frequency GMRT observations
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.
The Expanded Giant Metrewave Radio Telescope
With 30 antennas and a maximum baseline length of 25 km, the Giant Metrewave Radio Telescope (GMRT) is the premier low-frequency radio interferometer today. Patra et al. carried out a study of possible expansions of the GMRT, via adding new antennas and installing focal plane arrays (FPAs), to improve its point-source sensitivity, surface brightness sensitivity, angular resolution, field of view, and U-V coverage. They carried out array configuration studies, aimed at minimizing the number of new GMRT antennas required to obtain a well-behaved synthesized beam over a wide range of angular resolutions for full-synthesis observations. This was done via two approaches, tomographic projection and random sampling, to identify the optimal locations for the new GMRT antennas. We report results for the optimal locations of the antennas of an expanded array (the EGMRT), consisting of the existing 30 GMRT antennas, 30 new antennas at short distances, roughly 2.5 km from the GMRT array centre, and 26 additional antennas at relatively long distances, roughly 5-25 km from the array centre. The collecting area and the field of view of the proposed EGMRT array would be larger by factors of, respectively, roughly 3 and roughly 30 than those of the GMRT. Indeed, the EGMRT continuum sensitivity and survey speed with 550-850 MHz FPAs installed on the 45 antennas within a distance of ~ 2.5 km of the array centre would be far better than those of any existing interferometer, and comparable to the sensitivity and survey speed of Phase-1 of the Square Kilometre Array. In the figure, the left panel shows the root-mean-square (RMS) continuum noise of the EGMRT compared with that of modern radio interferometers (the uGMRT, the JVLA, LOFAR, MeerKAT, ASKAP, and the SKA-1) for a 9-hour full-synthesis integration. The green and magenta dashed lines show the RMS confusion noise for, respectively, the EGMRT and the uGMRT, at the different observing frequencies. It is clear that source confusion will be a limiting factor for the EGMRT only in its lowest frequency band (125-250 MHz), where the sensitivity is likely to anyway be limited by systematic effects, rather than thermal noise. The right panel shows the survey speed figure of merit (e.g. Dewdney et al. 2015), of the EGMRT compared with that of other present or planned radio interferometers. For the EGMRT, we have considered two possibilities: the open green stars (EGMRT) refer to single-pixel feeds on all 86 antennas, while the solid blue circles (EGMRT+FPA) assume FPAs covering 550-850 MHz installed on the 45 antennas within ~ 2.5 km of the central square.
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 VLA-GMRT look at 11 FR-II Quasars
Active Galactic Nuclei (AGNs) are a special class of galaxies that emit enormous amounts of energy from the nuclear region.  There are several variants of AGNs classified based on their observed properties.  Detailed observations of AGNs over two decades have suggested that AGNs are intrinsically similar objects, but may appear different due to different viewing angles; this idea is now known as the AGN unification scheme.  Vaddi et al. address the unification of radio-loud AGNs via statistical and spectral analysis approaches.  A sample of 11 steep-spectrum radio quasars and 13 Fanaroff-Riley-II radio galaxies that span similar luminosity and redshift ranges were used for this study. Matched resolution radio data for the quasars were obtained using the Jansky Very Large Array. The results are in general agreement with orientation-based AGN unification. However, the authors find that environmental effects cannot be ignored. The lack of correlation between the statistical orientation indicators such as misalignment angle and radio core prominence (see the figure), and the larger lobe distortions in quasars compared to radio galaxies suggest that additional intrinsic or environment effects are at play.
Angular momentum of dwarf galaxies
Mass and specific angular momentum are two fundamental physical parameters of galaxies. Kurapati et al. (2018) use high-resolution HI 21cm observations and broad band photometry to measure the baryonic mass (M) and baryonic specific angular momentum (j) of 11 dwarf galaxies that lie in the Lynx-Cancer void. They find that the specific angular momentum of void dwarf galaxies is similar to that of other dwarf galaxies in average density environments. However, all dwarf galaxies (regardless of environment) have significantly higher specific angular momentum than expected from an extrapolation of the relation between specific angular momentum and baryonic mass for large spiral galaxies. The figure shows the difference between the observed specific angular momentum of dwarf galaxies and the specific angular momentum expected from the bulge-less spiral relation, as a function of baryonic mass. The elevation in specific angular momentum occurs for dwarf galaxies with masses lower than roughly a billion solar masses. Galaxies above this mass threshold have relatively low baryonic specific angular momenta, following the relation obtained for massive galaxies with zero bulge fraction. Interestingly, the above mass threshold is very similar to the mass below which galaxy discs start to become systematically thick. Kurapati et al. examine the possibility that both these effects, viz. the thickening of disks and the increase in specific angular momentum, are results of feedback from star formation. Such feedback would preferentially remove the low angular momentum gas from the central parts of dwarfs (thus increasing the specific angular momentum of the system) and also inject mechanical energy into the system, leading to thicker discs. They find however, that the observed amount of observed star formation in their sample galaxies is insufficient to produce the observed increase in the specific angular momentum. It hence appears that some other, as yet unknown mechanism, plays a role in producing the observed enhancement in specific angular momentum.
Detection of the Galactic warm neutral medium in HI 21cm absorption
Patra et al. report a deep GMRT search for Galactic HI 21-cm absorption towards the quasar B0438-436, yielding the detection of wide, weak HI 21-cm absorption, with a velocity-integrated HI 21-cm optical depth of 0.0188 +/- 0.0036 km/s. Comparing this with the HI column density measured in the Parkes Galactic All-Sky Survey gives a column density-weighted harmonic mean spin temperature of 3760 +/- 365 K, one of the highest measured in the Galaxy. This is consistent with most of the HI along the sightline arising in the stable warm neutral medium. The low-peak HI 21-cm optical depth towards B0438-436 implies negligible self-absorption, allowing a multi-Gaussian joint decomposition of the HI 21-cm absorption and emission spectra. This yields a gas kinetic temperature T_k <= (4910 +/- 1900) K, and a spin temperature T_s = (1000 +/- 345) K for the gas that gives rise to the HI 21-cm absorption. The GMRT data are consistent with the HI 21-cm absorption arising from either the stable WNM, with T_s << T_k, T_k~5000 K, and little penetration of the background Lyman-alpha radiation field into the neutral hydrogen, or the unstable neutral medium, with T_sT_k~1000 K. The figure shows results of the multi-Gaussian joint decomposition of the (A) HI 21-cm emission and (B) HI 21-cm absorption spectra. The top panels show the best-fit model (solid curve) overlaid on the two spectra, while the bottom two panels show the residuals from the fit.
Probing Star Formation in Galaxies at z~1
Bera et al. carried out deep GMRT 610 MHz imaging of four fields of the DEEP2 Galaxy Redshift Survey, and stacked the radio emission from a sample of nearly 4000 blue star-forming galaxies at 0.7<z< 1.45 to detect the median rest-frame 1.4 GHz radio continuum emission of the galaxies. The authors used the local relation between total star formation rate (SFR) and radio 1.4 GHz luminosity to infer a median total SFR of (24.4 +/- 1.4) solar masses per year for blue star-forming galaxies at these redshifts. They detect the main-sequence relation between SFR and stellar mass, and find that the power-law index of the main sequence shows no change over z~0.7-1.45. They also find that the nebular line emission suffers less extinction than the stellar continuum, contrary to the situation in the local Universe; further, the ratio of nebular extinction to stellar extinction increases with decreasing redshift. They combined their results with earlier GMRT HI 21cm emission studies of the DEEP2 fields to obtain an upper limit of 0.87 Gyr to the atomic gas depletion time of star-forming galaxies at z~1.3. Neutral atomic gas thus appears to be a transient phase in high-z star-forming galaxies. The left panel of the figure shows the stacked rest-frame 1.4 GHz radio emission of the galaxies of the sample, detected at high statistical significance; the right panel shows a similar stack at neighbouring locations ("off-source") which shows no signal, indicating that the detected signal of the left panel is very unlikely to arise from systematic effects.
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.
Curvature in the spectrum of a remnant radio galaxy with the uGMRT
The origin of cosmic rays in the intra-cluster medium (ICM) has been attributed to re-acceleration of charged particles in shocks and turbulence. For these re-acceleration mechanisms to work, it is expected that there will be reservoirs of seed cosmic ray electrons in the ICM. Radio galaxies with jets and lobes are strong candidates for providing these seeds. Dr. Kale and collaborators have used the unique broad band observing capabilities of the recently operational upgraded Giant Metrewave Radio Telescope (uGMRT) to study an enigmatic "dead radio galaxy" or a "remnant radio relic" in the galaxy cluster Abell 4038. They have shown that the spectrum of the source varies considerably across its extent and undergoes extreme changes from high to low frequencies that are quantified in a parameter called the "spectral curvature". The authors fine that the assumption that the spectra of seed particles are simple power-laws may not be correct, given the extreme spectral curvature measured using the uGMRT images. Their study has recently been accepted for publication in the Monthly Notices of the Royal Astronomical Society.
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.
The angular momentum content of gas-rich dwarf galaxies
A galaxy's spin is intricately connected to its morphology --- spiral galaxies spin faster and hence are thinner whereas elliptical galaxies have lower specific angular momentum and are puffier. The mass and the angular momentum of a galaxy are related via their evolutionary history. Various researchers in the past have reported a power-law scaling relation between the mass and the specific angular momentum of large spiral galaxies. Chowdhury and Chengalur used archival GMRT, VLA and WSRT HI 21cm data of five gas-rich dwarf galaxies and found that the specific angular momentum in these smaller, less massive, dwarf galaxies is significantly higher than that expected from the earlier studies of spiral disks. The figure shows the location of these dwarf galaxies in the specific angular momentum - mass plane, and compares them with the distribution of spiral galaxies. All the five gas-rich dwarf galaxies lie outside the 95% probability band of the relation for spiral galaxies. The chance probability that the dwarf galaxies belong to the same angular momentum - mass distribution as the spirals is less than one part in a million. The authors suggest two mechanisms through which the dwarfs may acquire their higher specific angular momentum: (i) preferential outflow of low angular momentum gas due to stellar feedback, and (ii) cosmic cold mode accretion, which is known to dominate in less massive galaxies.
A fourth radio arc in Abell 2626
The supermassive black holes at the centres of active galaxies can lead to the formation of spectacular jets that are detectable in deep radio imaging studies. When such black holes are situated close to the centres of galaxy clusters, they experience a dense environment. The radio jets can be affected by the black hole itself and by the environment, leading to complex morphologies. A system of three concave arcs was earlier known towards the galaxy cluster Abell 2626. Kale & Gitti used the 610 MHz receivers of the GMRT to discover a fourth arc in the sytem, that completes an intriguing symmetric structure of four arcs around the central massive galaxy that itself has two active nuclei. The origin of the exotic source is as yet unknown, but may be a rare event of precessing jets from the double nuclei of the central galaxy or a similarly rare configuration of a gravitational lens. The image shows the GMRT radio image in blue, overlaid on X-ray (red) and optical (green) images.
A Giant Radio Galaxy at z ~ 0.57
Giant radio galaxies (GRGs) are radio galaxies whose linear extent is more than 1 Mpc. Most of the known GRGs are less than a billion light years away from us. The sharp decline in the number of GRGs at larger distances, i.e. higher redshifts, is a mystery because the number of normal radio sources is actually higher at high redshifts. We recently used the GMRT to carry out a deep 150 MHz study of a small region of the sky in the Lynx constellation, and discovered a large GRG, of size 7 million light years, at a distance of about 5 billion light years, i.e. a redshift of 0.57. We used the GMRT to carry out detailed imaging studies of the GRG, at 325 MHz, 610 MHz and 1420 MHz; the new data suggest that the object is probably a double-double radio galaxy. Further, the radio core of the galaxy shows an unusually steep spectrum, which may imply that there is yet another unresolved pair of lobes within the core, making this GRG a candidate triple-double radio galaxy. Further investigations of the central region of the GRG, to test if it is a re-started radio source, are now under way using the European Very Long Baseline Interferometry Network (EVN), which has the resolution to probe the central region very close to the supermassive black hole. The figure shows the GMRT 610 MHz image of the new GRG, overlaid on the optical SDSS gri-composite image. The optical host galaxy is shown separately in the rectangular box. The double-lobe structure on either side of the central core is clearly visible.
GMRT monitoring of the X-ray binary V404 Cygni during its June 2015 outburst
Chandra & Kanekar used the GMRT at 1280, 610, 325 and 235 MHz to monitor the black hole X-ray binary V404 Cygni during its 2015 June outburst, extending for a period of 2.5 weeks, and beginning on June 26.9 UT, a day after the strongest radio/X-ray outburst. They find the low-frequency radio emission of V404 Cygni to be extremely bright and fast-decaying in the outburst phase, with an inverted spectrum below 1.5 GHz and an intermediate X-ray state. The radio emission settles to a weak, quiescent state roughly 11 days after the outburst, with a flat radio spectrum and a soft X-ray state. Combining the GMRT measurements with flux density estimates from the literature, the authors identify a spectral turnover in the radio spectrum at ~1.5 GHz on June 26.9 UT (see the attached image), indicating the presence of a synchrotron self-absorbed emitting region. They use the measured flux density at the turnover frequency with the assumption of equipartition of energy between the particles and the magnetic field to infer the jet radius, magnetic field, minimum total energy, and transient jet power. The relatively low value of the jet power, despite V404 Cygni’s high black hole spin parameter, suggests that the radio jet power does not correlate with the spin parameter.
Discovery of a radio relic in the low mass galaxy cluster PLCK G200.9-28.2
Kale et al. used the Giant Metrewave Radio Telescope (GMRT), the XMM-Newton X-ray Observatory, and the Jansky Very Large Array to discover a new radio relic in the galaxy cluster PLCKG200.9-28.2 at z~0.22. Such arc-like radio relics are usually found at the periphery of massive colliding clusters, and are extremely rare, arising in fewer than 5% of merging clusters. Despite their rarity, radio relics are an excellent tracer of the shocks that are expected to be driven in the diffuse intra-cluster medium by violent cluster collisions. Indeed, it is very difficult to even detect these shocks at other wavelengths. So far, radio relics have been found only in the vicinity of merging massive clusters. The new radio relic detected by Kale et al. is very interesting because it arises in a cluster of low mass, the lowest mass at which such a relic has ever been seen! This demonstrates that violent mergers in low-mass clusters are capable of producing strong shock waves in their diffuse media. In the adjoining figure, the 235 MHz emission imaged with the GMRT is shown in red and the X-ray emission imaged with the XMM-Newton satellite observatory is shown in blue. The elongated source seen in red is the new radio relic.
GMRT imaging of a high-energy supernova remnant
Nayana et al. used the Giant Metrewave Radio Telescope (GMRT) to detect 325 and 610 MHz radio emission from HESS J1731-347, one of only five known very-high-energy (VHE; > 0.1 TeV) shell-type supernova remnants (SNRs). Multiple filaments of the SNR are clearly seen in the GMRT 610 and 325 MHz images, shown, respectively, in the left and right panels of the adjacent figure. However, the faintest feature in the GMRT bands corresponds to the peak in the VHE emission. This anti-correlation can be explained if the observed VHE gamma-ray emission has a leptonic origin. The individual filaments of the SNR (indicated by "1", "2", "3", and "4") have steep radio spectra, consistent with a non-thermal origin.
An AGN's rendezvous with a radio relic
van Weeren et al. used data from the Chandra X-ray Observatory, the Giant Metrewave Radio Telescope, the Jansky Very Large Array, and other telescopes to discover a cosmic event never seen before. Galaxy clusters contain multiple sources of radio emission, including active galactic nuclei (AGNs), radio halos and radio relics. A long-standing problem in studies of clusters is how low-Mach-number shocks can accelerate electrons efficiently to produce the observed radio relics. van Weeren et al. discovered, for the first time, a direct connection between a radio relic and an AGN (a radio galaxy) in the merging galaxy cluster Abell 3411-3412 by combining radio, X-ray and optical data. This discovery indicates that fossil relativistic electrons from AGNs are re-accelerated at cluster shocks. It also implies that radio galaxies play an important role in governing the non-thermal component of the intra-cluster medium in merging clusters. For the first time, two of the most powerful phenomena in the Universe have been clearly linked together in the same system. Image credits: X-ray: NASA/CXC/SAO/R. van Weeren et al; Optical: NAOJ/Subaru;