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The NCRA solar physics group presented the first firm observational evidence for the presence of ubiquitous impulsive nonthermal radio emissions from the quiet solar corona (Mondal et al., 2020). These have since been christened WINQSEs – Weak Impulsive Narrow-band Quiet Sun Emissions - and are the smoking guns for the weak underlying magnetic reconnection processes or nanoflares which were hypothesised to explain the many decades-old coronal heating problem (Parker, 1988). The quantity of interest, from a coronal heating perspective, is the amount of energy deposited in the corona by the reconnection processes giving rise to these WINQSEs. It is, however, very hard to estimate this energy from radio emissions which arise from non-linear coherent emission processes. On the other hand, such energy estimates are routinely derived using well-established techniques from emissions in the extreme ultraviolet (EUV) and soft X-ray parts of the spectrum, which arise due to thermal processes. Mondal (2021) presents the first attempt to identify the EUV counterparts of these radio transients and use them to estimate the energy deposited into the corona during the events. By a careful comparison of the radio and EUV light curves, the author first identifies the EUV brightening associated with a closely spaced group of WINQSEs and then estimates the flare energy of this brightening to be ~10^25 ergs. The figure shows the radio contours at 132 MHz overlaid on an AIA 171 A map. The two possible EUV brightening candidates are shown in white boxes, with the likely one shown in the solid white box. This is the weakest EUV transient event for which a radio counterpart has been clearly identified. This cluster of WINQSEs had a peak flux density of ~2 mSFU, and the 10^25 ergs estimate is about an order of magnitude larger than nanoflare energies. This work demonstrates that even with current instrumentation, it is possible to identify the EUV counterparts of clusters of WINQSEs, and that the energies involved are consistent with the expectations based on the nanoflare hypothesis, making this an exciting line of exploration.

Using the data from the Murchison Widefield Array and their pipeline tailored for solar imaging (AIRCARS; Mondal 2019), the NCRA solar physics group has been focusing on studies of weak solar bursts. Here, Mondal and Oberoi present a high-fidelity snapshot spectroscopic radio imaging study of a weak type I solar noise storm that took place during an otherwise exceptionally quiet time. The flux density of the noise storm source varied between ∼0.6–24 SFU, about two orders of magnitude weaker than earlier studies along similar lines. The type I radio emission is believed to arise due to electron beams energized during magnetic reconnection activity. They track the observed morphology of the burst source for about 70 minutes to study the details of the reconnection and electron acceleration process during such quiet times. During this time interval, the authors identify multiple instances where the source s integrated flux density and area are strongly anticorrelated with each other. The authors also find that the presence of anticorrelation at one frequency does not necessarily imply its presence at other neighbouring frequencies in the same time window. The observed anticorrelation is interpreted as evidence for presence of MHD sausage wave modes in the magnetic loops and strands along which these electron beams are propagating. Their observations suggest that the sites of these small-scale reconnections are distributed along the magnetic flux tube. The authors hypothesize that small scale reconnections produce electron beams which quickly get collisionally damped. Hence, the plasma emission produced by them spans only a narrow bandwidth and the features seen even in neighbouring bands 12 to 28 MHz apart must arise from independent electron beams. These observations suggest a scenario where sausage MHD modes are stochastically excited in quiescent coronal loops. These sausage modes change the density of the non-thermal electrons responsible for the radio emission, thereby producing the observed anticorrelation between the area and the integrated flux density of the noise storm source. The work provides strong evidence that even during very quiescent times, there is discernible magnetic activity in the vicinity of active regions and in coronal loops. It also suggests that MHD oscillations in coronal magnetic loops and strands are likely quite ubiquitous. The radio emission from the weak electron beams propagating through these loops and strands serves to light them up, allowing their detection.

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.

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.

Neutral atomic hydrogen (HI) is a key constituent of galaxies and is the primary fuel for star formation. Therefore, an understanding of galaxy evolution requires measurements of the HI content of galaxies at different cosmological epochs, to probe how the typical HI mass of galaxies changes with time. Unfortunately, the main tracer of HI in galaxies, the hyperfine spectral line at a wavelength of 21.1 cm, referred to as the "recentResults"HI 21cm line, is a very weak spectral line. This makes it very difficult to measure the HI mass of high-redshift galaxies with current radio telescopes, which has severely limited our understanding of critical issues in galaxy evolution. For example, the cosmological star-formation rate density of the Universe is observed to peak in the redshift range z~1-3 (approximately 8-11 billion years ago) and to then decline by a factor of ten to its current value in the local Universe. The cause of the decline is an important open question in galaxy evolution. Chowdhury et al. used approximately 400 hrs of GMRT observations to obtain a detection of the average HI 21cm emission signal from ~2800 star-forming galaxies at z~1.3. Panels [A] and [B] of the figure show the average HI 21cm emission spectrum and the average HI 21cm image, respectively; a detection can be clearly seen in both panels. This is the highest redshift at which the HI 21cm signal has so far been detected, coming from galaxies 9 billion years ago. The authors used the detection of the average HI 21cm emission to estimate the average HI mass of star-forming galaxies at z~1.3: they find that the average HI mass of galaxies at this epoch is roughly 2.5 times higher than the average mass in stars. This is very different from galaxies in the local Universe where the HI mass is typically less than half the stellar mass. However, the high-z galaxies also have very high star-formation rates; the authors combine the star-formation rates with the measured average HI mass to find that the atomic gas can fuel the star-formation activity for only around 2 billion years, without replenshment of the gas reservoir. This is much shorter than the timescale on which HI is consumed by galaxies in the local Universe. This indicates that a lack of HI fuel to maintain the high star-formation rate of galaxies at these redshifts is the likely cause of the observed decline in the cosmic star-formation activity at z<1. The new results extend to higher redshifts the group’s earlier detection of the average HI 21cm signal, from galaxies at z~1.0, i.e. roughly 8 billion years ago. Also, the two studies were carried out with different receivers and electronics signal chain: the current result used the original GMRT receivers and electronics, while Chowdhury et al. (2020) used the upgraded GMRT receivers and electronics. The new results are thus an important independent confirmation of the results of the earlier study.

Green Pea galaxies are extreme emission-line galaxies at low redshift, with low metallicity and dust content, strong nebular lines, compact or interacting morphology, and intense star formation activity, and which often show leakage of Lyman-continuum radiation. Green Peas are believed to be the best local analogs of the galaxies that drove cosmological reionization at z>6, and offer the exciting possibility of understanding conditions in the high-redshift galaxies by detailed studies of nearby objects. However, while detailed optical and UV imaging and spectroscopic studies have characterized the stellar, nebular and star-formation properties of Green Peas, little was hitherto known about the primary fuel for star-formation in these galaxies, the neutral atomic hydrogen (HI). As such, the cause of the intense starburst activity in the Green Peas was unclear. Kanekar et al. used the Arecibo Telescope and the Green Bank Telescope to carry out a deep search for HI 21cm emission from a large sample of Green peas, obtaining detections of HI 21cm detections and estimates of the HI mass in 19 galaxies, and strong upper limits on the HI mass in 21 systems. These are the first estimates of the atomic gas content of Green Pea galaxies. Kanekar et al. find that the HI-to-stellar mass ratio in Green Peas is consistent with trends identified in star-forming galaxies in the local Universe. However, the median HI depletion timescale in Green Peas is more than ten times lower than that obtained in local star-forming galaxies. This implies that Green Peas consume their atomic gas on very short timescales. Kanekar et al. also find evidence of bimodality in the Green Pea sample, with many Green Peas appearing gas-rich, suggesting recent gas accretion, and others appearing gas-poor, suggesting that all their atomic hydrogen has been eaten by star-formation. The left panel of the figure shows the HI mass of the Green Peas plotted against their optical B-band absolute magnitude (equivalent to their B-band luminosity); the solid line shows the relation between HI mass and B-band magnitude seen in normal galaxies in the local Universe, with the two dashed lines showing the spread around the relation. A number of Green Peas are seen to lie above and below the spread in the relation, indicating that some Green Peas have a higher HI mass than expected (i.e. are gas-rich), while others have a lower HI mass than expected (i.e. are gas-poor). The right panel shows the timescale on which the HI in the Green Peas would be consumed by star formation (green circles) plotted against their stellar mass. The dashed green line shows the median HI depletion timescale in the Green Pea sample, approximately 600 million years. The dashed blue line shows the median HI depletion timescale for the xGASS sample of normal nearby galaxies; this is seen to be a factor of 10 higher than that in the Green Peas. Note that both plots are in logarithmic units: a change by 1 unit corresponds to a factor of 10!

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.

The author presents a detailed analysis of deep upgraded Giant Metrewave Radio Telescope (uGMRT) images of the head-tail radio galaxy NGC 4869 in the Coma cluster. The uGMRT images have an angular resolution of ~6.3 arcsecs and ~2.2 arcsecs, at frequencies of 250-500 MHz and 1050-1450 MHz, respectively. The author also used archival GMRT data to image the source, with angular resolutions ranging from 4.9 arcsecs to 21.8 arcseconds at 610 MHz, 325 MHz, 240 MHz, and 150 MHz. The uGMRT images show that the radio morphology of NGC 4869 consists of five distinct regions, with the clear presence of a pinch at a distance of 38.8 kpc, and a ridge at a distance of ~94.2 kpc from the head of the radio galaxy. The sharp bend by ~ 70 degrees at ~97 kpc from the head is possibly due to projection effects. There is possibly re-acceleration of the synchrotron electrons and perhaps also magnetic field regeneration in the ~2.8 - 96.1 kpc region of the jet. The author reports a steep-spectrum sheath layer enveloping a flat-spectrum spine, hinting at a transverse velocity structure with a fast-moving spine surrounded by a slow-moving sheath layer. He also derives the lifetimes of the radiating electrons and equipartition parameters. The figure shows the uGMRT 250-500 MHz (green) and 1050-1450 MHz (red) images of NGC 4869, overlaid on a Chandra X-ray image. The red arrows indicate the location of the onset of flaring, i.e. the surface brightness edge. The two radio jets emanating from the apex of the host galaxy initially travel in opposite directions. As the galaxy plows through the dense intracluster gas, these jets form a trail behind the host galaxy due to interaction with the intracluster medium, forming a conical shaped feature centered on the nucleus. Subsequently, the two jets twist, wrap, overlap and eventually bend. The radio spectra show progressive spectral steepening with distance from the head (i.e. the radio core), due to synchrotron cooling. A plausible explanation for the characteristic feature, the ridge of emission perpendicular to the direction of tail, is the flaring of a straight, collimated radio jet as it crosses a surface brightness edge (due to Kelvin-Helmholtz instabilities).