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Metre-wavelength solar emission spans angular scales from a few arcminutes to a few degrees. The brightness temperature of these emissions also varies by several orders of magnitude. Often, the faint radio emission from the quiet solar corona or coronal mass ejections is present simultaneously with the very bright radio emissions from solar radio bursts. To understand the global coronal properties, one has to detect both types of emissions simultaneously. At radio wavelengths, one cannot use a coronagraph to block the bright emission. Hence, one requires high-dynamic-range imaging to detect faint emission in the presence of very bright emission. The combination of the unique array configuration of the Murchison Widefield Array (MWA) and the robust calibration and imaging pipeline, Automated Imaging Routine for the Compact Arrays for the Radio Sun (AIRCARS, Mondal et al. 2019), produces the best spectroscopic snapshot solar images at low radio frequencies available to date. The present work demonstrates that even uncalibrated data from the MWA have a certain degree of coherency, which allows AIRCARS to make a reasonable starting point for boot-strapping a self-calibration algorithm even without a dedicated calibrator observation. The left panel of the figure shows an image after applying the calibration solutions from night-time calibrator observation, while the right panel shows the image made directly from the uncalibrated data which provides a reasonable starting point.  The strength of this algorithm makes AIRCARS a state-of-the-art calibration and imaging pipeline for low-frequency solar imaging, which is expected to be highly suitable for the upcoming Square Kilometre Array and other future radio interferometers for producing high-dynamic-range and high-fidelity images of the Sun.

A ubiquitous presence of weak energy releases is one of the most promising hypotheses to explain coronal heating, referred to as the nanoflare hypothesis. The accelerated electrons associated with such weak heating events are also expected to give rise to coherent impulsive emission via plasma instabilities in the metrewave radio band, making this a promising spectral window to look for their presence. Recently, Mondal et al. (2019) had reported the presence of weak and impulsive emissions from quiet Sun regions which seem to meet the requirements of being radio counterparts of the hypothesized nanoflares. Detection of such low-contrast weak emission signals from the quiet Sun is challenging and, given their implications, it is important to confirm their presence. In this work, using data from the Murchison Widefield Array, Sharma et al. use an independent robust approach for their detection by separating the dominant, slowly varying component of emission from the weak impulsive one in the visibility domain. By imaging these so-called ‘residual visibilities’, they detect milli-Solar Flux Unit-level bursts taking place all over the Sun and characterize their brightness temperatures, distributions, durations, and associations with features seen in extreme-UV images. The top panel of the figure shows the number of instances in a 30 min period where the residual flux density in a given pixel exceeded 6\\sigma, where \\sigma is the rms noise in the map far from the Sun for frequencies ranging from 108 MHz to 240 MHz. These features are seen to be present all over the Sun, though some clustering around active regions is seen at the highest frequencies. The lower panel shows the mean brightness temperature (Tb) of all the emission features identified in the upper panel which lies in the range of a few kK, order a percent of the solar thermal bremsstrahlung. These are among the weakest detections of non-thermal solar radio emissions. The black circle marks the optical disc of the Sun and the green contour the 5\\sigma boundary of the radio Sun. Sharma et al. also constrain the energies of the nonthermal particles using inputs from the FORWARD coronal model along with some reasonable assumptions, and find them to lie in the subpico flare (~10^19-10^21 erg) range. They also report the discovery perhaps the weakest known type III radio burst yet and another emission feature showing the weakest known clear signature of the quasi-periodic pulsations.

Neutral atomic hydrogen (HI) is the primary fuel for star formation in galaxies. An understanding of galaxy evolution thus critically requires measurements of the atomic gas mass of galaxies over cosmological time. Unfortunately, the weakness of the HI 21 cm line, the only tracer of the HI mass of galaxies, has meant that, until very recently, nothing was known about the HI mass of high-redshift galaxies. Chowdhury et al. had used the upgraded Giant Metrewave Radio Telescope (GMRT) in 2020 to obtain the first measurement of the average HI mass of galaxies at z~1, nine billion years ago. The team has now followed this up with a much larger survey, the GMRT Cold-HI AT z~1 (CATz1) survey, a 510 hr upgraded GMRT survey aimed at characterizing HI in galaxies during and just after the epoch of peak star formation activity in the universe (often referred to as "recentResults"the epoch of galaxy assembly ), a key epoch in galaxy evolution. In the current paper, Chowdhury et al. describe the design, data analysis, and basic results of the GMRT-CATz1 survey. They combined ("recentResults"stacked ) the HI 21 cm emission signals of ~11,500 star-forming galaxies at z=0.74-1.45 to obtain a high (7.1-sigma) significance detection of the average HI 21 cm signal from the sample of galaxies. The detected HI 21 cm signal can be clearly seen in the adjacent figure in both [A] the stacked HI 21 cm image, and [B] the stacked HI 21 cm spectrum. The average HI mass of the galaxies inferred from the detected signal is a factor of 1.4 higher than the average stellar mass of the galaxies, and a factor of ~3.5 higher than the HI mass of galaxies with similar stellar masses in the local Universe. However, Chowdhury et al. find that even such a large HI reservoir will be able to sustain the high star-formation rate of these galaxies for only a short duration, ~1.7 Gyr. Chowdhury et al. thus find that although galaxies at z ~ 1 have a high HI mass, their short HI depletion timescale is likely to cause quenching of their star formation activity in the absence of rapid accretion of gas from the environment around the galaxies. The GMRT-CATz1 survey will allow detailed studies of the HI properties of high-redshift galaxies, providing, for the first time, an understanding of atomic gas in galaxies during and just after the epoch of galaxy assembly. A set of companion papers by Chowdhury et al. has already yielded exciting new insights on these issues.

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

Ordinary "recentResults"baryonic matter in galaxies is mostly in the form of stars and neutral atomic and molecular gas. Over the lifetime of a galaxy, neutral atomic gas gets converted to molecular gas which in turn gets converted to stars. A galaxy s baryonic composition is thus one of its fundamental properties, and an important indicator of its evolutionary stage. The distribution of the baryonic mass of galaxies in the early Universe between stars, atomic and molecular gas, has hence long been an open problem in galaxy evolution. Unfortunately, the weakness of the HI 21 cm line, the only direct tracer of the atomic gas mass of galaxies, has meant that, until very recently, the atomic gas masses of high-redshift galaxies were not known. Chowdhury et al. used their recent detection of the average HI 21 cm emission signals from a large sample of star-forming galaxies at z~1.0 and at z~1.3 to find that high-redshift galaxies, at the epoch of peak star-formation activity in the Universe, have a dramatically different baryonic composition from that of nearby galaxies. The adjacent figure shows the fraction of the average baryonic mass of galaxies in atomic gas (red), molecular gas (blue), and stars (yellow) at z~0, z~1.0, and z~1.3; the samples of galaxies at the three epochs have identical stellar mass distributions. The figure shows that the contribution of stars to the total baryonic mass has increased from approximately 16% at z~1.3 to roughly 60% in the nearby Universe. Conversely, the fraction of mass in molecular gas for such galaxies has declined from about 14% at z~1.3 to only 6% in the nearby Universe. Remarkably, Chowdhury et al. find that atomic gas dominates the baryonic mass of galaxies at z~1.3, with roughly 70% of the total baryonic mass in atomic gas, and only around 16% in stars. Overall, the study provides evidence for strong evolution in the baryonic composition of galaxies over the past 9 billion years, with early galaxies, at the peak of star-formation activity in the Universe, being predominantly made up of neutral gas.

A convenient and computationally efficient way of detecting pulsars in a time-domain search is to use the technique of Fourier transform. The Fourier transform distributes the power contained in the pulsar’s signal into the fundamental frequency bin and multiple higher harmonic bins. The incoherent harmonic sum aims to increase the pulsar’s signal-to-noise ratio (SNR) by accounting for power (either fully or partially) from an increasing number of harmonics. However, in such harmonic sum algorithms, there are limitations due to unfavourable memory access patterns (which significantly reduces data utilisation per cache-line) and the number of possible partial sums explored per a single fundamental bin. For example, the Lyne-Ashworth algorithm (described in Lorimer and Kramer 2004) used in the SIGPROC software package, sums only powers-of-two harmonics. The harmonic sum algorithm used in the PRESTO software package (Ransom 2002) also uses a subset of the harmonics. This paper reports a new harmonic sum algorithm based on a greedy approach and implementation of this on NVIDIA GPUs using the CUDA programming language. This algorithm determines which time samples to sum according to the short-term gains rather than finding the optimal sum of harmonics. The Greedy harmonic sum considering all harmonics, not only powers-of-two, achieves higher sensitivity with a performance similar to or higher than the standard harmonic sum algorithms. The figure presents a comparison between the Greedy harmonic sum algorithm and the PRESTO harmonic sum algorithm (which sums only even harmonics), its two modified versions, one that sums elements of all higher harmonics (PRESTOall) and another that performs the harmonic sum for all harmonic orders in addition to summing all higher harmonics (PRESTO+), and a simple harmonic sum which ignores the drift and sums only integer multiples of the fundamental bin.  The Greedy algorithm encounters minimum sensitivity loss as a function of pulse frequency similar to PRESTO+. The GMRT High Resolution Southern Sky (GHRSS) survey data analysis with Greedy Harmonic sum reports 10-30% more recovered SNRs than with PRESTO. The compute performance of the new algorithm in terms of the number of fundamental frequency bins processed per second is 10% faster than PRESTO and >50% faster than the updated version of PRESTO (PRESTO+). Thus, the new Greedy harmonic sum algorithm has lower signal loss and better-recovered SNR than the standard algorithm used in PRESTO while achieving similar or better performance in the number of processed fundamental frequency bins per second.

Standard pulsar radio emission models predict a critical value of period derivative (P_dot) corresponding to the spin period (P) of pulsars below which radio emission ceases. These critical values of period and period derivative trace a curve on the P-P_dot plane called the death-line. No radio pulsar should exist below this line. The discovery of long period pulsar J2144-3933 and its location on the P-P_dot plane has questioned all the existing radio emission models, but there are only a handful of such interesting objects. Over the last decade, the number of millisecond pulsars has increased four-fold, whereas there has been only a marginal increase in the number of long-period pulsars. Along with intrinsic and observational biases, susceptibility of conventional fast Fourier transform (FFT)-based searches to red noise can be the primary reason behind the lack of long period pulsars. Searching for periodic non-accelerated signals in the presence of ideal white noise using the fully phase-coherent fast-folding algorithm (FFA) is theoretically established as a more sensitive search method than the FFT search with incoherent harmonic summing. Some major pulsar surveys (e.g. SUPERB and PALFA) have implemented FFA search to get optimal sensitivity for long period pulsars. This paper reports a detailed comparative study of FFA and FFT search sensitivity under various noise conditions (ideal white noise, real telescope noise, and simulated red noise) and over a range of signal parameters (period, duty-cycle, and profile shape). Singh et al. find that the FFA search with appropriate de-reddening of the time series performs significantly better than the FFT search with spectral whitening for long-period pulsars under real noise conditions. They describe an implementation of an FFA-based search pipeline for the GMRT High Time Resolution Southern Sky (GHRSS) survey. With processing of 1500 square degrees of GHRSS sky, the paper reports the re-detection of 43 known pulsars and the discovery of 2 new pulsars. Panel (a) of the figure shows a comparison of FFA and FFT detection signal-to-noise (S/N) of these pulsars. All these pulsars are better detected in the FFA search and the long-period pulsars have a higher ratio of FFA to FFT detection significance. Five of these pulsars were missed by the FFT search. Panel (b) of the figure shows the time versus phase and folded profile plots for a newly-discovered pulsar J1517-31b, with a period of 1.1 s and at a DM of 61.7 pc-cm^{-3}. This pulsar, with a long period and an unusually narrow duty-cycle, was missed by the FFT search. The authors find that the FFA search can reduce the algorithmic bias against long-period pulsars. Increased observing time per pointing along with the implementation of the FFA search in major pulsar surveys will possibly recover the missing population of long-period pulsars and populate the region close to the death-line in the P-P_dot plane.

Only a minor fraction (~15%) of the known pulsars spin with millisecond periodicity. The intrinsic faint nature of millisecond pulsars (MSPs) have hindered the discovery of these objects. This paper reports the discovery of three MSPs: PSRs J1120-3618, J1646-2142, and J1828+0625 with the Giant Metrewave Radio Telescope (GMRT) at a frequency of 322 MHz using a 32 MHz observing bandwidth. These sources were discovered serendipitously while conducting deep observations to search for millisecond radio pulsations in the directions of unidentified Fermi Large Area Telescope (LAT) gamma-ray sources. Phase coherent timing models for these newly discovered MSPs were derived using ~5 yr of observations with the GMRT. These are plotted in the figure, where the red points denote the timing residuals at 322 MHz and blue at 607 MHz. PSR J1120-3618 has a 5.5 ms spin period and is in a binary system with an orbital period of 5.6 days and a minimum companion mass of 0.18 solar masses, PSR J1646-2142 is an isolated object with a spin period of 5.8 ms, and PSR J1828+0625 has a spin period of 3.6 ms and is in a binary system with an orbital period of 77.9 days and minimum companion mass of 0.27 solar masses. The two binaries have very low orbital eccentricities, in agreement with expectations for MSP-helium white dwarf systems. Using the GMRT 607 MHz receivers having a 32 MHz bandwidth, PSR J1646-2142 and PSR J1828+0625 were detected but not PSR J1120-3618. Spectral indices for these MSPs using the GMRT observations are reported in this paper. PSR J1646-2142 has a wide profile, with significant evolution between 322 and 607 MHz, whereas PSR J1120-3618 exhibits a single peaked profile at 322 MHz and PSR J1828+0625 exhibits a single peaked profile at both the observing frequencies. These MSPs do not have gamma-ray counterparts, indicating that these are not associated with the target Fermi LAT pointing. This emphasizes the significance of deep blind searches for MSPs. The serendipity of the discovery of these millisecond pulsars indicates a population of weak MSPs waiting to be discovered with deep enough blind searches.

Our current understanding of galaxies and galaxy evolution is based on studies of emission-selected galaxy samples. Such samples contain a luminosity bias, i.e. they are biased towards brighter galaxies, especially at high redshifts. However, it is possible to also identify high-redshift galaxies via their Ly-alpha absorption signature in the spectra of background quasars; such "recentResults"HI-absorption-selected galaxies do not have the above luminosity bias. In order to understand HI-absorption-selected galaxies and their connections to the emission-selected population, we must first detect them in emission and then characterise their stellar and gas properties. Kaur et al. used the Jansky Very Large Array to search for CO(1-0) emission from three such HI-absorption-selected galaxies, at z~2, that were earlier identified with ALMA using CO(3-2) or CO(4-3) emission. They detected CO(1-0) emission from two HI-selected galaxies, DLA0918+1636g at z=2.5832 and DLA1228-113g at z=2.1933; these are the first detections of CO(1-0) emission in high-z HI-absorption-selected galaxies. Kaur et al. infer molecular gas masses for the two detected galaxies that are ~1.5-2 times lower than earlier estimates based on the mid-J CO lines. Kaur et al. also used the JVLA data to determine the CO spectral line energy distribution in the three galaxies. They find that the J=3 and J=4 levels are thermally excited in DLA0918+1636g, while the data are consistent with thermal excitation of the J=3 level in DLA1228-113g. In the case of the third galaxy, DLA0551-366g, no CO(1-0) emission was detected, yielding lower limits on the excitation of the J=4, J=5, and J=6 levels. Kaur et al. also compared the CO excitation of the HI-selected galaxies with that of the Milky Way, main-sequence galaxies, and sub-mm galaxies at high redshifts. They find that the CO excitation in HI-selected galaxies is similar to that of massive main-sequence galaxies at z>2, but higher than that of main-sequence galaxies at z~1.5. They also compared the ratio of CO(3-2) and CO(1-0) line luminosities (r_31; see attached figure) in different types of galaxies, and find that all galaxies at z>2 have both a higher r_31 value (implying a higher excitation of the J=3 level) and a higher SFR surface density than the main-sequence galaxies at z~1.5. The higher CO excitation in galaxies at z>2 may thus arise due to their higher SFR surface densities, as suggested by earlier theoretical studies.

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