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

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

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 "recentResults"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.

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.

Explaining the presence of the million Kelvin corona sitting atop a 5800 Kelvin photosphere has been one of the longest-standing mysteries of solar physics. One of the hypotheses put forth for explaining this is the so called “nanoflare”-based coronal heating hypothesis (Parker, 1988). According to it, a large number magnetic reconnections keep taking place all of the over the Sun all the time; individually these small explosions involve only about a billionth of the energy of a large solar flare, but collectively they extract sufficient energy from the coronal magnetic fields to be able to heat and maintain the corona at a temperature of a million Kelvin. Considerable effort has been expended to look for observational evidence for the presence of these nanoflares in the X-ray and extreme ultraviolet bands, and has led to the conclusion that the observed distribution of even the weakest of the flares detected thus far is not consistent with the requirements for coronal heating. For the first time, Mondal et al. (2020) provide firm observational evidence for the presence of impulsive nonthermal radio emissions from the quiet solar corona, which form the smoking guns for the weak underlying magnetic reconnection processes. They meet all of the known criteria for coronal heating – they are found all through the quiet sun regions; and their radio flux density distribution has a power-law tail with a slope steeper than -2 at all frequencies (see figure). Mondal et al. estimate the energy that must be dumped in the corona to generate these impulsive emissions: this is consistent with the coronal heating requirements. These impulsive emissions have durations <1 second, their fractional time occupancy at a given region is <10%, and they show signs of clustering at small timescales. Additionally, the statistical properties of these impulsive emissions are very similar to those recently determined for magnetic switchbacks by the Parker Solar Probe. This study used data from the Murchison Widefield Array and was made possible by reliable detection of impulsive non-thermal solar emissions down to flux densities of a thousandth of a an SFU (1 SFU = 10,000 Jy), about two orders of magnitude fainter than earlier studies.

Prochaska et al. used the Atacama Large Millimeter/submillimeter Array (ALMA) to obtain high spatial resolution (~ 2 kpc) observations of [CII] 158-micron and dust-continuum emission from a galaxy at z=3.7978 selected by its strong Lyman-alpha absorption (a damped Ly-alpha absorber, DLA) against a background QSO. Their ALMA images reveal a pair of star-forming galaxies separated by approximately 6 kpc (projected) undergoing a major merger. Between these galaxies is a third emission component with highly elevated [CII] 158-micron A emission relative to the dust continuum (by a factor of ~2), which is likely to arise from stripped gas associated with the merger. This merger of two otherwise-normal galaxies is not accompanied by enhanced star-formation, contrary to mergers detected in most luminosity-selected samples. The DLA associated with the merger exhibits extreme kinematics, with a velocity width for the low-ionization metal lines of roughly 470 km/s, that spans the velocity spread revealed in the [CII] 158-micron emission. The authors propose that DLAs with high low-ionization metal line widths are a signpost of major mergers in normal galaxies at high redshifts, and use the distribution of the velocity widths of metal lines in high-z DLAs to provide a rough estimate of the fraction of z>3 galaxies that are undergoing a major merger. Figure: (Left) The zeroth-moment image showing the integrated [CII] 158-micron flux density of the galaxies A and B associated with the absorption-selected galaxy. The axes are labeled in physical units (kpc) at z=3.7978. The lowest contour corresponds to 3-sigma significance, with the contours increasing by a factor of sqrt(2). (right) The first-moment image showing the flux density-weighted velocity field, restricted to regions where the integrated flux density exceeds 2.5-sigma significance. There are two kinematically distinct components which we associate with a pair of merging galaxies, labeled A and B. There is additional emission between these components, referred to as component C, which appears to be gas stripped during the merger.

Massive disk galaxies like the Milky Way are expected to form at late times in traditional models of galaxy formation, but recent numerical simulations suggest that such galaxies could form as early as a billion years after the Big Bang through the accretion of cold material and mergers. Observationally, it has been difficult to identify disk galaxies in emission at high redshift in order to discern between competing models of galaxy formation. Neeleman et al. report Atacama Large Millimeter/submillimeter Array (ALMA) imaging, with a resolution of about 1.3 kiloparsecs, of the 158-micrometre emission line from singly ionized carbon ([CII]), the far-infrared dust continuum and the near-ultraviolet continuum emission from a galaxy at a redshift of 4.2603, identified by detecting its absorption of quasar light. The ALMA observations show that the emission arises from gas inside a cold, dusty, rotating disk with a rotational velocity of about 272 kilometres per second. The detection of emission from carbon monoxide in the galaxy yields a molecular mass that is consistent with the estimate from the ionized carbon emission of about 72 billion solar masses. The existence of such a massive, rotationally supported, cold disk galaxy when the Universe was only 1.5 billion years old favours formation through either cold-mode accretion or mergers, although its large rotational velocity and large content of cold gas remain challenging to reproduce with most numerical simulations. Figure: The top row shows the velocity-integrated [CII] flux density for the data (left panel), the constant rotational velocity model (middle panel) and the residual after subtracting the model from the data (right panel). The outer contour is at 3 sigma, where sigma is the standard deviation of the noise in the observations, with contours increasing in powers of sqrt(2). No negative contours at the same levels are observed in the image. The synthesized beam of the observations is shown in the bottom left corner of the leftmost plot. The bottom row shows the mean velocity of the [CII] emission, for the data (left panel), the model (middle) and the residuals (right). Velocities are relative to the systemic velocity of the [CII] emission, corresponding to z = 4.2603.

Measuring the physical parameters of Coronal Mass Ejections (CMEs), and particularly their entrained magnetic field, is crucial for understanding their physics and assessing their geo-effectiveness. At present, only remote sensing techniques can probe these quantities in the corona, the region where CMEs originate and acquire their defining characteristics. Radio observations offer a direct means for estimating the CME magnetic field by measuring the gyrosynchontron emission from CME plasma. Though simple in concept, this has proven to be challenging in practice, and there exist only a handful of successful examples in the literature. In this work, Mondal et al. measure various CME plasma parameters, including the magnetic field, by modeling the gyrosynchrotron emission from a CME. The radio imaging was done using the Murchison Widefield Array (MWA), and the high imaging dynamic range of these images allowed Mondal et al. to reliably detect these faint emissions. In fact, they were able to detect radio emission from a CME out to a larger distance (approximately 4.7 solar radii) than has been reported till date. The radio flux densities reported here are among the lowest measured in similar works. The MWA observations also provide much denser spectral sampling than has been available earlier, giving Mondal et al. the ability to more accurately constrain the model parameters. The present study is based on extensive flux density measurements of a slow, and otherwise unremarkable, CME. This suggests that new telescopes like the MWA should now be able to routinely detect the radio counterparts of CMEs and estimate their magnetic fields. The upper panel of the figure shows the average normalised radio contours (over the frequency range 108-145 MHz) superposed on a LASCO/C2 difference image. The green circles mark 3 and 4 solar radii. The contour levels start at 0.02% of the peak and increase in factors of two. The bottom panel shows the measured flux density from the region marked in yellow in the upper panel, along with the best-fit gyrosynchrotron spectral model.

Mulay et al. have used extreme UV imaging data from the AIA/SDO, radio imaging and spectra from the Murchison Widefield Array (MWA), and photospheric magnetic data from the HMI/SDO to carry out a detailed investigation of an active region jet and a nonthermal type III radio burst. The temperature of the jet spire and the footpoint regions were found to be similar to those reported in similar earlier studies, though the lower limits to the densities in these regions were estimated to be about an order of magnitude lower than the values reported earlier. A temporal and spatial correlation between the active region jet and the type III burst was established using high time resolution spectroscopic radio imaging. Observations of type III bursts are often used to establish the presence of open field lines. In the present instance, the MWA observations probe the regions ~0.13-0.52 R_Sun above the photosphere. A detailed examination of the data reveals that the nature of the observed radio emission is rather complex. A very interesting aspect of this work is that the observed and expected locations (based on the standard magnetic field extrapolation techniques) of radio burst sources do not match. The radio emission is very dynamic, changing across frequencies, and the differences between observed and expected emissions do not seem to follow a systematic pattern. A part of the observed differences can be explained by invoking the presence of significant propagation effects (refraction and scattering), which, in turn, provide evidence for large and dynamic density coronal inhomogeneities. The accompanying figure shows an AIA 171 Angstrom image at 03:51:47 UT with the red, green, and blue contours plotted over it (50, 55, 60, 65, 70, 75, 80, 85, 90, and 95% of the peak of the flux) representing observations at 101, 165, and 298 MHz, respectively. The yellow arrow indicates the location of the jet eruption. Orange and magenta lines indicate closed and open field lines, respectively, as given by extrapolation models. The red, green, and blue squares show the expected locations of radio sources at 101, 165, and 298 MHz, respectively. The different panels show the radio emission during each of the four episodes type III bursts, spanning a period of about 12 min.

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 "recentResults"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.