Active galactic nuclei with powerful radio jets play a key role in galaxy evolution through their ability to regulate the cold gas reservoirs that fuel star formation. Jet-driven feedback can heat, compress, or expel atomic and molecular gas, thereby reshaping the interstellar medium and altering star formation efficiency. However, the physical coupling between AGN activity and the multi-phase interstellar medium remains poorly constrained, particularly in radio-loud systems where mechanical feedback and multiphase outflows are expected to dominate. SKA will provide major advances in the study of cold gas in AGN host galaxies through sensitive observations of H i emission and absorption, together with access to selected low-frequency molecular transitions within the SKA 1 frequency range, including OH, H_2CO, CH_3OH, and, at high redshift, low-J transitions of CO, HCN, and HCO^+ in rare bright systems. Combined with radio continuum measurements, these tracers will provide direct constraints on gas mass, kinematics, turbulence, and inflow/outflow signatures, enabling detailed studies of feedback-regulated cold gas reservoirs in AGN environments. In this chapter, we examine how SKA1 observations of neutral hydrogen, complemented by molecular-line and radio continuum studies, can be used to quantify multiphase gas flows and feedback energetics in molecular-gas-rich radio galaxies. SKA surveys will enable population-level studies of AGN-driven feedback, providing a new framework for understanding how radio jets regulate the cold interstellar medium and star formation across cosmic time.

1LHAASO J2108 + 5153u, a PeVatron candidate detected by Large High Altitude Air Shower Observatory (LHAASO), has no known association in any other wavelength range. In this work we attempted to identify any possible association by observing the source region in low frequency radio band. 1LHAASO J2108 + 5153u was observed by upgraded Giant Metrewave Radio Telescope (uGMRT) at 650 MHz frequency. The data were analysed to map and spatially correlate the sources in the field of view with the LHAASO detected source. We identified a new extended source within the LHAASO PSF showing a distinct jet and core structure in radio band. The exact nature of the source could not be identified with the present observation. It can be a microquasar and the particles can be accelerated to PeV energies in the microquasar jet.

1LHAASO J2108+5153u, a PeVatron candidate detected by Large High Altitude Air Shower Observatory (LHAASO), has no known association in any other wavelength range. In this work we attempted to identify any possible association by observing the source region in low frequency radio band. 1LHAASO J2108+5153u was observed by upgraded Giant Metrewave Radio Telescope (uGMRT) at 650 MHz frequency. The data were analysed to map and spatially correlate the sources in the field of view with the LHAASO detected source. We identified a new extended source within the LHAASO PSF showing a distinct jet and core structure in radio band. The exact nature of the source could not be identified with the present observation. It can be a microquasar and the particles can be accelerated to PeV energies in the microquasar jet.

Using archival data, we have made an HI absorption study of the 7^' halo surrounding the Sgr A complex, observed towards the Galactic centre (GC) region. We find strong HI absorption near velocities of -53 km s^-1, which is due to the 3-kpc arm, placing it beyond 5 kpc from us. We further examined the HI absorption properties towards 5 different parts of the 7^' halo. Absorption by +50 km s^-1 GC cloud is seen towards only 3 parts of the halo, but not towards the other 2 regions. Strong emissions in CO and CS are, however, identified toward all the above 5 parts of the halo by the +50 km s^-1 GC molecular cloud. This does show that the 7^' halo is partly behind, and partly in front of the +50 km s^-1 cloud. To our knowledge, this, for the first time clearly shows the 7^' halo to be located at the same distance as the +50 km s^-1 molecular cloud, i.e., at the GC region.

Using archival data, we have made an H I absorption study of the 7 ' halo surrounding the Sgr A complex, observed toward the Galactic center (GC) region. We find strong H I absorption near velocities of -53 km s-1, which is due to the 3 kpc arm, placing it beyond 5 kpc from us. We further examined the H I absorption properties toward five different parts of the 7 ' halo. Absorption by the +50 km s-1 GC cloud is seen toward only three parts of the halo, but not toward the other two regions. Strong emissions in CO and CS are, however, identified toward all the above five parts of the halo by the +50 km s-1 GC molecular cloud. This does show that the 7 ' halo is partly behind and partly in front of the 50 km s-1 cloud. To our knowledge, this for the first time clearly shows the 7 ' halo to be located at the same distance as the +50 km s-1 molecular cloud, i.e., at the GC region.

We present the results of deep radio observations of seven nearby large galaxies observed using the upgraded Giant Metrewave Radio Telescope (uGMRT) 0.3-0.5 GHz receivers with an angular resolution of similar to 10 ''. The achieved sensitivities of these observations range from approximate to 15 to 50 mu Jy beam-1, which is a factor of approximate to 3-4 lower than the previous observations at these frequencies. For two galaxies (NGC 3344 and NGC 3627) with moderate inclination angles, significant diffuse emissions are seen for the first time. The detected radio halos in the vertical direction are significantly larger in our 0.4 GHz maps compared to the observations at similar to 1.5 GHz for four nearly edge-on galaxies-NGC 3623, NGC 4096, NGC 4594, and NGC 4631. For these four galaxies, significantly larger halos are also detected along the galaxy disk. For NGC 3623 and NGC 4594, we could detect elongated radio disks that were not seen before. We also present new uGMRT images of NGC 3344 and NGC 3623 at 1.3 GHz and a new VLA image of NGC 3627 at 1.5 GHz. We fitted an exponential function to the flux densities along different cross-cuts and found a significantly wider distribution at the 0.4 GHz uGMRT images compared to the high-frequency images at similar to 1.5 GHz. Using maps at 0.144, 0.4, and similar to 1.5 GHz, we made spectral index maps of the seven sample galaxies and found a steepening of the spectrum up to a value of similar to-1.5 in the halo regions of the galaxies.

Recent observations by the Large High Altitude Air Shower Observatory (LHAASO) detected Ultra High Energy (UHE) photons in the range 100 TeV to 1.4 PeV from twelve sources including Crab nebula. The detection of these photons demands the presence of at least PeV energy particle in the source. It is important to understand particle acceleration and radiation emission processes in such source. One of those twelve sources, LHAASO J2108+5157 does not show any association or counterparts at any other wavelength. In search of counterpart, we surveyed the region with Giant Metrewave Radio Telescope (GMRT) at 650 MHz frequency. GMRT observation revel radio emission from an extended source within the PSF of LHAASO which shows disk-jet morphology. Considering the spatial association and extent of the source, it is plausible that particle acceleration to PeV energies originates from this source.

Empirical studies of cold gas content are essential for comprehending the star formation activities and evolution in galaxies. However, it is not straightforward to understand these processes because they depend on various physical properties of the interstellar medium. Massive Faranoff–Riley I/II type radio galaxies rich in molecular hydrogen with lower star formation activities are known as radio molecular hydrogen emission galaxies (MOHEGs). We present a study of neutral hydrogen-gas-associated radio MOHEGs at redshifts <0.2 probed via the H i 21 cm absorption line. Neutral hydrogen is detected in 70% of these galaxies, which are located at a distance of 8–120 kpc from the neighboring galaxies. These galaxies show a scarcity of H i gas as compared to merging galaxies at similar redshifts. We found no strong correlation between N (H i ), N _H , and the galaxy properties, regardless of whether the H i is assumed to be cold or warm, indicating that the atomic gas probably plays no important role in star formation. The relation between the total hydrogen gas surface density and the star formation surface density deviates from the standard Kennicutt–Schmidt law. Our study highlights the importance of H i studies and offers insights into the role of atomic and molecular hydrogen gas in explaining the properties of these galaxies. In the upcoming H i 21 cm absorption surveys with next-generation radio telescopes such as the Square Kilometre Array and pathfinder instruments, it may be possible to provide better constraints for these correlations.

The X-ray source CXOU J163802.6-471358 is thought to be a pulsar wind nebula (PWN), as it shows an extended, approximate to 40 arcsec trail from a compact source. Here, we present Giant Metrewave Radio Telescope observations of this source at 330 and 1390 MHz, which reveal a remarkable linear radio trail approximate to 90 arcsec in extent. Although the radio trail points back to the supernova remnant (SNR) G338.1+0.4, approximate to 50 arcmin from CXOU J163802.6-471358, associating it with this remnant would require a very large velocity for the pulsar. There are no known Galactic SNRs close to the PWN and radio trail. No pulsar has yet been identified in CXOU J163802.6-471358, but if one could be found, this would allow more quantitative studies of the PWN and radio trail to be made.

We have estimated the magnetic field strengths of a sample of seven galaxies using their nonthermal synchrotron radio emission at meter wavelengths, and assuming energy equipartition between magnetic fields and cosmic-ray particles. We tested for deviation of magnetic fields from energy equipartition with cosmic-ray particles, and found that deviations of ∼25% are typical for the sample galaxies. Spatially resolved star formation rates (SFRs) were estimated for the seven galaxies along with five galaxies studied previously. For the combined sample of 12 galaxies, the equipartition magnetic fields ( B eq ) are correlated with the SFR surface densities (Σ SFR ) at sub-kiloparsec scales with B eq ∝ Σ SFR 0.31 ± 0.06 , consistent with model predictions. We estimated gas densities ( ρ gas ) for a subsample of seven galaxies using archival observations of the CO rotational transitions and the atomic hydrogen (H i ) 21 cm line and studied the spatially resolved correlation between the magnetic fields and ρ gas . Magnetic fields and gas densities are found to be correlated at sub-kiloparsec scale as B eq ∝ ρ gas 0.40 ± 0.09 . This is broadly consistent with models, which typically predict B ∝ ρ gas 0.5 .

This work gives an update to existing reconstructions of the Galactic Faraday rotation sky by processing almost all Faraday rotation data sets available at the end of the year 2020. Observations of extra-Galactic sources in recent years have, among other regions, further illuminated the previously under-constrained southern celestial sky, as well as parts of the inner disc of the Milky Way. This has culminated in an all-sky data set of 55,190 data points, which is a significant expansion on the 41,330 used in previous works, hence making an updated separation of the Galactic component a promising venture. The increased source density allows us to present our results in a resolution of about $1.3\cdot 10^{-2}\, \mathrm{deg}^2$ ($46.8\,\mathrm{arcmin}^2$), which is a twofold increase compared to previous works. As for previous Faraday rotation sky reconstructions, this work is based on information field theory, a Bayesian inference scheme for field-like quantities which handles noisy and incomplete data. In contrast to previous reconstructions, we find a significantly thinner and pronounced Galactic disc with small-scale structures exceeding values of several thousand $\mathrm{rad}\,\mathrm{m}^{-2}$. The improvements can mainly be attributed to the new catalog of Faraday data, but are also supported by advances in correlation structure modeling within numerical information field theory. We furthermore give a detailed discussion on statistical properties of the Faraday rotation sky and investigate correlations to other data sets.

ABSTRACT We have observed seven nearby large-angular-sized galaxies at 0.33 GHz using Giant Metrewave Radio Telescope with an angular resolution of ∼10 arcsec and sub-mJy sensitivity. Using archival higher frequency data at 1.4 or ∼6 GHz, we have then determined their spatially resolved non-thermal spectrum. As a general trend, we find that the spectral indices are comparatively flat at the galaxy centres and gradually steepen with increasing galactocentric distances. Using archival far-infrared (FIR) MIPS 70-${\mu }\mathrm{m}$ data, we estimate the exponent of radio–FIR correlation. One of the galaxies (NGC 4826) was found to have an exponent of the correlation of ∼1.4. Average exponent from 0.33-GHz data for the rest of the galaxies was 0.63 ± 0.06 and is significantly flatter than the exponent 0.78 ± 0.04 obtained using 1.4-GHz data. This indicates cosmic-ray electron (CRe) propagation to have reduced the correlation between FIR and 0.33-GHz radio. Assuming a model of simple isotropic diffusion of CRe, we find that the scenario can explain the frequency-dependent CRe propagation length-scales for only two galaxies. Invoking streaming instability could, however, explain the results for the majority of the remaining ones.

AX J1745.6-2901 is a high-inclination (eclipsing) transient neutron star (NS) low-mass X-ray binary showcasing intense ionized Fe K absorption. We present here the analysis of 11 XMM-Newton and 15 NuSTAR new data sets (obtained between 2013 and 2016), therefore tripling the number of observations of AX J1745.6-2901 in outburst. Thanks to simultaneous XMM-Newton and NuSTAR spectra, we greatly improve on the fitting of the X-ray continuum. During the soft state, the emission can be described by a disc blackbody (kT similar to 1.1-1.2 keV and inner disc radius r(DBB) similar to 14 km), plus hot (kT similar to 2.2-3.0 keV) blackbody radiation with a small emitting radius (r(BB) similar to 0.5-0.8 km) likely associated with the boundary layer or NS surface, plus a faint Comptonization component. Imprinted on the spectra are clear absorption features created by both neutral and ionized matter. Additionally, positive residuals suggestive of an emission Fe K alpha disc line and consistent with relativistic ionized reflection are present during the soft state, while such residuals are not significant during the hard state. The hard-state spectra are characterized by a hard (Gamma similar to 1.9-2.1) power law, showing no evidence for a high energy cut-off (kT(e) > 60-140 keV) and implying a small optical depth (tau < 1.6). The new observations confirm the previously witnessed trend of exhibiting strong Fe K absorption in the soft state that significantly weakens during the hard state. Optical (GROND) and radio (GMRT) observations suggest for AX J1745.6-2901 a standard broad-band spectral energy distribution as typically obsrved in accreting NSs.

We report the discovery of a diffuse radio emission around PSR J0855-4644 using an upgraded GMRT (uGMRT) observation at 1.35 GHz. The radio emission is spatially coincident with the diffuse X-ray pulsar wind nebula (PWN) seen with XMM-Newton but is much larger in extent compared to the compact axisymmetric PWN seen with Chandra. The morphology of the emission, with a bright partial ring-like structure and two faint tail-like features strongly resembles a bow shock nebula, and indicates a velocity of 100 km s(-1) through the ambient medium. We conclude that the emission is most likely to be associated with the radio PWN of PSR J0855-4644. From the integrated flux density, we estimate the energetics of the PWN.

The Venusian surface has been studied by measuring radar reflections and thermal radio emission over a wide spectral region of several centimeters to meter wavelengths from the Earth-based as well as orbiter platforms. The radiometric observations, in the decimeter (dcm) wavelength regime showed a decreasing trend in the observed brightness temperature (T-b) with increasing wavelength. The thermal emission models available at present have not been able to explain the radiometric observations at longer wavelength (dcm) to a satisfactory level. This paper reports the first interferometric imaging observations of Venus below 620 MHz. They were carried out at 606, 332.9 and 239.9 MHz using the Giant Meterwave Radio Telescope (GMRT). The T-b values derived at the respective frequencies are 526 K, 409 K and <426 K, with errors of similar to 7% which are generally consistent with the reported T-b values at 608 MHz and 430 MHz by previous investigators, but are much lower than those derived from high-frequency observations at 1.38-22.46 GHz using the VLA. (C) 2017 Elsevier Inc. All rights reserved.

HESS J1731-347 also known as SNR G353.6-0.7 is one of the five known shell-type supernova remnants (SNRs) emitting in the very high energy (VHE, energy > 0.1 TeV) gamma-ray domain. We observed this TeV SNR with the Giant Metrewave Radio Telescope (GMRT) in 1390, 610 and 325 MHz bands. In this paper, we report the discovery of 325 and 610 MHz radio counterparts of the SNR HESS J1731-347 with the GMRT. Various filaments of the SNR are clearly seen in the 325 and 610 MHz bands. However, the faintest feature in the radio bands corresponds to the peak in VHE emission. We explain this anti-correlation in terms of a possible leptonic origin of the observed VHE gamma-ray emission. We determine the spectral indices of the bright individual filaments, which were detected in both the 610 and the 325 MHz bands. Our values range from-1.11 to-0.15, consistent with the non-thermal radio emission. We also report a possible radio counterpart of a nearby TeV source HESS J1729-345 from the 843 MHz Molonglo Galactic Plane Survey and the 1.4 GHz Southern Galactic Plane Survey maps. The positive radio spectral index of this possible counterpart suggests a thermal origin of the radio emission of this nearby TeV source.

The Giant Metrewave Radio Telescope (GMRT) is today a frontline international facility for lowfrequency radio astronomy, that has produced several exciting and important new results in the 15 years that it has been operational. To keep the GMRT competitive in the global arena in the future, a major upgrade of the observatory is nearing completion that will increase its sensitivity by up to three times and make it a more powerful and versatile facility. We describe the main goals of this upgrade, highlight the technical features and challenges, outline the science potential and update the current status of this venture.

Although originally discovered as a radio-quiet gamma-ray pulsar, J1732-3131 has exhibited intriguing detections at decameter wavelengths. We report an extensive follow-up of the pulsar at 327 MHz with the Ooty radio telescope. Using the previously observed radio characteristics, and with an effective integration time of 60 h, we present a detection of the pulsar at a confidence level of 99.82 per cent. The 327 MHz mean flux density is estimated to be 0.5-0.8 mJy, which establishes the pulsar to be a steep spectrum source and one of the least luminous pulsars known to date. We also phase-aligned the radio and gamma-ray profiles of the pulsar, and measured the phase-offset between the main peaks in the two profiles to be 0.24 +/- 0.06. We discuss the observed phase-offset in the context of various trends exhibited by the radio-loud gamma-ray pulsar population, and suggest that the gamma-ray emission from J1732-3131 is best explained by outer magnetosphere models. Details of our analysis leading to the pulsar detection, and measurements of various parameters and their implications relevant to the pulsar's emission mechanism are presented.