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1. Radio Follow-up Observations of a Weak-line Quasar Exhibiting Remarkable X-Ray Variability

   https://doi.org/10.3847/1538-4357/ae1957  

   Chhipa, Ayushi; Vivek, M; J, Nayana A; Kharb, P; Brandt, W N; Jagannathan, Preshanth; Baghel, Janhavi; Ezhikode, Savithri H; Ishwara-Chandra, C H (December 2025, The Astrophysical Journal)

   Abstract SDSS J1539+3954 (z ≈ 1.935), a radio-quiet weak-line quasar (WLQ), exhibited exceptional X-ray variability in 2019–2020, with its X-ray flux increasing by over 20 times from 2013 to 2019 and subsequently dropping by at least a factor of 9 in 2020. Motivated by the empirical correlations between X-ray and radio emission in the cores of active galactic nuclei (AGNs), we carried out a follow-up radio study in the 0.3–10 GHz range using the Giant Metrewave Radio Telescope (GMRT; 2020, 2022, 2024) and Very Large Array (VLA; 2022), and analyzed archival Very Large Array Sky Survey (VLASS) 3 GHz data (2017–2023) to investigate the source’s radio properties and potential connection with the X-ray behavior. Our observations reveal a compact radio source with a spectral index of −0.65 ± 0.15 in the frequency range 0.3–1.4 GHz and −1.09 ± 0.16 in 3–10 GHz. While the source was undetected in VLA-FIRST (1994) and VLASS epochs, the GMRT and VLA observations show no statistically significant variability over the monitored period. The absence of detectable changes in the radio flux, despite strong X-ray variability, suggests no direct connection between the X-ray variability and the radio emission, consistent with the thick-disk-plus-outflow (TDO) model for WLQs. However, the sensitivity limit of the surveys prevents us from drawing definitive conclusions regarding variability on longer timescales between the VLA-FIRST and GMRT epochs. We further explore possible mechanisms driving the radio emission from this source. Our analysis rules out small-scale jets and coronal emission as the primary drivers of the radio emission, suggesting that extended emission from AGN winds and star formation is the more plausible mechanism. 

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2. Radio spectral energy distribution of low- z metal-poor extreme starburst galaxies: novel insights on the escape of ionizing photons

   https://doi.org/10.1093/mnras/stag492  

   Bait, Omkar; Schaerer, Daniel; Izotov, Yuri I; Sebastian, Biny (April 2026, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Recent optical surveys have identified a rare population of low-z ($$z \sim 0.01 \!-\! 0.06$$) extreme star-forming galaxies (xSFGs) characterized by very low metallicity, strong emission lines, extremely high specific star-formation rate, low stellar mass, and strong Ly $$\alpha$$ emission. Their global properties resemble recently discovered $$z\gt 6$$ reionization-era star-forming galaxies. We present new multifrequency radio continuum observations of eight xSFGs using the upgraded Giant Metrewave Radio Telescope at 1.25 GHz, the Karl G. Jansky Very Large Array at $1.5, 3.0, 6.0, 10.0$, and 15.0 GHz, along with archival LOw Frequency ARray data at 150 MHz for several sources. These data allow construction of the radio spectral energy distribution (radio-SED) from $$\sim 1$$ GHz (down to 150 MHz for some sources) to 15 GHz, spanning nearly two orders of magnitude in frequency. We find that xSFGs exhibit a flat spectral index between 6 and 15 GHz, while a subset shows spectral turnovers at $$0.3 \!-\! 3$$ GHz. Our Bayesian radio-SED modelling indicates that these features are consistent with a high thermal fraction combined with free–free absorption, requiring high emission measures in some systems. By comparing modelled thermal radio emission with observed H$$\beta {}$$ line flux density, we find evidence for dust in several xSFGs. Finally, we confirm a previously reported correlation between Lyman continuum escape fraction ($$f_\mathrm{esc}^\mathrm{LyC}$$), ionization state, and radio spectral index, particularly among strong leakers. 

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3. SuperCLASS – II. Photometric redshifts and characteristics of spatially resolved μJy radio sources

   https://doi.org/10.1093/mnras/staa657  

   Manning, Sinclaire M; Casey, Caitlin M; Hung, Chao-Ling; Battye, Richard; Brown, Michael L; Jackson, Neal; Abdalla, Filipe; Chapman, Scott; Demetroullas, Constantinos; Drew, Patrick; et al (June 2020, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT We present optical and near-infrared imaging covering a ∼1.53 deg2 region in the Super-Cluster Assisted Shear Survey (SuperCLASS) field, which aims to make the first robust weak lensing measurement at radio wavelengths. We derive photometric redshifts for ≈176 000 sources down to $$i^\prime _{\rm AB}\sim 24$$ and present photometric redshifts for 1.4 GHz expanded Multi-Element Radio Linked Interferometer Network (e-MERLIN) and Karl G. Jansky Very Large Array (VLA) detected radio sources found in the central 0.26 deg2. We compile an initial catalogue of 149 radio sources brighter than S1.4 > 75 μJy and find their photometric redshifts span 0 < zphot < 4 with radio luminosities between 1021 and 1025 W Hz−1, with medians of $$\langle z \rangle \, =0.55$$ and $$\langle L_{1.4}\rangle \, =1.9\times 10^{23}$$ W Hz−1, respectively. We find 95 per cent of the μJy radio source sample (141/149) have spectral energy distributions (SEDs) best fit by star-forming templates while 5 per cent (8/149) are better fit by active galactic nuclei (AGN). Spectral indices are calculated for sources with radio observations from the VLA and Giant Metrewave Radio Telescope (GMRT) at 325 MHz, with an average spectral slope of α = 0.59 ± 0.04. Using the full photometric redshift catalogue, we construct a density map at the redshift of the known galaxy clusters, z = 0.20 ± 0.08. Four of the five clusters are prominently detected at $$\gt 7\, \sigma$$ in the density map and we confirm the photometric redshifts are consistent with previously measured spectra from a few galaxies at the cluster centres. 

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4. Relativistic Jet Motion in the Radio-quiet LINER Galaxy KISSR 872

   https://doi.org/10.3847/1538-4357/ad168e  

   Kharb, Preeti; Blackman, Eric G; Clausen-Brown, Eric; Das, Mousumi; Schwartz, Daniel A; Siemiginowska, Aneta; Subramanian, Smitha; Vaddi, Sravani (February 2024, The Astrophysical Journal)

   Abstract We report superluminal jet motion with an apparent speed ofβ_app= 1.65 ± 0.57 in the radio-quiet (RQ) low-ionization nuclear emission-line region (LINER) galaxy KISSR 872. This result comes from two-epoch phase-referenced very long baseline interferometry observations at 5 GHz. The detection of bulk relativistic motion in the jet of this extremely radio-faint active galactic nucleus (AGN), with a total 1.4 GHz flux density of 5 mJy in the 5.″4 resolution Very Large Array FIRST survey image and 1.5 mJy in the ∼5 mas resolution Very Long Baseline Array image, is the first of its kind in an RQ LINER galaxy. The presence of relativistic jets in lower accretion rate objects like KISSR 872, with an Eddington ratio of 0.04, reveals that even RQ AGN can harbor relativistic jets and provides evidence of their universality over a wide range of accretion powers. 

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5. Radio Analysis of SN2004C Reveals an Unusual CSM Density Profile as a Harbinger of Core Collapse

   https://doi.org/10.3847/1538-4357/ac8c26  

   DeMarchi, Lindsay; Margutti, R.; Dittman, J.; Brunthaler, A.; Milisavljevic, D.; Bietenholz, Michael F.; Stauffer, C.; Brethauer, D.; Coppejans, D.; Auchettl, K.; et al (October 2022, The Astrophysical Journal)

   Abstract We present extensive multifrequency Karl G. Jansky Very Large Array (VLA) and Very Long Baseline Array (VLBA) observations of the radio-bright supernova (SN) IIb SN 2004C that span ∼40–2793 days post-explosion. We interpret the temporal evolution of the radio spectral energy distribution in the context of synchrotron self-absorbed emission from the explosion’s forward shock as it expands in the circumstellar medium (CSM) previously sculpted by the mass-loss history of the stellar progenitor. VLBA observations and modeling of the VLA data point to a blastwave with average velocity ∼0.06 c that carries an energy of ≈10 49 erg. Our modeling further reveals a flat CSM density profile ρ CSM ∝ R −0.03±0.22 up to a break radius R br ≈ (1.96 ± 0.10) × 10 16 cm, with a steep density gradient following ρ CSM ∝ R −2.3±0.5 at larger radii. We infer that the flat part of the density profile corresponds to a CSM shell with mass ∼0.021 M ☉ , and that the progenitor’s effective mass-loss rate varied with time over the range (50–500) × 10 −5 M ☉ yr −1 for an adopted wind velocity v w = 1000 km s −1 and shock microphysical parameters ϵ e = 0.1, ϵ B = 0.01. These results add to the mounting observational evidence for departures from the traditional single-wind mass-loss scenarios in evolved, massive stars in the centuries leading up to core collapse. Potentially viable scenarios include mass loss powered by gravity waves and/or interaction with a binary companion. 

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