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1. Radio imaging of gravitationally lensed radio-quiet quasars

   https://doi.org/10.1093/mnras/stae916  

   Jackson, Neal ; Badole, Shruti ; Dugdale, Thomas ; Stacey, Hannah R. ; Hartley, Philippa ; McKean, J. P. ( March 2024 , Monthly Notices of the Royal Astronomical Society)

   We present 6 GHz Very Large Array radio images of 70 gravitational lens systems at 300 mas resolution, in which the source is an optically selected quasar, and nearly all of which have two lensed images. We find that about in half of the systems (40/70, with 33/70 secure), one or more lensed images are detected down to our detection limit of 20 μJy beam−1, similar to previous investigations and reinforcing the conclusion that typical optically selected quasars have intrinsic GHz radio flux densities of a few μJy (∼1023 W Hz−1 at redshifts of 1–2). In addition, for 10 cases it is likely that the lensing galaxies are detected in the radio. Available detections of, and limits on the far-infrared luminosities from the literature, suggest that nearly all of the sample lie on the radio-FIR correlation typical of star-forming galaxies, and that their radio luminosities are at least compatible with the radio emission being produced by star formation processes. One object, WISE2329−1258, has an extra radio component that is not present in optical images, and is difficult to explain using simple lens models. In-band spectral indices, where these can be determined, are generally moderately steep and consistent with synchrotron processes either from star formation/supernovae or AGNs. Comparison of the A/B image flux ratios at radio and optical wavelengths suggests a 10 per cent level contribution from finite source effects or optical extinction to the optical flux ratios, together with sporadic larger discrepancies that are likely to be due to optical microlensing.

    

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2. A Radio Study of Persistent Radio Sources in Nearby Dwarf Galaxies: Implications for Fast Radio Bursts

   https://doi.org/10.3847/1538-4357/ad6568  

   Dong_董, Y_雨欣 ; Eftekhari, T. ; Fong, W. ; Bhandari, S. ; Berger, E. ; Ould-Boukattine, O_S ; Hessels, J_W_T ; Sridhar, N. ; Reines, A. ; Margalit, B. ; et al ( September 2024 , The Astrophysical Journal)

   We present 1–12 GHz Karl G. Jansky Very Large Array observations of nine off-nuclear persistent radio sources (PRSs) in nearby (z≲ 0.055) dwarf galaxies, along with high-resolution European VLBI Network observations for one of them at 1.7 GHz. We explore the plausibility that these PRSs are associated with fast radio burst (FRB) sources by examining their properties—physical sizes, host-normalized offsets, spectral energy distributions (SEDs), radio luminosities, and light curves—and compare them to those of the PRSs associated with FRB 20121102A and FRB 20190520B, two known active galactic nuclei (AGN), and one likely AGN in our sample with comparable data, as well as other radio transients exhibiting characteristics analogous to FRB-PRSs. We identify a single source in our sample, J1136+2643, as the most promising FRB-PRS, based on its compact physical size and host-normalized offset. We further identify two sources, J0019+1507 and J0909+5655, with physical sizes comparable to FRB-PRSs, but which exhibit large offsets and flat spectral indices potentially indicative of a background AGN origin. We test the viability of neutron star wind nebula and hypernebula models for J1136+2643 and find that the physical size, luminosity, and SED of J1136+2643 are broadly consistent with these models. Finally, we discuss the alternative interpretation that the radio sources are instead powered by accreting massive black holes, and we outline future prospects and follow-up observations for differentiating between these scenarios.

    

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3. On the Fast Radio Burst and Persistent Radio Source Populations

   https://doi.org/10.3847/1538-4357/ac4c42  

   Law, Casey J. ; Connor, Liam ; Aggarwal, Kshitij ( March 2022 , The Astrophysical Journal)

   The first fast radio burst (FRB) to be precisely localized was associated with a luminous persistent radio source (PRS). Recently, a second FRB/PRS association was discovered for another repeating source of FRBs. However, it is not clear what makes FRBs or PRS or how they are related. We compile FRB and PRS properties to consider the population of FRB/PRS sources. We suggest a practical definition for PRS as FRB associations with luminosity greater than 10²⁹erg s⁻¹Hz⁻¹that are not attributed to star formation activity in the host galaxy. We model the probability distribution of the fraction of FRBs with PRS for repeaters and nonrepeaters, showing there is not yet evidence for repeaters to be preferentially associated with PRS. We discuss how FRB/PRS sources may be distinguished by the combination of active repetition and an excess dispersion measure local to the FRB environment. We use CHIME/FRB event statistics to bound the mean per-source repetition rate of FRBs to be between 25 and 440 yr⁻¹. We use this to provide a bound on the density of FRB-emitting sources in the local universe of between 2.2 × 10²and 5.2 × 10⁴Gpc⁻³assuming a pulsar-like beamwidth for FRB emission. This density implies that PRS may comprise as much as 1% of compact, luminous radio sources detected in the local universe. The cosmic density and phenomenology of PRS are similar to that of the newly discovered, off-nuclear “wandering” active galactic nuclei (AGN). We argue that it is likely that some PRS have already been detected and misidentified as AGN.

    

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4. The radio emission in radio-quiet quasars: the VLBA perspective

   https://doi.org/10.1093/mnras/stad2289  

   Chen, Sina ; Laor, Ari ; Behar, Ehud ; Baldi, Ranieri_D ; Gelfand, Joseph_D ( July 2023 , Monthly Notices of the Royal Astronomical Society)

   The origin of the radio emission in radio-quiet quasars (RQQ) is not established yet. We present new VLBA observations at 1.6 and 4.9 GHz of 10 RQQ (9 detected), which together with published earlier observations of 8 RQQ (5 detected), forms a representative sample of 18 RQQ drawn from the Palomar–Green sample of low z (< 0.5) AGN. The spectral slope of the integrated emission extends from very steep (α < −1.98) to strongly inverted (α = +2.18), and the slopes of 9 of the 14 objects are flat (α > −0.5). Most objects have an unresolved flat-spectrum core, which coincides with the optical Gaia position. The extended emission is generally steep-spectrum, has a low brightness temperature (< 107 K), and is displaced from the optical core (the Gaia position) by ∼ 5–100 pc. The VLBA core flux is tightly correlated with the X-ray flux, and follows a radio to X-ray luminosity relation of log LR/LX ≃ −6, for all objects with a black hole mass log MBH/M⊙ < 8.5. The flatness of the core emission implies a compact source size (≲ 0.1 pc), which likely originates from the accretion disc corona. The mas-scale extended emission is optically thin and of clumpy structure, and is likely produced by an outflow from the center. Radio observations at higher frequencies can further test the accretion disc coronal emission interpretation for the core emission in RQQ.

    

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5. Blind Radio Tomography

   https://doi.org/10.1109/TSP.2018.2799169  

   Romero, Daniel ; Lee, Donghoon ; Giannakis, Georgios B. ( April 2018 , IEEE Transactions on Signal Processing)