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1. A Nonrepeating Fast Radio Burst in a Dwarf Host Galaxy

   https://doi.org/10.3847/1538-4357/acc178  

   Bhandari, Shivani ; Gordon, Alexa C. ; Scott, Danica R. ; Marnoch, Lachlan ; Sridhar, Navin ; Kumar, Pravir ; James, Clancy W. ; Qiu, Hao ; Bannister, Keith W. ; T. Deller, Adam ; et al ( May 2023 , The Astrophysical Journal)

   We present the discovery of an as yet nonrepeating fast radio burst (FRB), FRB 20210117A, with the Australian Square Kilometre Array Pathfinder (ASKAP), as a part of the Commensal Real-time ASKAP Fast Transients Survey. The subarcsecond localization of the burst led to the identification of its host galaxy atz= 0.214(1). This redshift is much lower than what would be expected for a source dispersion measure (DM) of 729 pc cm⁻³, given typical contributions from the intergalactic medium and the host galaxy. Optical observations reveal the host to be a dwarf galaxy with little ongoing star formation—very different to the dwarf host galaxies of the known repeating FRBs 20121102A and 20190520B. We find an excess DM contribution from the host and attribute it to the FRB’s local environment. We do not find any radio emission from the FRB site or host galaxy. The low magnetized environment and the lack of a persistent radio source indicate that the FRB source is older than those found in other dwarf host galaxies, establishing the diversity of FRB sources in dwarf galaxy environments. We find our observations to be fully consistent with the “hypernebula” model, where the FRB is powered by an accretion jetmore »from a hyperaccreting black hole. Finally, our high time resolution analysis reveals burst characteristics similar to those seen in repeating FRBs. We encourage follow-up observations of FRB 20210117A to establish any repeating nature.

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2. The First JWST Spectrum of a GRB Afterglow: No Bright Supernova in Observations of the Brightest GRB of all Time, GRB 221009A

   https://doi.org/10.3847/2041-8213/acc2c1  

   Levan, A. J. ; Lamb, G. P. ; Schneider, B. ; Hjorth, J. ; Zafar, T. ; de Ugarte Postigo, A. ; Sargent, B. ; Mullally, S. E. ; Izzo, L. ; D’Avanzo, P. ; et al ( March 2023 , The Astrophysical Journal Letters)

   Abstract We present James Webb Space Telescope (JWST) and Hubble Space Telescope (HST) observations of the afterglow of GRB 221009A, the brightest gamma-ray burst (GRB) ever observed. This includes the first mid-IR spectra of any GRB, obtained with JWST/Near Infrared Spectrograph (0.6–5.5 micron) and Mid-Infrared Instrument (5–12 micron), 12 days after the burst. Assuming that the intrinsic spectral slope is a single power law, with F ν ∝ ν − β , we obtain β ≈ 0.35, modified by substantial dust extinction with A V = 4.9. This suggests extinction above the notional Galactic value, possibly due to patchy extinction within the Milky Way or dust in the GRB host galaxy. It further implies that the X-ray and optical/IR regimes are not on the same segment of the synchrotron spectrum of the afterglow. If the cooling break lies between the X-ray and optical/IR, then the temporal decay rates would only match a post-jet-break model, with electron index p < 2, and with the jet expanding into a uniform ISM medium. The shape of the JWST spectrum is near-identical in the optical/near-IR to X-SHOOTER spectroscopy obtained at 0.5 days and to later time observations with HST. The lack of spectral evolutionmore »suggests that any accompanying supernova (SN) is either substantially fainter or bluer than SN 1998bw, the proto-type GRB-SN. Our HST observations also reveal a disk-like host galaxy, viewed close to edge-on, that further complicates the isolation of any SN component. The host galaxy appears rather typical among long-GRB hosts and suggests that the extreme properties of GRB 221009A are not directly tied to its galaxy-scale environment.« less
3. The thermal and non-thermal components within and between galaxy clusters Abell 399 and Abell 401

   https://doi.org/10.1093/mnras/stac3015  

   Radiconi, Federico ; Vacca, Valentina ; Battistelli, Elia ; Bonafede, Annalisa ; Capalbo, Valentina ; Devlin, Mark J. ; Di Mascolo, Luca ; Feretti, Luigina ; Gallardo, Patricio A. ; Gill, Ajay ; et al ( October 2022 , Monthly Notices of the Royal Astronomical Society)

   We measure the local correlation between radio emission and Compton-y signal across two galaxy clusters, Abell 399 and Abell 401, using maps from the Low Frequency Array and the Atacama Cosmology Telescope  + Planck. These data sets allow us to make the first measurement of this kind at ∼arcmin resolution. We find that the radio brightness scales as Fradio ∝ y1.5 for Abell 401 and Fradio ∝ y2.8 for Abell 399. Furthermore, using XMM–Newton data, we derive a sublinear correlation between radio and X-ray brightness for both the clusters ($F_{\mathrm{radio}} \propto F_{\rm X}^{0.7}$). Finally, we correlate the Compton-y and X-ray data, finding that an isothermal model is consistent with the cluster profiles, $y \propto F_{\rm X}^{0.5}$. By adopting an isothermal–β model, we are able, for the first time, to jointly use radio, X-ray, and Compton-y data to estimate the scaling index for the magnetic field profile, B(r) ∝ ne(r)η in the injection and re-acceleration scenarios. Applying this model, we find that the combined radio and Compton-y signal exhibits a significantly tighter correlation with the X-ray across the clusters than when the data sets are independently correlated. We find η ∼ 0.6–0.8. These results are consistent with the upper limit we derive for the scaling index of the magnetic field usingmore »rotation measure values for two radio galaxies in Abell 401. We also measure the radio, Compton-y, and X-ray correlations in the filament between the clusters but conclude that deeper data are required for a convincing determination of the correlations in the filament.

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4. Puzzling large-scale polarization in the galaxy cluster Abell 523

   https://doi.org/10.1093/mnras/stac1421  

   Vacca, Valentina ; Govoni, Federica ; Murgia, Matteo ; Perley, Richard A. ; Feretti, Luigina ; Giovannini, Gabriele ; Carretti, Ettore ; Gastaldello, Fabio ; Cova, Filippo ; Marchegiani, Paolo ; et al ( July 2022 , Monthly Notices of the Royal Astronomical Society)

   Large-scale magnetic fields reveal themselves through diffuse synchrotron sources observed in galaxy clusters such as radio halos. Total intensity filaments of these sources have been observed in polarization as well, but only in three radio halos out of about 100 currently known. In this paper we analyse new polarimetric Very Large Array data of the diffuse emission in the galaxy cluster Abell 523 in the frequency range 1–2 GHz. We find for the first time evidence of polarized emission on scales of ∼2.5 Mpc. The total intensity emission is observed only in the central part of the source, likely due to observational limitations. To look for total intensity emission beyond the central region, we combine these data with single-dish observations from the Sardinia Radio Telescope and we compare them with multifrequency total intensity observations obtained with different instruments, including the LOw Frequency ARray and the Murchison Widefield Array. By analysing the rotation measure properties of the system and utilizing numerical simulations, we infer that this polarized emission is associated with filaments of the radio halo located in the outskirts of the system, in the peripheral region closest to the observer.
5. The Host Galaxy of the Recoiling Black Hole Candidate in 3C 186: An Old Major Merger Remnant at the Center of a z = 1 Cluster

   https://doi.org/10.3847/1538-4357/ac6a58  

   Morishita, T. ; Chiaberge, M. ; Hilbert, B. ; Lambrides, E. ; Blecha, L. ; Baum, S. ; Bianchi, S. ; Capetti, A. ; Castignani, G. ; Macchetto, F. D. ; et al ( June 2022 , The Astrophysical Journal)

   Abstract 3C 186, a radio-loud quasar at z = 1.0685, was previously reported to have both velocity and spatial offsets from its host galaxy, and has been considered as a promising candidate for a gravitational wave recoiling black hole triggered by a black hole merger. Another possible scenario is that 3C 186 is in an ongoing galaxy merger, exhibiting a temporary displacement. In this study, we present analyses of new deep images from the Hubble Space Telescope WFC3-IR and Advanced Camera for Surveys, aiming to characterize the host galaxy and test this alternative scenario. We carefully measure the light-weighted center of the host and reveal a significant spatial offset from the quasar core (11.1 ± 0.1 kpc). The direction of the confirmed offset aligns almost perpendicularly to the radio jet. We do not find evidence of a recent merger, such as a young starburst in disturbed outskirts, but only marginal light concentration in F160W at ∼30 kpc. The host consists of mature (≳200 Myr) stellar populations and one compact star-forming region. We compare with hydrodynamical simulations and find that those observed features are consistently seen in late-stage merger remnants. Taken together, those pieces of evidence indicate that the system ismore »not an ongoing/young merger remnant, suggesting that the recoiling black hole scenario is still a plausible explanation for the puzzling nature of 3C 186.« less