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1. A repeating fast radio burst associated with a persistent radio source

   https://doi.org/10.1038/s41586-022-04755-5  

   Niu, C.-H. ; Aggarwal, K. ; Li, D. ; Zhang, X. ; Chatterjee, S. ; Tsai, C.-W. ; Yu, W. ; Law, C. J. ; Burke-Spolaor, S. ; Cordes, J. M. ; et al ( June 2022 , Nature)

   Abstract The dispersive sweep of fast radio bursts (FRBs) has been used to probe the ionized baryon content of the intergalactic medium 1 , which is assumed to dominate the total extragalactic dispersion. Although the host-galaxy contributions to the dispersion measure appear to be small for most FRBs 2 , in at least one case there is evidence for an extreme magneto-ionic local environment 3,4 and a compact persistent radio source 5 . Here we report the detection and localization of the repeating FRB 20190520B, which is co-located with a compact, persistent radio source and associated with a dwarf host galaxy of high specific-star-formation rate at a redshift of 0.241 ± 0.001. The estimated host-galaxy dispersion measure of approximately $${903}_{-111}^{+72}$$ 903 − 111 + 72 parsecs per cubic centimetre, which is nearly an order of magnitude higher than the average of FRB host galaxies 2,6 , far exceeds the dispersion-measure contribution of the intergalactic medium. Caution is thus warranted in inferring redshifts for FRBs without accurate host-galaxy identifications. 

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2. 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 jet 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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3. Fast Radio Bursts: An Extragalactic Enigma

   https://doi.org/10.1146/annurev-astro-091918-104501  

   Cordes, James M. ; Chatterjee, Shami ( August 2019 , Annual Review of Astronomy and Astrophysics)

   We summarize our understanding of millisecond radio bursts from an extragalactic population of sources. Fast radio bursts (FRBs) occur at an extraordinary rate, thousands per day over the entire sky with radiation energy densities at the source about ten billion times larger than those from Galactic pulsars. We survey FRB phenomenology, source models and host galaxies, coherent radiation models, and the role of plasma propagation effects in burst detection. The FRB field is guaranteed to be exciting: New telescopes will expand the sample from the current ∼80 unique burst sources (and only a few secure localizations and redshifts) to thousands, with burst localizations that enable host-galaxy redshifts emerging directly from interferometric surveys. ▪ FRBs are now established as an extragalactic phenomenon. ▪ Only a few sources are known to repeat. Despite the failure to redetect other FRBs, they are not inconsistent with all being repeaters. ▪ FRB sources may be new, exotic kinds of objects or known types in extreme circumstances. Many inventive models exist, ranging from alien spacecraft to cosmic strings, but those concerning compact objects and supermassive black holes have gained the most attention. A rapidly rotating magnetar is a promising explanation for FRB 121102 along with the persistent source associated with it, but alternative source models are not ruled out for it or other FRBs. ▪ FRBs are powerful tracers of circumsource environments, “missing baryons” in the intergalactic medium (IGM), and dark matter. ▪ The relative contributions of host galaxies and the IGM to propagation effects have yet to be disentangled, so dispersion measure distances have large uncertainties. 

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4. The ultranarrow FRB20191107B, and the origins of FRB scattering

   https://doi.org/10.1093/mnras/stac1720  

   Gupta, V. ; Flynn, C. ; Farah, W. ; Bailes, M. ; Deller, A. T. ; Day, C. K. ; Lower, M. E. ( June 2022 , Monthly Notices of the Royal Astronomical Society)

   We report the detection of FRB20191107B with UTMOST radio telescope at a dispersion measure (DM) of 714.9 pc cm−3. The burst consists of three components, the brightest of which has an intrinsic width of only 11.3 μs and a scattering tail with an exponentially decaying time-scale of 21.4 μs measured at 835 MHz. We model the sensitivity of UTMOST and other major fast radio burst (FRB) surveys to such narrow events. We find that $\gt 60{{\ \rm per\, cent}}$ of FRBs like FRB20191107B are being missed, and that a significant population of very narrow FRBs probably exists and remains underrepresented in these surveys. The high DM and small scattering time-scale of FRB20191107B allows us to place an upper limit on the strength of turbulence in the intergalactic medium, quantified as scattering measure (SM), of SMIGM < 8.4 × 10−7 kpc m−20/3. Almost all UTMOST FRBs have full phase information due to real-time voltage capture, which provides us with the largest sample of coherently dedispersed single burst FRBs. Our 10.24 μs time resolution data yields accurately measured FRB scattering time-scales. We combine the UTMOST FRBs with 10 FRBs from the literature and find no obvious evidence for a DM-scattering relation, suggesting that IGM is not the dominant source of scattering in FRBs. We support the results of previous studies and identify the local environment of the source in the host galaxy as the most likely region that dominates the observed scattering of our FRBs.

    

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5. Searching for the Sources of Excess Extragalactic Dispersion of FRBs

   https://doi.org/10.3847/1538-4357/ace324  

   Simha, Sunil ; Lee, Khee-Gan ; Prochaska, J. Xavier ; Khrykin, Ilya S. ; Huang, Yuxin ; Tejos, Nicolas ; Marnoch, Lachlan ; Ata, Metin ; Bernales, Lucas ; Bhandari, Shivani ; et al ( August 2023 , The Astrophysical Journal)

   The FLIMFLAM survey is collecting spectroscopic data of field galaxies near fast radio burst (FRB) sight lines to constrain key parameters describing the distribution of matter in the Universe. In this work, we leverage the survey data to determine the source of the excess extragalactic dispersion measure (DM), compared to Macquart relation estimates of four FRBs: FRB20190714A, FRB20200906A, FRB20200430A, and FRB20210117A. By modeling the gas distribution around the foreground galaxy halos and galaxy groups of the sight lines, we estimate DM_halos, their contribution to the FRB DMs. The FRB20190714A sight line shows a clear excess of foreground halos which contribute roughly two-thirds of the observed excess DM, thus implying a sight line that is baryon dense. FRB20200906A shows a smaller but nonnegligible foreground halo contribution, and further analysis of the intergalactic medium is necessary to ascertain the true cosmic contribution to its DM. FRB20200430A and FRB20210117A show negligible foreground contributions, implying a large host galaxy excess and/or progenitor environment excess.

    

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