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1. Radio Scattering Horizons for Galactic and Extragalactic Transients

   https://doi.org/10.3847/1538-4357/ac75ba  

   Ocker, Stella Koch ; Cordes, James M. ; Chatterjee, Shami ; Gorsuch, Miranda R. ( July 2022 , The Astrophysical Journal)

   Radio wave scattering can cause severe reductions in detection sensitivity for surveys of Galactic and extragalactic fast (∼ms duration) transients. While Galactic sources like pulsars undergo scattering in the Milky Way interstellar medium (ISM), extragalactic fast radio bursts (FRBs) can also experience scattering in their host galaxies and other galaxies intervening in their lines of sight. We assess Galactic and extragalactic scattering horizons for fast radio transients using a combination of NE2001 to model the dispersion measure and scattering time (τ) contributed by the Galactic disk, and independently constructed electron density models for the Galactic halo and other galaxies’ ISMs and halos that account for different galaxy morphologies, masses, densities, and strengths of turbulence. For source redshifts 0.5 ≤z_s≤ 1, an all-sky, isotropic FRB population has simulated values ofτ(1 GHz) ranging from ∼1μs to ∼2 ms (90% confidence, observer frame) that are dominated by host galaxies, althoughτcan be ≫2 ms at low Galactic latitudes. A population atz_s= 5 has 0.01 ≲τ≲ 300 ms at 1 GHz (90% confidence), dominated by intervening galaxies. About 20% of these high-redshift FRBs are predicted to haveτ> 5 ms at 1 GHz (observer frame), and ≳40% of FRBs betweenz_s∼ 0.5–5 haveτ≳ 1 ms forν≤ 800 MHz. Our scattering predictions may be conservative if scattering from circumsource environments is significant, which is possible under specific conditions. The percentage of FRBs selected against from scattering could also be substantially larger than we predict if circumgalactic turbulence causes more small-scale (≪1 au) density fluctuations than observed from nearby halos.

    

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2. The Large Dispersion and Scattering of FRB 20190520B Are Dominated by the Host Galaxy

   https://doi.org/10.3847/1538-4357/ac6504  

   Koch Ocker, Stella ; Cordes, James M. ; Chatterjee, Shami ; Niu, Chen-Hui ; Li, Di ; McKee, James W. ; Law, Casey J. ; Tsai, Chao-Wei ; Anna-Thomas, Reshma ; Yao, Ju-Mei ; et al ( May 2022 , The Astrophysical Journal)

   The repeating fast radio burst FRB 20190520B is localized to a galaxy atz= 0.241, much closer than expected given its dispersion measure DM = 1205 ± 4 pc cm⁻³. Here we assess implications of the large DM and scattering observed from FRB 20190520B for the host galaxy’s plasma properties. A sample of 75 bursts detected with the Five-hundred-meter Aperture Spherical radio Telescope shows scattering on two scales: a mean temporal delayτ(1.41 GHz) = 10.9 ± 1.5 ms, which is attributed to the host galaxy, and a mean scintillation bandwidth Δν_d(1.41 GHz) = 0.21 ± 0.01 MHz, which is attributed to the Milky Way. Balmer line measurements for the host imply an Hαemission measure (galaxy frame) EM_s= 620 pc cm⁻⁶× (T/10⁴K)^0.9, implying DM_Hαof order the value inferred from the FRB DM budget,${\mathrm{DM}}_{\mathrm{h}}={1121}_{-138}^{+89}$pc cm⁻³for plasma temperatures greater than the typical value 10⁴K. Combiningτand DM_hyields a nominal constraint on the scattering amplification from the host galaxy$\tilde{F}G={1.5}_{-0.3}^{+0.8}{({\mathrm{pc}}^2\mathrm{km})}^{-1/3}$, where$\tilde{F}$describes turbulent density fluctuations andGrepresents the geometric leverage to scattering that depends on the location of the scattering material. For a two-screen scattering geometry whereτarises from the host galaxy and Δν_dfrom the Milky Way, the implied distance between the FRB source and dominant scattering material is ≲100 pc. The host galaxy scattering and DM contributions support a novel technique for estimating FRB redshifts using theτ–DM relation, and are consistent with previous findings that scattering of localized FRBs is largely dominated by plasma within host galaxies and the Milky Way.

    

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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. Four new fast radio bursts discovered in the Parkes 70-cm pulsar survey archive

   https://doi.org/10.1093/mnras/stac2101  

   Crawford, F. ; Hisano, S. ; Golden, M. ; Kikunaga, T. ; Laity, A. ; Zoeller, D. ( August 2022 , Monthly Notices of the Royal Astronomical Society)

   We present four new fast radio bursts discovered in a search of the Parkes 70-cm pulsar survey data archive for dispersed single pulses and bursts. We searched dispersion measures (DMs) ranging between 0 and 5000 pc cm−3 with the HEIMDALL and FETCH detection and classification algorithms. All four of the fast radio bursts (FRBs) discovered have significantly larger widths (>50 ms) than almost all of the FRBs detected and catalogued to date. The large pulse widths are not dominated by interstellar scattering or dispersive smearing within channels. One of the FRBs has a DM of 3338 pc cm3, the largest measured for any FRB to date. These are also the first FRBs detected by any radio telescope so far, predating the Lorimer Burst by almost a decade. Our results suggest that pulsar survey archives remain important sources of previously undetected FRBs and that searches for FRBs on time-scales extending beyond ∼100 ms may reveal the presence of a larger population of wide-pulse FRBs.

    

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5. The First CHIME/FRB Fast Radio Burst Catalog

   https://doi.org/10.3847/1538-4365/ac33ab  

   The CHIME/FRB Collaboration ; Amiri, Mandana ; Andersen, Bridget C. ; Bandura, Kevin ; Berger, Sabrina ; Bhardwaj, Mohit ; Boyce, Michelle M. ; Boyle, P. J. ; Brar, Charanjot ; Breitman, Daniela ; et al ( December 2021 , The Astrophysical Journal Supplement Series)

   We present a catalog of 536 fast radio bursts (FRBs) detected by the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst (CHIME/FRB) Project between 400 and 800 MHz from 2018 July 25 to 2019 July 1, including 62 bursts from 18 previously reported repeating sources. The catalog represents the first large sample, including bursts from repeaters and nonrepeaters, observed in a single survey with uniform selection effects. This facilitates comparative and absolute studies of the FRB population. We show that repeaters and apparent nonrepeaters have sky locations and dispersion measures (DMs) that are consistent with being drawn from the same distribution. However, bursts from repeating sources differ from apparent nonrepeaters in intrinsic temporal width and spectral bandwidth. Through injection of simulated events into our detection pipeline, we perform an absolute calibration of selection effects to account for systematic biases. We find evidence for a population of FRBs—composing a large fraction of the overall population—with a scattering time at 600 MHz in excess of 10 ms, of which only a small fraction are observed by CHIME/FRB. We infer a power-law index for the cumulative fluence distribution of$\alpha =-1.40\pm 0.11(\mathrm{stat.}{)}_{-0.09}^{+0.06}(\mathrm{sys.})$, consistent with the −3/2 expectation for a nonevolving population in Euclidean space. We find thatαis steeper for high-DM events and shallower for low-DM events, which is what would be expected when DM is correlated with distance. We infer a sky rate of$[820\pm 60(\mathrm{stat.}{)}_{-200}^{+220}(\mathrm{sys.})]/\mathrm{sky}/\mathrm{day}$above a fluence of 5 Jy ms at 600 MHz, with a scattering time at 600 MHz under 10 ms and DM above 100 pc cm⁻³.

    

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