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1. An FRB Sent Me a DM: Constraining the Electron Column of the Milky Way Halo with Fast Radio Burst Dispersion Measures from CHIME/FRB

   https://doi.org/10.3847/1538-4357/acbbd0  

   Cook, Amanda M. ; Bhardwaj, Mohit ; Gaensler, B. M. ; Scholz, Paul ; Eadie, Gwendolyn M. ; Hill, Alex S. ; Kaspi, Victoria M. ; Masui, Kiyoshi W. ; Curtin, Alice P. ; Dong, Fengqiu Adam ; et al ( March 2023 , The Astrophysical Journal)

   The CHIME/FRB project has detected hundreds of fast radio bursts (FRBs), providing an unparalleled population to statistically probe the foreground media that they illuminate. One such foreground medium is the ionized halo of the Milky Way (MW). We estimate the total Galactic electron column density from FRB dispersion measures (DMs) as a function of Galactic latitude using four different estimators, including ones that assume spherical symmetry of the ionized MW halo and ones that imply more latitudinal variation in density. Our observation-based constraints of the total Galactic DM contribution for ∣b∣ ≥ 30°, depending on the Galactic latitude and selected model, span 87.8–141 pc cm⁻³. This constraint implies upper limits on the MW halo DM contribution that range over 52–111 pc cm⁻³. We discuss the viability of various gas density profiles for the MW halo that have been used to estimate the halo’s contribution to DMs of extragalactic sources. Several models overestimate the DM contribution, especially when assuming higher halo gas masses (∼3.5 × 10¹²M_⊙). Some halo models predict a higher MW halo DM contribution than can be supported by our observations unless the effect of feedback is increased within them, highlighting the impact of feedback processes in galaxy formation.

    

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2. The FRB 20190520B Sight Line Intersects Foreground Galaxy Clusters

   https://doi.org/10.3847/2041-8213/acefb5  

   Lee, Khee-Gan ; Khrykin, Ilya S. ; Simha, Sunil ; Ata, Metin ; Huang, Yuxin ; Prochaska, J. Xavier ; Tejos, Nicolas ; Cooke, Jeff ; Nagamine, Kentaro ; Zhang, Jielai ( August 2023 , The Astrophysical Journal Letters)

   The repeating fast radio burst FRB 20190520B is an anomaly of the FRB population thanks to its high dispersion measure (DM = 1205 pc cm⁻³) despite its low redshift ofz_frb= 0.241. This excess has been attributed to a large host contribution of DM_host≈ 900 pc cm⁻³, far larger than any other known FRB. In this paper, we describe spectroscopic observations of the FRB 20190520B field obtained as part of the FLIMFLAM survey, which yielded 701 galaxy redshifts in the field. We find multiple foreground galaxy groups and clusters, for which we then estimated halo masses by comparing their richness with numerical simulations. We discover two separateM_halo> 10¹⁴M_⊙galaxy clusters atz= 0.1867 and 0.2170 that are directly intersected by the FRB sight line within their characteristic halo radiusr₂₀₀. Subtracting off their estimated DM contributions, as well that of the diffuse intergalactic medium, we estimate a host contribution of${\mathrm{D}\mathrm{M}}_{\mathrm{h}\mathrm{o}\mathrm{s}\mathrm{t}}={430}_{-220}^{+140}$or${280}_{-170}^{+140}\mathrm{p}\mathrm{c}{\mathrm{c}\mathrm{m}}^{-3}$(observed frame), depending on whether we assume that the halo gas extends tor₂₀₀or 2 ×r₂₀₀. This significantly smaller DM_host—no longer the largest known value—is now consistent with Hαemission measures of the host galaxy without invoking unusually high gas temperatures. Combined with the observed FRB scattering timescale, we estimate the turbulent fluctuation and geometric amplification factor of the scattering layer to be$\tilde{F}G\approx 4.5–11{({\mathrm{pc}}^2\mathrm{km})}^{-1/3}$, suggesting that most of the gas is close to the FRB host. This result illustrates the importance of incorporating foreground data for FRB analyses both for understanding the nature of FRBs and to realize their potential as a cosmological probe.

    

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3. Measuring the Variance of the Macquart Relation in Redshift–Extragalactic Dispersion Measure Modeling

   https://doi.org/10.3847/1538-4357/ad2705  

   Baptista, Jay ; Prochaska, J. Xavier ; Mannings, Alexandra G. ; James, C. W. ; Shannon, R. M. ; Ryder, Stuart D. ; Deller, A. T. ; Scott, Danica R. ; Glowacki, Marcin ; Tejos, Nicolas ( April 2024 , The Astrophysical Journal)

   The Macquart relation describes the correlation between the dispersion measure (DM) of fast radio bursts (FRBs) and the redshiftzof their host galaxies. The scatter of the Macquart relation is sensitive to the distribution of baryons in the intergalactic medium including those ejected from galactic halos through feedback processes. The variance of the distribution in DMs from the cosmic web (DM_cosmic) is parameterized by a fluctuation parameterF. In this work, we present a new measurement ofFusing 78 FRBs of which 21 have been localized to host galaxies. Our analysis simultaneously fits for the Hubble constantH₀and the DM distribution due to the FRB host galaxy. We find that the fluctuation parameter is degenerate with these parameters, most notablyH₀, and use a uniform prior onH₀to measure${\log }_{10}F>-0.86$at the 3σconfidence interval and a new constraint on the Hubble constant$H_0={85.3}_{-8.1}^{+9.4}\mathrm{km}{\mathrm{s}}^{-1}{\mathrm{Mpc}}^{-1}$. Using a synthetic sample of 100 localized FRBs, the constraint on the fluctuation parameter is improved by a factor of ∼2. Comparing ourFmeasurement to simulated predictions from cosmological simulation (IllustrisTNG), we find agreement between redshifts 0.4 <z andz< 2.0. However, atz< 0.4, the simulations underpredictF, which we attribute to the rapidly changing extragalactic DM excess distribution at low redshift.

    

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4. An X-Ray Census of Fast Radio Burst Host Galaxies: Constraints on Active Galactic Nuclei and X-Ray Counterparts

   https://doi.org/10.3847/1538-4357/acf843  

   Eftekhari, T. ; Fong, W. ; Gordon, A. C. ; Sridhar, N. ; Kilpatrick, C. D. ; Bhandari, S. ; Deller, A. T. ; Dong 董, Y. 雨. 欣. ; Rouco Escorial, A. ; Heintz, K. E. ; et al ( November 2023 , The Astrophysical Journal)

   We present the first X-ray census of fast radio burst (FRB) host galaxies to conduct the deepest search for active galactic nuclei (AGN) and X-ray counterparts to date. Our sample includes seven well-localized FRBs with unambiguous host associations and existing deep Chandra observations, including two events for which we present new observations. We find evidence for AGN in two FRB host galaxies based on the presence of X-ray emission coincident with their centers, including the detection of a luminous (L_X≈ 5 × 10⁴²erg s⁻¹) X-ray source at the nucleus of FRB 20190608B’s host, for which we infer an SMBH mass ofM_BH∼ 10⁸M_⊙and an Eddington ratioL_bol/L_Edd≈ 0.02, characteristic of geometrically thin disks in Seyfert galaxies. We also report nebular emission-line fluxes for 24 highly secure FRB hosts (including 10 hosts for the first time), and assess their placement on a BPT diagram, finding that FRB hosts trace the underlying galaxy population. We further find that the hosts of repeating FRBs are not confined to the star-forming locus, contrary to previous findings. Finally, we place constraints on associated X-ray counterparts to FRBs in the context of ultraluminous X-ray sources (ULXs), and find that existing X-ray limits for FRBs rule out ULXs brighter thanL_X≳ 10⁴⁰erg s⁻¹. Leveraging the CHIME/FRB catalog and existing ULX catalogs, we search for spatially coincident ULX–FRB pairs. We identify a total of 28 ULXs spatially coincident with the localization regions for 17 FRBs, but find that the DM-inferred redshifts for the FRBs are inconsistent with the ULX redshifts, disfavoring an association between these specific ULX–FRB pairs.

    

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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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