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1. A Measurement of Circumgalactic Gas around Nearby Galaxies Using Fast Radio Bursts

   https://doi.org/10.3847/1538-4357/acbc7d  

   Wu, Xiaohan ; McQuinn, Matthew ( March 2023 , The Astrophysical Journal)

   The distribution of gas in the circumgalactic medium (CGM) of galaxies of all types is poorly constrained. Foreground CGMs contribute an extra amount to the dispersion measure (DM) of fast radio bursts (FRBs). We measure this DM excess for the CGMs of 10¹¹–10¹³M_⊙halos using the CHIME/FRB first data release, a halo mass range that is challenging to probe in any other way. Because of the uncertainty in the FRBs’ angular coordinates, only for nearby galaxies is the localization sufficient to confidently associate them with intersecting any foreground halo. Thus we stack on galaxies within 80 Mpc, optimizing the stacking scheme to approximately minimize the stack’s variance and marginalize over uncertainties in FRB locations. The sample has 20–30 FRBs intersecting halos with masses of 10¹¹–10¹²M_⊙and also of 10¹²–10¹³M_⊙, and these intersections allow a marginal 1σ–2σdetection of the DM excess in both mass bins. The bin of 10¹¹–10¹²M_⊙halos also shows a DM excess at 1–2 virial radii. By comparing data with different models for the CGM gas profile, we find that all models are favored by the data up to 2σlevel compared to the null hypothesis of no DM excess. With 3000 more bursts from a future CHIME data release, wemore »project a 4σdetection of the CGM. Distinguishing between viable CGM models by stacking FRBs with CHIME-like localization would require tens of thousands of bursts.

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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 unusuallymore »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. Fast Radio Bursts as Probes of Magnetic Fields in Galaxies at z < 0.5

   https://doi.org/10.3847/1538-4357/ace7bb  

   Mannings, Alexandra G. ; Pakmor, Rüdiger ; Prochaska, J. Xavier ; van de Voort, Freeke ; Simha, Sunil ; Shannon, R. M. ; Tejos, Nicolas ; Deller, Adam ; Rafelski, Marc ( September 2023 , The Astrophysical Journal)

   We present a sample of nine fast radio bursts (FRBs) from which we derive magnetic field strengths of the host galaxies represented by normal,z< 0.5 star-forming galaxies with stellar massesM_*≈ 10⁸–10^10.5M_⊙. We find no correlation between the FRB rotation measure (RM) and redshift, which indicates that the RM values are due mostly to the FRB host contribution. This assertion is further supported by a significant positive correlation (Spearman test probabilityP_S< 0.05) found between the RM and the estimated host dispersion measure (DM_host; with Spearman rank correlation coefficientr_S= +0.75). For these nine galaxies, we estimate their magnetic field strengths projected along the sight line ∣B_∥∣, finding a low median value of 0.5μG. This implies the magnetic fields of our sample of hosts are weaker than those characteristic of the solar neighborhood (≈6μG), but relatively consistent with a lower limit on the observed range of ≈2–10μG for star-forming disk galaxies, especially as we consider reversals in theB-field, and that we are only probing B_∥. We compare to RMs from simulated galaxies of the Auriga project—magneto-hydrodynamic cosmological zoom simulations—and find that the simulations predict the observed values to within a 95% confidence interval. Upcoming FRB surveys will provide hundreds of new FRBsmore »with high-precision localizations, RMs, and imaging follow-up to support further investigation into the magnetic fields of a diverse population ofz< 1 galaxies.

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4. An Updated Reference Frame for the Galactic Inner Parsec

   https://doi.org/10.3847/1538-4357/acec71  

   Darling, Jeremy ; Paine, Jennie ; Reid, Mark J. ; Menten, Karl M. ; Sakai, Shoko ; Ghez, Andrea ( September 2023 , The Astrophysical Journal)

   Infrared observations of stellar orbits about Sgr A* probe the mass distribution in the inner parsec of the Galaxy and provide definitive evidence for the existence of a massive black hole. However, the infrared astrometry is relative and is tied to the radio emission from Sgr A* using stellar SiO masers that coincide with infrared-bright stars. To support and improve this two-step astrometry, we present new astrometric observations of 15 stellar SiO masers within 2 pc of Sgr A*. Combined with legacy observations spanning 25.8 yr, we reanalyze the relative offsets of these masers from Sgr A* and measure positions and proper motions that are significantly improved compared to the previously published reference frame. Maser positions are corrected for epoch-specific differential aberration, precession, nutation, and solar gravitational deflection. Omitting the supergiant IRS 7, the mean position uncertainties are 0.46 mas and 0.84 mas in R.A. and decl., and the mean proper motion uncertainties are 0.07 mas yr⁻¹and 0.12 mas yr⁻¹, respectively. At a distance of 8.2 kpc, these correspond to position uncertainties of 3.7 and 6.9 au and proper motion uncertainties of 2.7 and 4.6 km s⁻¹. The reference frame stability, the uncertainty in the variance-weighted mean proper motionmore »of the maser ensemble, is 8μas yr⁻¹(0.30 km s⁻¹) in R.A. and 11μas yr⁻¹(0.44 km s⁻¹) in decl., which represents a 2.3-fold improvement over previous work and a new benchmark for the maser-based reference frame.

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5. The host galaxy and persistent radio counterpart of FRB 20201124A

   https://doi.org/10.1093/mnras/stac465  

   Ravi, Vikram ; Law, Casey J. ; Li, Dongzi ; Aggarwal, Kshitij ; Bhardwaj, Mohit ; Burke-Spolaor, Sarah ; Connor, Liam ; Lazio, T. Joseph W. ; Simard, Dana ; Somalwar, Jean ; et al ( April 2022 , Monthly Notices of the Royal Astronomical Society)

   The physical properties of fast radio burst (FRB) host galaxies provide important clues towards the nature of FRB sources. The 16 FRB hosts identified thus far span three orders of magnitude in mass and specific star formation rate, implicating a ubiquitously occurring progenitor object. FRBs localized with ∼arcsecond accuracy also enable effective searches for associated multiwavelength and multi-time-scale counterparts, such as the persistent radio source associated with FRB 20121102A. Here we present a localization of the repeating source FRB 20201124A, and its association with a host galaxy (SDSS J050803.48+260338.0, z = 0.098) and persistent radio source. The galaxy is massive (${\sim}3\times 10^{10}\, \text{M}_{\odot }$), star-forming (few solar masses per year), and dusty. Very Large Array and Very Long Baseline Array observations of the persistent radio source measure a luminosity of 1.2 × 1029 erg s−1 Hz−1, and show that is extended on scales ≳50 mas. We associate this radio emission with the ongoing star formation activity in SDSS J050803.48+260338.0. Deeper, high-resolution optical observations are required to better utilize the milliarcsecond-scale localization of FRB 20201124A and determine the origin of the large dispersion measure (150–220 pc cm−3) contributed by the host. SDSS J050803.48+260338.0 is an order of magnitude more massive than any galaxy or stellar system previously associated with a repeating FRB source, butmore »is comparable to the hosts of so far non-repeating FRBs, further building the link between the two apparent populations.

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