Magnetic fields in galactic environments probed by Fast Radio Bursts

TL;DR

This work uses a Bayesian framework to decompose the observed RM of 14 localized FRBs into contributions from the Milky Way, the IGM, intervening foreground halos, and the FRB host (both its halo and local ISM/progenitor). By leveraging a spectroscopic foreground-halo dataset and modeling each RM component with a modified NFW (mNFW) halo profile, the authors constrain the magnetic-field strengths: $B_{ m host}^{ m local} \\approx 5.4\mu G$, $B_{ m host}^{\rm halo} \lesssim 4.8\mu G$, and $B_{ m fg}^{\rm halo} \lesssim 4.3\mu G$, while jointly inferring a halo baryon fraction $f_{ m gas} = 0.45^{+0.21}_{-0.19}$ and CGM fraction $f_{ m cgm}^{\rm sample} = 0.14^{+0.07}_{-0.06}$. The results align with prior extragalactic RM measurements and demonstrate that foreground/host halos contribute non-negligibly to RM and must be accounted for in analyses of future FRB RM samples. The study also highlights a degeneracy between the IGM baryon fraction $f_{ m igm}$ and $f_{ m gas}$, underscoring the need for improved constraints on the diffuse baryon content to sharpen measurements of galactic magnetic fields via FRBs.

Abstract

FRBs constitute a unique probe of various astrophysical and cosmological environments via their characteristic dispersion and rotation (RM) measures that encode information about the ionized gas traversed by the FRB sightlines. In this work, we analyse observed RM measured for 14 localized FRBs at $0.05 \lesssim z \lesssim 0.5$, to infer total magnetic fields in various galactic environments. Additionally, we calculate $f_{\rm gas}$ - the average fraction of halo baryons in the ionized CGM. We build a spectroscopic dataset of FRB foreground galaxy halos, acquired with VLT/MUSE and FLIMFLAM survey. We develop a novel Bayesian algorithm and use it to correlate the individual intervening halos with the observed RM. This approach allows us to disentangle the magnetic fields present in various environments traversed by the FRB. Our analysis yields the first direct FRB constraints on the strength of magnetic fields in the ISM and halos of the FRB host galaxies, as well as in halos of foreground galaxies. We find that the average magnetic field in the ISM of FRB hosts is $B_{\rm host}^{\rm local} = 5.44^{+1.13}_{-0.87}μ{\rm G}$. Additionally, we place upper limits on average magnetic field in FRB host halos, $B_{\rm host}^{\rm halo} < 4.81μ{\rm G}$, and in foreground intervening halos, $B_{\rm f/g}^{\rm halo} < 4.31μ{\rm G}$. Moreover, we estimate the average fraction of cosmic baryons inside $10 \lesssim \log_{10} \left( M_{\rm halo} / M_{\odot}\right) \lesssim 13.1$ halos $f_{\rm gas} = 0.45^{+0.21}_{-0.19}$. We find that the magnetic fields inferred in this work are in good agreement with previous measurements. In contrast to previous studies that analysed FRB RMs and have not considered contributions from the halos of the foreground and/or FRB host galaxies, we show that they can contribute a non-negligible amount of RM and must be taken into account when analysing future FRB samples.

Figures (8)

• Figure 1: ( Left): Distribution of absolute values of $\mathrm{RM_{eg}}$ as a function of FRB redshift in observer frame. The black dots show the FRB sample analysed in this work, whereas the grey triangles are the DSA-110 subsample taken from sherman2023 and the blue diamonds illustrate the FRBs from the mannings2023 sample, which are not part of this work. ( Right): Distribution of $\mathrm{RM_{eg}}$ values in rest-frame of FRB hosts as a function of the corresponding $\mathrm{DM_{host}^{direct}}$ and $\mathrm{DM_{host}^{ism}}$ values from lucas2025 (only for sources for which corresponding MUSE data are available).
• Figure 2: Impact parameter $b_{\rm impact}$ of identified foreground halos with respect to their virial radius, $r_{200}$, along each FRB sight line, plotted as a function of the impact parameter. The colour map illustrates the mean mass of the halos, whereas the associated uncertainty is propagated into the error bars on their corresponding $r_{200}$. The dashed line illustrates the maximum distance, $b_{\rm impact} = r_{200}$, at which a foreground halo can be intersected by the FRB sight line and potentially contribute to $\mathrm{RM_{obs}}$ if a halo extends to 1 virial radius, $r_{200}$. In addition, we also show the case where the halos extend to two virial radii, $2 \times r_{200}$, with the dash-dotted lines.
• Figure 3: Example of typical distributions of RM values for FRB $20211212{\rm A}$: ( left) from the mNFW model of a single foreground galactic halo (see Section \ref{['sec:rm_halos']}); ( middle) from the mNFW model of the corresponding host-galaxy halo (see Section \ref{['sec:rm_host']}); and ( right) from the local environment in the ISM of the host galaxy and/or FRB progenitor (see Section \ref{['sec:rm_local']}). The positive values of the histograms correspond to the RM values for the case when the line-of-sight component of the magnetic field is aligned with the direction of the FRB sight line (parallel), whereas the negative parts of the histograms, illustrate the case where the line-of-sight component of the magnetic field is directed away from the observer (anti-parallel). The dashed maroon lines show the KDE fit to the resulting RM distributions (see Section \ref{['sec:infernece']}). Note the different range of RM values in the panels.
• Figure 4: Resulting distribution of $\mathrm{RM_{model}}$ values given by Eq. (\ref{['eq:rmmodel']}), estimated by combining contributions from each environment along the FRB sight line (see Section \ref{['sec:rm_model']} and Figure \ref{['fig:rm_parts']} for details). The vertical dashed magenta line illustrates the value of the $\mathrm{RM_{obs}}$ for FRB $20211212$A, while the horizontal magenta dotted line shows the value of the corresponding log-likelihood value ($\log\mathcal{L_{\rm FRB}} = -6.41$) evaluated from the KDE fit to the $\mathrm{RM_{model}}$ distribution at the value of $\mathrm{RM_{obs}}$, given the combination of model parameters $\Theta$ (see discussion in Section \ref{['sec:infernece']} for more details).
• Figure 5: MCMC inference result for sample of 14 FRBs in our dataset (see Table \ref{['tab:mtab']}). The orange (blue) contours correspond to the inferred $68\%$ ($95\%$) confidence intervals. The diagonal panels show the corresponding marginalised $1$D posterior probabilities of the model parameters.