2026A&A...706A..11K

Fast radio bursts (FRBs) are extragalactic, bright, millisecond radio pulses emitted by unknown sources. FRBs constitute a unique probe of various astrophysical and cosmological environments via their characteristic dispersion (DM) and Faraday rotation (RM) measures that encode information about the ionised gas traversed by the radio waves along the FRB line of sight. In this work, we analysed the observed RM measured for 14 localised FRBs in the 0.05 ≲ z_frb ≲ 0.5 redshift range, in order to infer the total magnetic field, B, in various galactic environments. Additionally, we calculated f_gas - the average fraction of baryons in the ionised CGM. We built a spectroscopic dataset of FRB foreground galaxy halos, acquired with VLT/MUSE observations and by the FLIMFLAM collaboration. We developed a novel Bayesian statistical algorithm and used it to correlate information on the individual intervening halos with the observed RM_obs. This approach allowed us to disentangle the magnetic fields present in various environments traversed by the FRB sight lines. Our analysis yields the first direct FRB constraints on the strength of magnetic fields in the interstellar medium (ISM) ($ B_{\mathrm{host}}^{\mathrm{local}} $) and in the halos ($ B_{\mathrm{host}}^{\mathrm{halo}} $) of FRB host galaxies, as well as in the halos of foreground galaxies and groups ($ B_{\mathrm{fg}}^{\mathrm{halo}} $). Assuming no field reversals, we find that the average magnetic field strength in the ISM of the FRB host galaxies is [****UnclosedBrace??****]$ B_{\mathrm{host}}^{\mathrm{local}} = 5.4^{+1.1}_{-0.9}μG. Additionally, we placed an upper limit on the average magnetic field strength in FRB host halos, \verbatim{$ B_{\mathrm{host}}^{\mathrm{halo}} \lesssim 4.8μG, and in foreground intervening halos, \verbatim{$ B_{\mathrm{fg}}^{\mathrm{halo}} \lesssim 4.3μG. Moreover, we estimated the average fraction of cosmic baryons inside 10 ≲ log_10_({\em M}_halo_/{\em M}_⊙_) ≲ 13.1 halos to be \verbatim{$ f_{\mathrm{gas}} = 0.45^{+0.21}_{-0.19} $}. We find that the magnetic field strengths 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 halos can contribute a non-negligible amount of RM and must be taken into account when analysing future FRB samples.\[****ClosingBrace(s)Inserted****]}}}$ B_{\mathrm{host}}^{\mathrm{local}} = 5.4^{+1.1}_{-0.9}μG. Additionally, we placed an upper limit on the average magnetic field strength in FRB host halos, \verbatim{$ B_{\mathrm{host}}^{\mathrm{halo}} \lesssim 4.8μG, and in foreground intervening halos, \verbatim{$ B_{\mathrm{fg}}^{\mathrm{halo}} \lesssim 4.3μG. Moreover, we estimated the average fraction of cosmic baryons inside 10 ≲ log_10_({\em M}_halo_/{\em M}_⊙_) ≲ 13.1 halos to be \verbatim{$ f_{\mathrm{gas}} = 0.45^{+0.21}_{-0.19} $}. We find that the magnetic field strengths 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 halos can contribute a non-negligible amount of RM and must be taken into account when analysing future FRB samples.\