A New Limit on Intergalactic Magnetic Fields on Subkiloparsec Scales from Fast Radio Bursts

We use the measured scattering timescales of fast radio bursts (FRBs) from the CHIME catalog to derive an upper limit on the magnetic field on subkiloparsec scales in the intergalactic medium (IGM). A nonmagnetized, photoionized IGM is insufficient to explain the turbulent scattering at all redshifts, with a warm-hot component being marginally consistent with the data at z ∼ 1. Accounting for the lower envelope of the temporal smearing distribution with a nonzero magnetic field leads to upper limits B < 10–30 nG on scales of 0.07–0.20 kpc in the IGM at z ∼ 1–2. Our work introduces a novel technique to constrain small-scale magnetic fields in the IGM, in a regime unexplored by the rotation and dispersion measures of FRBs.