Constraining the Milky Way's Dispersion Measure Using FRB and X-ray Data

Abstract

The dispersion measures (DMs) of fast radio bursts (FRBs) are a valuable tool for probing the baryonic content of the intergalactic and the circumgalactic medium of the intervening galaxies along the sightlines. However, interpreting the DMs is complicated by the contributions from the hot gas in and around our Milky Way. This study examines the relationship between DM_MW, derived from localized FRBs, and the Galaxy's hot gas, using X-ray absorption and emission data from O VII and O VIII. We find evidence for a positive correlation between DM_MW and O VII absorption, reflecting contributions from both the disk and halo components. This conclusion is supported by two lines of evidence: (1) No correlation between DM_MW and O VII/O VIII emission, which primarily traces dense disk regions; and (2) the comparison with electron density models, where DM_MW aligns with models that incorporate both disk and halo components but significantly exceeds predictions from pure disk-only models, emphasizing the halo's role. Furthermore, the lack of correlation with O VIII absorption suggests that the primary temperature of the Galaxy's hot gas is likely around 2 x 10^6 K or less, as traced by O VII absorption, while gas at higher temperatures (~3 x 10^6 K to 5 x 10^6 K) is present but less abundant. Our findings provide insights into the Milky Way's gas distribution and improve DM_MW estimates for future cosmological studies.