A Datadriven Technique Using Millisecond Transients to Measure the Milky Way Halo
Abstract
We introduce a new technique to constrain the lineofsight integrated electron density of our Galactic halo ${\mathrm{DM}}_{\mathrm{MW},\mathrm{halo}}$ through analysis of the observed dispersion measure distributions of pulsars ${\mathrm{DM}}_{\mathrm{pulsar}}$ and fast radio bursts (FRBs) ${\mathrm{DM}}_{\mathrm{FRB}}$ . We model these distributions, correcting for the Galactic interstellar medium, with kernel density estimation—wellsuited to the small data regime—to find lower/upper bounds to the corrected ${\mathrm{DM}}_{\mathrm{pulsar}}$ / ${\mathrm{DM}}_{\mathrm{FRB}}$ distributions: $\max \left[{\mathrm{DM}}_{\mathrm{pulsar}}\right]\approx 7\pm 2\,(\mathrm{stat})\pm 9\,(\mathrm{sys})\,\mathrm{pc}\,{\mathrm{cm}}^{3}$ and $\min \left[{\mathrm{DM}}_{\mathrm{FRB}}\right]\approx {63}_{21}^{+27}\,(\mathrm{stat})\pm 9\,(\mathrm{sys})\,\mathrm{pc}\,{\mathrm{cm}}^{3}$ . Using bootstrap resampling to estimate uncertainties, we set conservative limits on the Galactic halo dispersion measure $2\lt {\mathrm{DM}}_{\mathrm{MW},\mathrm{halo}}\lt 123\,\mathrm{pc}\,{\mathrm{cm}}^{3}$ (95% c.l.). The upper limit is especially conservative because it may include a nonnegligible contribution from the FRB host galaxies and a nonzero contribution from the cosmic web. It strongly disfavors models where the Galaxy has retained the majority of its baryons with a density profile tracking the presumed dark matter density profile. Last, we perform Monte Carlo simulations of larger FRB samples to validate our technique and assess the sensitivity of ongoing and future surveys. We recover bounds of several tens of $\mathrm{pc}\,{\mathrm{cm}}^{3}$ that may be sufficient to test whether the Galaxy has retained a majority of its baryonic mass. We estimate that a sample of several thousand FRBs will significantly tighten constraints on ${\mathrm{DM}}_{\mathrm{MW},\mathrm{halo}}$ and offer a valuable complement to other analyses.
 Publication:

The Astrophysical Journal
 Pub Date:
 June 2020
 DOI:
 10.3847/20418213/ab930a
 arXiv:
 arXiv:2005.06256
 Bibcode:
 2020ApJ...895L..49P
 Keywords:

 Radio transient sources;
 Circumgalactic medium;
 Astrostatistics techniques;
 2008;
 1879;
 1886;
 Astrophysics  Astrophysics of Galaxies;
 Astrophysics  High Energy Astrophysical Phenomena
 EPrint:
 15 pages, 6 figures