I can see your halo: Constraining the Milky Way halo DM with FRB population studies

TL;DR

This work tackles the problem of constraining the Milky Way halo dispersion measure DM_MW,halo by leveraging a population analysis of FRBs. Using the zDM framework, the authors jointly fit FRB population parameters and DM_MW,halo for a sample of 98 high-latitude FRBs with 32 redshifts, adopting an isotropic halo model as the default. The primary result is a mean halo contribution of $DM_{ m MW,halo} = 68^{+27}_{-24}$ pc cm$^{-3}$, broadly consistent with X-ray based estimates and larger FRB studies that sample more sightlines. They find that including nearby, low-DM FRBs can strongly affect the inferred value, underscoring the value of wide sky coverage and well-localized events for robust halo constraints, while also noting no strong preference among halo models given current data. The study highlights the importance of expanding localized FRB samples to measure halo fluctuations and potential directional dependencies in the Galactic halo DM budget.

Abstract

Fast radio bursts (FRBs) probe the electron column density along the line of sight and hence can be used to probe foreground structures. One such structure is the Galactic halo. In this work, we use a total of 98 high Galactic latitude ($|b| > 20^\circ$) FRBs detected by ASKAP, Parkes, DSA and FAST with 32 associated redshifts to constrain the dispersion measure (DM) contribution from the Galactic halo. We simultaneously fit unknown FRB population parameters, which show correlations with the Galactic halo but are not completely degenerate. We primarily use an isotropic model for the halo, but find no evidence favouring a particular halo model. We find DM$_{\rm MW,halo}$=$68^{+27}_{-24}$pc/cm$^3$, which is in agreement with other results within the literature. Previous constraints on DM$_{\rm MW,halo}$ with FRBs have used a few, low-DM FRBs. However, this is highly subject to fluctuations between different lines of sight, and hence using a larger number of sightlines as we do is more likely to be representative of the true average contribution. Nevertheless, we show that individual FRBs can still skew the data significantly and hence will be important in the future for more precise results.

Figures (4)

• Figure 1: Results from the MCMC analysis including FAST, DSA and CRAFT FRBs. The parameters are identical to those described in Table \ref{['table:params']}.
• Figure 2: Shown is the correlation between DM$_{\rm MW,halo}$ and $\mu_{\rm host}$ from our MCMC analysis. Overplotted in orange is the expected degeneracy, calculated according to Equation \ref{['eq:DM']}.
• Figure 3: A slice through DM$_{\rm MW,halo}$ when including or excluding FRB 20220319D. All other parameters are kept constant at their best fit values from Figure \ref{['fig:MCMCbase']}. When excluding FRB 20220319D we obtain DM$_{\rm MW,halo}$=55 pc cm$^{-3}$ and when including it we obtain DM$_{\rm MW,halo}$=45 pc cm$^{-3}$.
• Figure 4: Residual DMs ($\Delta \mathrm{DM}$) of the localised FRBs used in our analysis given different halo models. This represents the scatter around the Macquart relation. The three halo models considered were an isotropic halo, an empirical halo from X-ray observations Das2021 and an isotropic halo with an additional disk-like component Yamasaki2020. The point from Das2021 at the bottom of the plot marked with a green cross is considered an outlier as the estimated DM$_{\rm MW}$ is $1750^{+4550}_{-1370}$ pc cm$^{-3}$ and this estimation comes from a point 16.5 degrees away from the FRB position on the sky and hence is considered unreliable.