Investigating the Anisotropy of Dispersion Measure Contribution from the Galactic Halo by Using Fast Radio Bursts

TL;DR

By expanding $DM_{halo}$ on the sphere with real spherical harmonics, the authors reconstruct the all-sky distribution of the Galactic halo dispersion measure from FRBs and test for anisotropy. They model $DM_{halo}$ as a truncated harmonic series and fit coefficients using a Bayesian FRB likelihood that incorporates $P_{ISM}$, $P_{host}$, and $P_{cos}$ across a full and a refined FRB sample, with ISM models NE2001 and YMW16. The analysis finds a significant dipole in $DM_{halo}$ toward $(l,b) \approx (130^{\circ},+5^{\circ})$ for the full sample (and a near-consistent dipole toward $(l,b) \approx (141^{\circ},+51^{\circ})$ for the refined sample), corresponding to $DM_{halo}$ peaks of ~63 pc cm$^{-3}$ against a mean ~36 pc cm$^{-3}$. Model comparisons via AIC and Bayesian evidence generally prefer the dipole model (ell_max=1), though the Bayes factor is modest, suggesting the result is not yet definitive. If real, the dipole may reflect Local Group intragroup medium or CGM-wind effects, underscoring the need for larger, more uniform FRB samples to confirm and interpret the anisotropy.

Abstract

We propose a data-driven approach to reconstruct the all-sky distribution of the dispersion measure contribution from the Galactic halo ($\mathrm{DM_{halo}}$) through a spherical harmonic expansion, enabling an investigation of its possible anisotropies. Based on the NE2001 model and using 92 localized and 574 unlocalized non-repeating fast radio bursts (FRBs) at Galactic latitudes $|b|>15^\circ$, we find a significant dipole anisotropy in $\mathrm{DM_{halo}}$, pointing toward $(l=130^\circ,\, b=+5^\circ)$ with a $1σ$ uncertainty of approximately $28^\circ$. The $\mathrm{DM_{halo}}$ value in this direction is $63\pm9~\mathrm{pc~cm^{-3}}$, exceeding the all-sky mean by about $2.6σ$. This result is not significantly affected by the choice of Galactic ISM models. Furthermore, even when using a refined sample of 62 localized FRBs (excluding CHIME detections, repeaters, and unlocalized events), the dipole anisotropic structure persists, with a direction of $(l=141^\circ,\, b=+51^\circ)$ and a larger 1$σ$ uncertainty of $\sim 44^\circ$. Model comparisons using the Akaike Information Criterion and Bayesian evidence yield consistent preferences, and together they suggest that current FRB data slightly favor the existence of a dipole structure in $\mathrm{DM_{halo}}$. If this feature is not a statistical fluctuation or systematic error, its physical origin requires further investigation. Future FRB samples with larger sizes and more complete sky coverage will be essential to confirm or refute this possible anisotropic structure.

Figures (6)

• Figure 1: Longitude-averaged $\mathrm{DM_{halo}}$ at different Galactic latitudes. Blue and orange points with $1\sigma$ error bars show $\langle\mathrm{DM_{halo}}\rangle_\mathrm{lon}$ for the $\ell_{\max}=1$ and $\ell_{\max}=2$ models, respectively. Gray squares denote localized FRBs, and green stars denote unlocalized FRBs, where $\overline{\mathrm{DM}}_\mathrm{ISM}$ is calculated from the NE2001 model.
• Figure 2: Predicted $\mathrm{DM_{halo}}$ as a function of Galactic longitude for different latitude slices. The left column shows the results for the $\ell_{\max}=1$ model, and the right column shows those for the $\ell_{\max}=2$ model. The gray shadows denote the all-sky mean values $\langle \mathrm{DM_{halo}} \rangle$ with 1$\sigma$ CL for both models.
• Figure 3: All-sky Mollweide projections of the $\mathrm{DM_{halo}}$ models expanded using spherical harmonics with $\ell_{\max}=1$ (left) and $\ell_{\max}=2$ (right). Gray squares denote localized FRBs, while stars represent unlocalized FRBs with $\mathrm{DM_{obs}}-\overline{\mathrm{DM}}_\mathrm{ISM}<250~\mathrm{pc~cm^{-3}}$, where $\overline{\mathrm{DM}}_\mathrm{ISM}$ is calculated from the NE2001 model. The region within $|b|<15^\circ$ is masked out, and the directions of the LMC and M31 are also shown.
• Figure 4: All-sky Mollweide projections of the $\mathrm{DM_{halo}}$ models expanded using spherical harmonics with $\ell_{\max}=1$. Only 62 localized FRBs are used (Gray squares).
• Figure 5: 1D posterior PDFs and 2D confidence regions (with 1–2$\sigma$ contours) for $a_{\ell,m}$, shown in red for $\ell_{\max}=0$, blue for $\ell_{\max}=1$, and gray for $\ell_{\max}=2$. All results are derived from the full sample based on the NE2001 model.