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

Abstract We propose a data-driven approach to reconstruct the all-sky distribution of the dispersion measure contribution from the Galactic halo (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 nonrepeating fast radio bursts (FRBs) at Galactic latitudes ∣ b ∣ > 15°, we find a significant dipole anisotropy in DM halo , pointing toward ( l = 130°, b = +5°) with a 1 σ uncertainty of approximately 28°. The DM halo value in this direction is 63 ± 9 pc cm −3 , exceeding the all-sky mean by about 2.6 σ . This result is not significantly affected by the choice of Galactic interstellar medium 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°, b = +51°) and a larger 1 σ uncertainty of ∼44°. 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 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.