A comparison of Galactic electron density models using PyGEDM

Abstract

Galactic electron density distribution models are crucial tools for estimating the impact of the ionised interstellar medium on the impulsive signals from radio pulsars and fast radio bursts. The two prevailing Galactic electron density models (GEDMs) are YMW16 (Yao et al. 2017, ApJ, 835, 29) and NE2001 (Cordes & Lazio 2002, arXiv e-prints, pp astro–ph/0207156). Here, we introduce a software package PyGEDM which provides a unified application programming interface for these models and the YT20 (Yamasaki & Totani 2020, ApJ, 888, 105) model of the Galactic halo. We use PyGEDM to compute all-sky maps of Galactic dispersion measure (DM) for YMW16 and NE2001 and compare the large-scale differences between the two. In general, YMW16 predicts higher DM values towards the Galactic anticentre. YMW16 predicts higher DMs at low Galactic latitudes, but NE2001 predicts higher DMs in most other directions. We identify lines of sight for which the models are most discrepant, using pulsars with independent distance measurements. YMW16 performs better on average than NE2001, but both models show significant outliers. We suggest that future campaigns to determine pulsar distances should focus on targets where the models show large discrepancies, so future models can use those measurements to better estimate distances along those line of sight. We also suggest that the Galactic halo should be considered as a component in future GEDMs, to avoid overestimating the Galactic DM contribution for extragalactic sources such as FRBs.

An FRB Sent Me a DM: Constraining the Electron Column of the Milky Way Halo with Fast Radio Burst Dispersion Measures from CHIME/FRB

“…19 Both models have been shown to fail in predicting the DMs of certain populations, like high-latitude pulsars, pulsars in H II regions, and several relatively local pulsars (Chatterjee et al 2009). Price et al (2021) gave a comprehensive review and comparison of these two models.…”

Short Timescale Evolution of the Polarized Radio Jet during V404 Cygni's 2015 Outburst

“…Assuming a constant Galactic magnetic field, the Galactic RM would be dominated by distances well beyond the ∼2.4 kpc distance to V404 Cyg. For both standard models of electron distributions (Cordes & Lazio 2003;Yao et al 2017) the P GEDM tool (Price et al 2021) indicates much larger dispersion measures (which are proxies for the integrated electron column density) at 10 kpc (269±16 pc cm −3 ) than at 2.4 kpc (32 ± 4 pc cm −3 ). Thus, our measured value of ≈ − 100 rad m −2 suggests, either an inversion (or multiple inversions) of the Galactic magnetic field along our line of sight, or an intrinsic rotation measure component local to the source.…”

Short time-scale evolution of the polarized radio jet during V404 Cygni’s 2015 outburst

“…4 kpc distance to V404 Cyg. For both standard models of electron distributions (Cordes & Lazio 2003 ;Yao, Manchester & Wang 2017 ) the PYGEDM tool (Price, Flynn & Deller 2021 ) indicates much larger dispersion measures (which are proxies for the integrated electron column density) at 10 kpc (269 ± 16 pc cm −3 ) than at 2.4 kpc (32 ± 4 pc cm −3 ). Thus, our measured value of ≈ − 100 rad m −2 suggests, either an inversion (or multiple inversions) of the Galactic magnetic field along our line of sight, or an intrinsic rotation measure component local to the source.…”