NE2025: An Updated Electron Density Model for the Galactic Interstellar Medium

TL;DR

The paper presents NE2025, an updated Galactic electron density model that revises NE2001’s large-scale structure to improve distances and scattering predictions for pulsars, FRBs, and related sources. It leverages 171 precise distances (parallax and globular clusters) and 568 scattering measurements to refit the thick disk, spiral arms, thin disk, and Galactic Center components, and adds clumps to account for discrete overdensities. NE2025 achieves about a 20× improvement in median distance accuracy and a 100% improvement in scattering predictions (DM-based) relative to NE2001, with a 15× improvement in distance accuracy relative to YMW16; the Galactic Center’s scattering is reduced by three orders of magnitude compared to NE2001. The model preserves NE2001’s framework while delivering substantial performance gains and is publicly available on Github and via the Python NE2001p interface, enabling broader ISM studies and improved predictions for pulsars, FRBs, and related Galactic foregrounds.

Abstract

Free electrons in the Galactic interstellar medium (ISM) disperse and scatter coherent radio waves, by amounts that depend on the distance to the radio source. Models of the Galactic electron density are thus widely used to predict distances and scattering of compact radio sources (including pulsars, fast radio bursts (FRBs), and long-period transients), in addition to mitigating ISM foregrounds in Galactic and extragalactic studies. We use a sample of 171 precise pulsar distances, based entirely on parallaxes and globular cluster associations, as well as scattering measurements of 568 pulsars, active galactic nuclei, and masers, to update the NE2001 Galactic electron density model. We refit the thick and thin disks and three of the spiral arms. The new parameters for these large-scale components significantly repartition free electrons between the thick disk and spiral arms, thereby correcting NE2001's systematic underestimation of pulsar distance and scattering. Sightlines with excessive dispersion and scattering are used to identify new clumps that are added to the model, in addition to refining clumps that were already included (e.g., Cygnus, Vela, and Gum). The Galactic Center component is revised, yielding scattering time predictions that are $10^3$ times smaller than the Galactic Center in NE2001. The updated model, NE2025, provides a factor of $20\times$ improvement in median distance prediction accuracy and $100\%$ median improvement in scattering predictions based on DM, relative to NE2001. There is a $15\times$ improvement in median distance prediction accuracy relative to YMW16. NE2025 is available on Github and the Python Package Interface.

Figures (9)

• Figure 1: Electron density in the Galactic plane ($Z = 0$) for NE2001 (left) and NE2025 (right). The color stretch is the same for both panels.
• Figure 2: Comparison between the sky distributions of distance measurements used to calibrate NE2001 and NE2025. Left: Pulsar parallaxes (red), globular cluster associations (blue), and other distance measures (yellow; e.g., H i kinematic distances and supernova remnant (SNR) associations) used to calibrate NE2001 are shown on top of an all-sky H$\alpha$ intensity map (grey) from 2003ApJS..146..407F. Right: All parallaxes and globular cluster associations used to update NE2001.
• Figure 3: Histogram of distance measurements used in this work. Globular clusters (light blue) provide the largest distances in the sample, whereas parallaxes (dark blue) predominantly yield distances $<3$ kpc.
• Figure 4: Comparison between predicted and observed distances (top row) and DMs (bottom row) for NE2001 (left; purple points) and NE2025 (right; orange points). Black diagonal lines indicate where the predictions match the measurements.
• Figure 5: Left: Dispersion measure vs. Galactic longitude for all Galactic pulsars at latitudes $|b|<5^\circ$. The maximum DM predicted by NE2001 is shown by the black curve, and is evaluated at a resolution of $1^\circ$ in longitude for $b=0^\circ$. Specific regions of interest for the updated model, including H ii regions and a hole at $l\approx-20^\circ$, are highlighted in orange. Right: A Gaussian kernel-density estimator (KDE) of the probability distribution function for the DMs shown in the righthand panel. The KDE is evaluated using logarithmic bins in DM.