Locating the intense interstellar scattering towards the inner Galaxy

TL;DR

This study uses high-resolution radio observations to locate multiple scattering regions affecting pulsar signals in the inner Galaxy, revealing multiple scattering screens and the significant role of HII regions in interstellar scattering.

Contribution

It identifies multiple scattering screens along the line of sight and links scattering to HII regions, refining understanding of interstellar scattering sources in the inner Galaxy.

Findings

Multiple scattering screens are required to explain observations of GC pulsars.

The dominant scattering screen near the GC is closer than previously thought.

HII regions contribute over 25% to pulsar dispersion measures.

Abstract

We use VLBA+VLA observations to measure the sizes of the scatter-broadened images of 6 of the most heavily scattered known pulsars: 3 within the Galactic Centre (GC) and 3 elsewhere in the inner Galactic plane. By combining the measured sizes with temporal pulse broadening data from the literature and using the thin-screen approximation, we locate the scattering medium along the line of sight to these 6 pulsars. At least two scattering screens are needed to explain the observations of the GC sample. We show that the screen inferred by previous observations of SGR J1745-2900 and Sgr A*, which must be located far from the GC, falls off in strength on scales < 0.2 degree. A second scattering component closer to (< 2 kpc) or even (tentatively) within (< 700 pc) the GC produces most or all of the temporal broadening observed in the other GC pulsars. Outside the GC, the scattering locations for all three pulsars are ~2 kpc from Earth, consistent with the distance of the Carina-Sagittarius or Scutum spiral arm. For each object the 3D scattering origin coincides with a known HII region (and in one case also a supernova remnant), suggesting that such objects preferentially cause the intense interstellar scattering seen towards the Galactic plane. We show that the HII regions should contribute > 25% of the total dispersion measure (DM) towards these pulsars, and calculate reduced DM distances. Those distances for other pulsars lying behind HII regions may be similarly overestimated.