Locating the intense interstellar scattering towards the inner Galaxy

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.

16 pages, 10 figures, MNRAS, in press

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• Two years of pulsar observations with the Ultra-Wideband Receiver on the Parkes radio telescope
• Modelling annual scintillation arc variations in PSR J1643-1224 using the Large European Array for Pulsars
• Radio Detection of PSR J1813-1749 in HESS J1813-178: The Most Scattered Pulsar Known
• Multi-wavelength Variability of Sagittarius A* in July 2019
• Scintillation can explain the spectral structure of the bright radio burst from SGR 1935+2154
• Multi-epoch searches for relativistic binary pulsars and fast transients in the Galactic Centre
• Observations of Radio Magnetars with the Deep Space Network