A single HII region model of the strong interstellar scattering towards Sgr A*

Abstract

Until recently, the strong interstellar scattering observed towards the Galactic center (GC) black hole, Sgr A*, was thought to come from dense gas within the GC region. The pulse broadening towards the transient magnetar SGR J1745-2900 near Sgr A* has shown that the source of the scattering is instead located much closer to Earth, possibly in a nearby spiral arm. We show that a single HII region along the line of sight, 1.5-4.8 kpc away from Earth with density $n_e$ of a few 100 cm${}^{-3}$ and radius R = 1.8-3.2 pc can explain the observed angular broadening of Sgr A*. Clouds closer to the GC overproduce the observed DM, providing an independent location constraint that agrees with that from the magnetar pulse broadening. Our model predicts that sources within 10 pc should show the same scattering origin as the magnetar and Sgr A*, while the nearest known pulsars with separations > 20 pc should not. The radio spectrum of Sgr A* should show a cutoff from free-free absorption at 0.2-1 GHz. For a magnetic field strength $B=15-70\mu$G, the HII region could produce the rotation measure of the magnetar, the largest of any known pulsar, without requiring the gas near Sgr A* to be strongly magnetised.

arXiv comment6 pages, 5 figures, resubmitted to MNRAS after referee report

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• An extreme magneto-ionic environment associated with the fast radio burst source FRB 121102
• The Scattering and Intrinsic Structure of Sagittarius A* at Radio Wavelengths
• Spectral Energy Distribution of the inner accretion flow around Sgr A* -- Clue for a weak outflow in the innermost region
• Locating the intense interstellar scattering towards the inner Galaxy
• Probing the Interstellar Dust towards the Galactic Centre: Dust Scattering Halo around AX J1745.6-2901
• Accretion of Magnetized Stellar Winds in the Galactic Centre: Implications for Sgr A* and PSR J1745-2900
• Chandra large-scale mapping of the Galactic center: Probing high-energy structures around the central molecular zone
• NE2025: An Updated Electron Density Model for the Galactic Interstellar Medium
• Exploring the Interstellar Medium Using an Asymmetric X-ray Dust Scattering Halo