A single H II region model of the strong interstellar scattering towards Sgr A*
Abstract
Until recently, the strong interstellar scattering observed towards the Galactic centre (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 H II region along the line of sight, 1.5-4.8 kpc away from Earth with density ne 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 disperson measure, 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 cut-off from free-free absorption at 0.2 ≲ ν ≲ 1 GHz. For a magnetic field strength B ≃ 15-70 μG, the H II 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 magnetized.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- May 2017
- DOI:
- 10.1093/mnras/stx103
- arXiv:
- arXiv:1612.04819
- Bibcode:
- 2017MNRAS.467.3642S
- Keywords:
-
- scattering;
- pulsars: individual (J745-2900);
- H II regions;
- Galaxy: centre;
- Astrophysics - Astrophysics of Galaxies;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 6 pages, 5 figures, resubmitted to MNRAS after referee report