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 \simeq 100 \hskip { 2.0 pt } cm ^ { -3 } and radius R \simeq 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 \lesssim 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 \lesssim \nu \lesssim 1 GHz .
For a magnetic field strength B \simeq 15 - 70 \hskip { 2.0 pt } \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 .