K-shell emission of neutral iron line from sagittarius B2 excited by subrelativistic protons

Abstract

We investigated the emission of the Kα iron line from massive molecular clouds in the galactic center (GC). We assumed that at present the total flux of this emission consists of a time-variable component generated by primary, X-ray photons ejected by Sagittarius A* (Sgr A*) in the past, and a relatively weak quasi-stationary component excited by the impact of protons that were generated by star accretion onto the central black hole. The level of background emission was estimated from a rise of the 6.4 keV line intensity in the direction of several molecular clouds, which we interpreted as being a stage when the X-ray front ejected by Sgr A* entered into these clouds. Regarding the 6.4 keV emission before this intensity jump, we interpreted it as emission generated by subrelativistic cosmic rays there. The cross-section of Kα vacancies produced by protons differs from that of electrons or X-rays. Therefore, we expect that this processes can be distinguished from an analysis of the equivalentwidth of the iron line; also, the time variations of the width can be predicted. The line intensity from the clouds depends on their distance from Sgr A* and the coefficient of spatial diffusion near the galactic center. We expect that in a few years the line intensity for the cloud G0.11-0.11, which is relatively close to Sgr A*, will decrease to a level of ≲10% from its present value. For the cloud Sagittarius B2 (Sgr B2) the situation is more intricate. If the diffusion coefficient is D ≳ 10 27cm 2 s -1, the expected stationary flux should be about 10% of its level in 2000. In the opposite case the line intensity from Sgr B2 should drop down to zero because the protons do not reach the cloud. © 2011. Astronomical Society of Japan.