Nuclear interaction gamma-ray lines from the Galactic center region

Abstract

Aims. The accretion of stars onto the central supermassive black hole at the center of the Milky Way is predicted to generate large fluxes of subrelativistic ions in the Galactic center region. We analyze the intensity, shape, and spatial distribution of de-excitation gamma-ray lines produced by nuclear interactions of these energetic particles with the ambient medium. Methods. We first estimated the amount and mean kinetic energy of particles released from the central black hole during star disruption. We then calculated the energy and spatial distributions of these particles in the Galactic center region from a kinetic equation. These particle distributions were then used to derive the characteristics of the main nuclear interaction gamma-ray lines. Results. Because the time period of star capture by the supermassive black hole is expected to be shorter than the lifetime of the ejected fast particles against Coulomb losses, the gamma-ray emission is predicted to be stationary. We find that the nuclear deexcitation lines should be emitted from a region with a maximum 5 • angular radius. The total gamma-ray line flux below 8 MeV is calculated to be ∼10 −4 photons cm −2 s −1 . The most promising lines for detection are those at 4.44 and ∼6.2 MeV, with a predicted flux in each line of ∼10 −5 photons cm −2 s −1 . Unfortunately, it is unlikely that this emission can be detected with the INTEGRAL observatory. But the predicted line intensities appear to be within reach of future gamma-ray space instruments. A future detection of de-excitation gamma-ray lines from the Galactic center region would provide unique information on the high-energy processes induced by the central supermassive black hole and the physical conditions of the emitting region.

THE FERMI BUBBLE AS A SOURCE OF COSMIC RAYS IN THE ENERGY RANGE >1015eV

“…By matching with the observed flux in the knee region we find that the conversion efficiency from shocks in the bubble to CRs is about 10%, which is quite consistent with numerical simulations. Other input parameters in this model, such as the capture timescale τ cap ∼ 3 × 10 4 yr, the mean rate of energy releasė W ∼ 3 × 10 40 erg s −1 per capture, and the injected wind speed u ∼ 10 8 cm s −1 , have been estimated and used in other observed phenomena in the GC (e.g., Cheng et al 2006Cheng et al , 2007Dogiel et al 2009aDogiel et al , 2009bDogiel et al , 2009cDogiel et al , 2011.…”

“…In a series of papers (Cheng et al 2006(Cheng et al , 2007Dogiel et al 2009aDogiel et al , 2009bDogiel et al , 2009cDogiel et al , 2011, we developed a model of energy release in the GC due to star accretion onto the central black hole for the interpretation of X-ray and gamma-ray emission from the GC. Our goal was to explain these observational data in the framework of a single model.…”

On the origin of the 6.4keV line in the Galactic Center region

“…We see that the required proton luminosity in the GC region, ≤ 10 40 erg s −1 , seems to be quite reasonable if supplied by SNs in the GC (see Crocker et al, 2011). As Cheng et al (2007Cheng et al ( , 2011 and Dogiel et al (2009c) showed, this luminosity of subrelativistic and relativistic CRs in the GC can also be provided by processes of star accretion onto the central black hole, though as we mentioned these estimates cannot be considered as reliable.…”

The origin of K-shell Emission of Neutral Iron Line from Sgr B2

“…As a result, a flux of subrelativistic protons is ejected from the GC, which heats the central region with the necessary power. These protons can also produce 6.4 keV line emission from molecular clouds which is, however, stationary because the lifetime of these protons τ p ∼ 10 7 yr [3] is much longer than the characteristic time of star capture by the central black hole (τ c ∼ 10 5 yr) [1].…”