I. Evidence for energy release in the Galactic center:

1. De-Excitation Gamma-ray Line Emission from the Galactic Center. D.O. Chernyshov (MIPT), V.A. Dogiel (FIAN/ISAS), V. Tatischeff (CNRS), K.-S. Cheng (HKU), C.-M. Ko (NCU), W.H. Ip (NCU) • II. Energy source in the Galactic center • One likely scenario is that the Galactic nucleus was brighter in the past, possibly caused by an accretion onto the massive back hole; • The estimated frequency of star accretion is 4.5 10-5 years-1 (Syer and Ulmer 1999 ); • Approximately 50% of the star mass is not accreted but becomes unbound receiving an additional angular momentum transferred from the captured material (Ayal et al. 2000); • The average kinetic energy per baryon of the unbound material in case of capture of solar mass star when 50% of the star mass is unbounded is where b is penetration parameter. Parameter b is not well constrained by theory (Alexander 2005); • 6. For accretion process to take place it’s required that b << 1. For value of penetration parameter b ~ 0.1 the escape energy should be order of 10-100 MeV. • 7. Fast protons with energies about 10..100 MeV can also excite nuclei of particles from background plasma. The excitation can lead to triggering of nuclear reaction or to formation of de-excitation line. • I.Evidence for energy release in the Galactic center: • CHANDRA resolved only a weak X-ray point source at the position of Sgr A* with a flux Lx ~1033 erg/s. This "X-ray quiet" Sgr A* is in a sharp contrast to the high activity of the surrounding medium; • ASCA, CHANDRA and SUZAKU found emission of 6.5 keV plasma from the central ~1o -radius region which cannot be heated by SNs. The required energy input is ~1041 erg/s (see e.g. Koyama et al. 1996, Muno et al. 2001); • SUZAKU observed a flux of nonthermal X-ray emission from the Galactic center. The total flux in he range 14-40 keV is 4 1036 erg/s (Yuasa et al. 2008); • The molecular gas in the Galactic buldge is much warmer (T~100 K) than in other part of the Galactic disk (T~20 K). No evident sources of heating are observed there. This heating may be associated with a high density of subrelativistic cosmic rays near the Galactic center (Yuzef-Zadeh et al. 2007) which can produce also the observed 6.4 keV flux from molecular clouds. • It’s likely that all these processes can be somehow connected to accretion processes on central black hole. IV. Intensity of de-excitation lines and spectrum III. Propagation of subrelativistic protons. The spectrum of protons can be calculated using the equation where are Coulomb losses. The characteristic time of cooling due to energy losses for protons with energy 10..100 MeV is for ambient density 0.2 cm-3. The diffusion coefficient is about D = 1027 cm2/s. Thus protons can propagate for distance Predicted fluxes from GC region of carbon (a) and oxygen (b) lines as a function of injection energy Radial profiles of carbon (a) and oxygen (b) lines as a function of angular distance from Galactic Center for different values of injection energy. For Eesc < 100 MeV/n and n = 0.2 cm-3 the gamma-ray lines will be emitted from region of maximum 5 angular radius. Such an emission would appear as small-scale diffuse emission for gamma- ray instruments like SPI. Expected spectrum of gamma-ray emission and main gamma-ray lines. V. Conclusion Our predicted fluxes are below currently available sensitivity limits of INTEGRAL. After ~3 Ms of exposure of the GC region, the SPI sensitivity for detection of a narrow line at energy ~5 MeV was 3.2x10-5 cm-2s-1. (Teegarden & Watanabe,2006). But for a broad line of ~200 keV FWHM the sensitivity limit increases by a factor of ~8. Thus it’s unlikely that nuclear lines mentioned above can be detected with SPI. But future gamma­ray missions may be able to test our predictions. Thus, the GRIPS mission proposed for ESA's ''Cosmic Vision'' program (Greiner J. et al., 2008) could achieve after 5 years in orbit more than an order of magnitude sensitivity improvement over COMPTEL (in 9 years), which would allow a clear detection of the predicted gamma­ray line emission at 4.44 MeV and 6.2 MeV from the GC region. The Advanced Compton Telescope project proposed as a future NASA mission aimed at even better sensitivity, near 10-6 photons cm-2 s-1 for 3% broad lines. A future detection of the predicted gamma­ray lines with such instruments would provide unique information on the high­energy processes induced by the central black hole, as well as on the physical conditions of the emitting region. We note that nuclear interactions of subrelativistic ions with ambient material can also synthesize nuclear radioisotopes, whose decay can inject positrons into the GC region. From the radioisotope production yields given by Kozlovsky et al. (1987), we estimate that for Eesc ~ 100 MeV the number of positrons produced by this mechanism is < 5 times the number of gamma­rays emitted in the 4.44 and 6.2 MeV lines. This limit is more than an order of magnitude lower than the positron annihilation rate measured with INTEGRAL/SPI . This work was accepted for publication in A&A