* Astronomy

Title: A deep Chandra look at the low L_B elliptical NGC821: not only wind and X-ray binaries
Authors: S. Pellegrini (1), A. Baldi (2), D.W. Kim (2), G. Fabbiano (2), R. Soria (2,3), A. Siemiginowska (2), M. Elvis (2) ((1) Astronomy Dept., Bologna University, Italy; (2) Harvard-Smithsonian Center for Astrophysics, Cambridge, MA (USA); (3) Mullard Space Science Laboratory, University College London, UK)

The nearby, low L_B elliptical galaxy NGC821, known to host a central massive black hole but inactive at all wavelengths, has been observed with deep Chandra pointings, for a total exposure of 230 ksec, in order to search for nuclear emission and gas available for accretion. Within its optical image, 41 X-ray sources are detected, with spectral properties typical of low mass X-ray binaries (LMXBs); their X-ray luminosity function (XLF) was derived down to a 0.3-8 keV luminosity of 3X10^{37} erg/s. At the position of the galactic centre a source of L(0.3-8 keV)=6X10^{38} erg/s is detected for the first time. This source is surrounded by three other sources with a spectral shape typical of LMXBs and luminosities on the brightest end of the XLF. Only one is consistent with being pointlike, and one of them resembles a jet-like feature. Diffuse emission is detected out to a radius of ~30" and comes mostly from unresolved LMXBs, with a possible minor contribution from other types of stellar sources. Hydrodynamical simulations specific for NGC821 show that the hot gas is driven out of the galaxy in a wind sustained by type Ia supernovae, but also that it is accreting towards the centre from within a very small inner region. A companion paper (Pellegrini et al. 2006) presents further observational results obtained with Spitzer and the VLA, and discusses possible accretion modalities for this central massive black hole.

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