Astronomy and Astrophysics – Astrophysics
Scientific paper
2004-10-06
Astronomy and Astrophysics
Astrophysics
8 pages, 3 figures, proceedings of the "6th Italian Conference on Active Galactic Nuclei", Volterra, 25-28 May 2004
Scientific paper
We report the results of the Chandra ACIS-S observations of the elliptical galaxy NGC 821, which harbors a supermassive nuclear black hole (of 3.5x10^7 solar masses), but does not show sign of AGN activity. A small, 8.5" long (~1 kpc at the galaxy's distance of 23 Mpc), S-shaped, jet-like feature centered on the nucleus is detected in the 38 ksec ACIS-S integrated exposure of this region. The luminosity of this feature is L_X ~ 2.6 x 10^39 erg/s (0.3-10 keV), and its spectrum is hard (described by a power-law of Gamma = 1.8^{+0.7}_{-0.6}; or by thermal emission with kT > 2 keV). We discuss two possibilities for the origin of this feature: (1) a low-luminosity X-ray jet, or (2) a hot shocked gas. In either case, it is a clear indication of nuclear activity, detectable only in the X-ray band. Steady spherical accretion of the mass losses from the central stellar cusp within the accretion radius, when coupled to a high radiative efficiency, already provides a power source exceeding the observed radiative losses from the nuclear region.
Baldi Alessandro
Elvis Martin
Fabbiano Giuseppina
McDowell Jonathan C.
Pellegrini Sara
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