Chandra discovery of activity in the quiescent nuclear black hole of NGC821

We report the results of the Chandra ACIS-S observations of the elliptical galaxy NGC 821, which harbors a supermassive nuclear black hole (of 3.5 × 10 7 M⊙), 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 LX � 2.6 × 10 39 ergs s −1 (0.3-10 keV), and its spectrum is hard (described by a power-law of = 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. Subject headings: galaxies: NGC 821 - galaxies: nuclei - X-ray: galaxies