ChandraObservations of the Quiescent Nuclear Black Hole of NGC 821: Evidence of Nuclear Activity?

We report the results of the {\it Chandra} ACIS-S observations of theelliptical galaxy NGC 821, which harbors a supermassive nuclear black hole (of$3.5 \times 10^7 M_{\odot}$), but does not show sign of AGN activity. A small,8.5$^{\prime\prime}$ long ($\sim 1$ kpc at the galaxy's distance of 23 Mpc),S-shaped, jet-like feature centered on the nucleus is detected in the 38 ksecACIS-S integrated exposure of this region. The luminosity of this feature is$L_X \sim 2.6 \times 10^{39} \rm ergs s^{-1}$ (0.3-10 keV), and its spectrum ishard (described by a power-law of $\Gamma = 1.8^{+0.7}_{-0.6}$; or by thermalemission with $kT >2$ keV). We discuss two possibilities for the origin of thisfeature: (1) a low-luminosity X-ray jet, or (2) a hot shocked gas. In eithercase, it is a clear indication of nuclear activity, detectable only in theX-ray band. Steady spherical accretion of the mass losses from the centralstellar cusp within the accretion radius, when coupled to a high radiativeefficiency, already provides a power source exceeding the observed radiativelosses from the nuclear region.Comment: 18 pages, 4 figures, 3 JPEG submitted to Ap