Chandra's Discovery of Activity in the Quiescent Nuclear Black Hole of NGC 821

Abstract

We report the results of the {\it Chandra} ACIS-S observations of the elliptical 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 ksec ACIS-S integrated exposure of this region. The luminosity of this feature is $L_X\sim 2.6\times 10^{39}\mathrm{ergs}{\mathrm{s}}^{-1}$ (0.3-10 keV), and its spectrum is hard (described by a power-law of $\Gamma =1.8_{-0.6}^{+0.7}$; 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.

18 pages, 4 figures, 3 JPEG submitted to ApJ

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