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

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 L X ∼ 2 . 6 × 10 39 ergs s − 1 (0.3-10 keV), and its spectrum is hard (described by a power-law of Γ = 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.