2004ApJ...616..730F

We report the results of the Chandra ACIS-S observations of the elliptical galaxy NGC 821, which harbors a supermassive nuclear black hole (3.5x10⁷ M_☉) but does not show signs of active galactic nucleus activity. A small, 8".5 long (∼1 kpc at the galaxy's distance of 23 Mpc), possibly S-shaped, jetlike feature centered on the nucleus is detected in the 38 ks ACIS-S integrated exposure of this region. The luminosity of this feature is L_X∼2.6x10³⁹ ergs/s (0.3-10 keV), and its spectrum is hard (described by a power law of Γ=1.8^+0.7_–0.6or 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 with a high radiative efficiency, already provides a power source exceeding the observed radiative losses from the nuclear region. A third possibility, that this feature may arise from a fortuitous distribution of luminous X-ray binaries in NGC 821, is also discussed.