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

A. Baldi·G. Fabbiano·S. Pellegrini·A. Siemiginowska·M. Elvis·A. Zezas·J. C. McDowell
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Abstract

We report the results of the Chandra ACIS-S observations of the elliptical galaxy NGC 821, which harbors a supermassive nuclear black hole (of 3.5x10^7 solar masses), 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 x 10^39 erg/s (0.3-10 keV), and its spectrum is hard (described by a power-law of Gamma = 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.

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A. Baldi, G. Fabbiano, S. Pellegrini, A. Siemiginowska, M. Elvis, A. Zezas, J. C. McDowell. 2004-10-06. Chandra discovery of activity in the quiescent nuclear black hole of NGC821. https://arxiv.org/abs/astro-ph/0410169

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