The broad-band spectrum of 3C 120 observed by BeppoSAX

Abstract

We study a broad-band X-ray observation by BeppoSAX of the broad-line radio galaxy 3C 120. The primary X-ray continuum is well described by a power law with the photon index of Gamma=1.85+-0.05 and a high-energy break or a cutoff. The obtained e-folding energy of about100-300 keV corresponds to kT of about 50 keV in thermal-Compton models. A soft X-ray excess is found at a statistical significance of >98%$ Our physical best-fit model of the excess is optically-thin emission from an extended plasma (with a luminosity of 2% of the total X-ray luminosity), which interpretation is supported by an independent finding of an extended soft X-ray halo in 3C 120. We find a moderate Compton reflection component together with a moderately broad Fe K alpha line with an equivalent width fully consistent with the strength of reflection, indicating their origin in an optically-thick accretion disk extending relatively close to the central black hole. We observe strong spectral variability during our 2-day observation with Gamma varying from 1.7 to 2.0 and correlated with the soft X-ray flux. The variability is very well modeled by thermal Comptonization in a hot plasma irradiated by a variable UV flux. Both a hot patchy corona outflowing with a mildly relativistic velocity away from a cold disk and a hot inner accretion flow overlapping with the cold disk are viable geometries. The presence of an outflow in the first case and an overlap between the hot and cold phases in the second case are required by energy balance and the observed strength of Compton reflection.

ApJ, in press, 16 pages, p. 9 is in color

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