3C 33: another case of photoionized soft X-ray emission in radio galaxies

Abstract

We investigate the X-ray spectral properties of 3C 33 by analyzing all the observations available in the Chandra and XMM-Newton archives. In this paper we present a complete X-ray analysis of the nuclear emission of this narrow-line radio galaxy. The broad-band spectrum of 3C 33 is complex. Its hard part resembles that of Seyfert 2 galaxies, which have a heavily obscured nuclear continuum (N_H ~ 10²³ cm^-2) and a prominent Fe Kα line. This represents the nuclear radiation observed directly in transmission through a cold circumnuclear gas. On the other hand an unabsorbed continuum plus emission lines seem to reproduce the soft part of the spectrum (0.5-2 keV) well, suggesting that the jet does not contribute significantly to the X-ray emission. We discuss the possible collisional or photoionized origin of the gas that emits the soft X-ray lines. Our results, supported by those from optical spectroscopy, favor the photoionization scenario.