Correlated multiwavelength emission from the X-ray-bright Seyfert galaxy III Zw 2

Abstract

The X-ray-bright Seyfert 1 galaxy III Zw 2 was observed with XMM–Newton in 2000 July. Its X-ray spectrum can be described by a power law of photon index Γ= 1.7 and an extremely broad (FWHM∼ 140 000 km s −1 ) Fe KΑ line at 6.44 keV. The iron line has an equivalent width of ∼800 eV. To study the long-term X-ray behaviour of the source we have analysed 25 yr of data, from 1975 to 2000. There is no evidence of significant intrinsic absorption within the source or of a soft X-ray excess in the XMM or archival data. We do not detect rapid X-ray variability (a few × 10 3 s ) during any of the individual observations; however, on longer time-scales (a few years) the X-ray light curve shows 10-fold flux variations. We infer a black hole mass of ∼10 9 M ⊙ (from HΒ FWHM) for III Zw 2 which is much higher than some previous estimates. A comparison of X-ray variability with light curves at other wavelengths over a 25-yr period reveals correlated flux variations from radio to X-ray wavelengths. We interpret the variable radio to optical emission as synchrotron radiation, self-absorbed in the radio/millimetre region, and the X-rays mainly as a result of Compton up-scattering of low-energy photons by the population of high-energy electrons that give rise to the synchrotron radiation.