X-ray observations of the bright Seyfert galaxy IC 4329A

Abstract

We have performed a detailed analysis of the broad-band (0.1-25 keV) EXOSAT observations of. the high luminosity, nearby Seyfert type I galaxy IC 4329A. The observations were carried out on two occasions, 1984 July and 1985 July. The X-ray luminosity decreased by ~35% over one year. A significant short-term variability where the intensity decreased by ~12% on a time scale of ~2 x 10⁴s was detected in the 1985 data. The observed X-ray spectra are all well fitted by simple power-law models with nearly 10 times larger absorption than the Galactic value, indicating significant absorbing matter local to the source, consistent with the reddening observed in the optical spectrum. We do not find any evidence for low-energy excess in the spectra. The photon index Γ is measured to be 1.85 ± 0.06 (90% confidence) in 1984 and 1.77^+0.06_-0.05 (90% confidence) in 1985. We find a Fe line emission at 6.4 keV with an equivalent width of 169⁺¹²⁴_-113 eV in the 1984 data when the source was bright. The results are used to constrain the mass of the putative supermassive black hole in the nucleus of IC 4329A. The implications of the results on the matter surrounding the active galactic nucleus in IC 4329A are discussed.