Serendipitous discovery of an AGN showing rapid X-ray variability.

Abstract

A serendipitous X-ray source, discovered by EXOSAT in the field of the hot, helium-rich degenerate star PG1159-035, has been identified with the 15.1-mag galaxy ZWG 013.024. The object is classified as Seyfert 1 and has a redshift of 0.019, implying a distance of 117 Mpc. ZWG 013.024 has an unusually soft medium-energy (0.5-6 keV) X-ray spectrum (photon index 2.1) and is variable on a time-scale of 7-14 hr. Variations in the low-energy telescope (thin lexan filter) and medium-energy array count rates are correlated and have similar amplitudes, a factor ~3. The softness ratio is constant throughout the observation indicating that there is no associated spectral variability. This object appears to be a further addition to the small group of rapidly variable AGNs.