On the nature of the ultraluminous X-ray transient in Cen~A (NGC 5128)

Abstract

We combine 9 ROSAT, 9 Chandra, and 2 XMM-Newton observations of the Cen~A galaxy to obtain the X-ray light curve of 1RXH J132519.8-430312 (=CXOU J132519.9$-$430317) spanning 1990 to 2003. The source reached a peak 0.1-2.4 keV flux F_X>10^{-12} ergs cm^{-2} s^{-1} during a 10~day span in 1995 July. The inferred peak isotropic luminosity of the source therefore exceeded 3 10^{39} ergs s^{-1}, which places the source in the class of ultra-luminous X-ray sources. Coherent pulsations at 13.264 Hz are detected during a second bright episode (F_X >3 times 10^{-13} ergs cm^{-2} s^{-1}) in 1999 December. The source is detected and varies significantly within three additional observations but is below the detection threshold in 7 observations. The X-ray spectrum in 1999 December is best described as a cut-off power law or a disk-blackbody (multi-colored disk). We also detect an optical source, m_F555W ~ 24.1 mag, within the Chandra error circle of 1RXH J132519.8-430312 in HST images taken 195~days before the nearest X-ray observation. The optical brightness of this source is consistent with a late O or early B star at the distance of Cen A. If the optical source is the counterpart, then the X-ray and optical behavior of 1RXH J132519.8-430312 are similar to the transient Be/X-ray pulsar A 0538-66.