On the Nature of the Ultraluminous X-Ray Transient in Cen A (NGC 5128)

Abstract

We combine nine ROSAT, nine Chandra, and two 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 1990-2003. 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 3×10³⁹ ergs s^-1, which places the source in the class of ultraluminous X-ray sources. Coherent pulsations at 13.264 Hz are detected during a second bright episode (F_X>3×10^-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 seven observations. The X-ray spectrum in 1999 December is best described as a cutoff power law or a disk blackbody (multicolored disk). We also detect an optical source, m_F555W~24.1 mag, within the Chandra error circle of 1RXH J132519.8-430312 in Hubble Space Telescope 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 is similar to the transient Be/X-ray pulsar A0538-66.