Discovery of a bright transient ultraluminous X-ray source, Suzaku J1305-4931 in NGC 4945

Abstract

An X-ray source, Suzaku J1305-4931, was discovered in the south-west arm of a nearby Seyfert II galaxy, NGC 4945, at 0.5-10 keV flux of 2.2 × 10 -12 erg Cm-2 s-1 during a Suzaku observation conducted on 2006 January 15-17. It was undetectable in a shorter observation on 2005 August 22-23, with an upper limit of 1.7 × 10-14 erg cm-2 s-1. At a distance of 3.7 Mpc, the bolometric luminosity of the source became Lbol = 4.4 × 1039 α erg s-1, where α = (cos 60°/cos i) and i is the disk inclination. The time-averaged X-ray spectrum of the source is described by a multi-color disk model, with an innermost disk temperature of Tin = 1 .69-0.05+0.06 keV. It varied by a factor of 2 in intensity, following a clear correlation of Lbol ∝ T in4. The innermost disk radius is inferred to stay constant at Rin = 79-3.9+4.0α1/2 km, suggesting the presence of a standard accretion disk. Relating Rin with the last stable orbit around a non-rotating black hole yields a rather low black-hole mass, ∼ 9α1/2 solar masses, which would imply that the source is shining at a considerable super-Eddington luminosity. These results can be better interpreted by invoking sub-Eddington emission from a rapidly spinning black hole with 20-130 solar masses. © 2008. Astronomical Society of Japan.