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

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 L_bol = 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 T_in = 1 .69_-0.05^+0.06 keV. It varied by a factor of 2 in intensity, following a clear correlation of L_bol ∝ T _in⁴. The innermost disk radius is inferred to stay constant at R_in = 79_-3.9^+4.0α1/2 km, suggesting the presence of a standard accretion disk. Relating R_in 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.