This paper reports the discovery of a bright X-ray transient source , Suzaku J1305-4913 , in the south-west arm of the nearby Seyfert II galaxy NGC 4945 .
It was detected at a 0.5 – 10 keV flux of 2.2 \times 10 ^ { -12 } erg cm ^ { -2 } s ^ { -1 } during the Suzaku observation conducted on 2006 January 15 – 17 , but was undetectable in a shorter observation on 2005 August 22 –23 , with an upper limit of 1.7 \times 10 ^ { -14 } erg cm ^ { -2 } s ^ { -1 } ( 90 % confidence level ) .
At a distance of 3.7 Mpc , the bolometric luminosity of the source becomes L _ { bol } = 4.4 \times 10 ^ { 39 } \alpha erg s ^ { -1 } , where \alpha = ( \cos 60 ^ { \circ } / \cos i ) and i is the disk inclination .
Therefore , the source is classified into so-called ultraluminous X-ray sources ( ULXs ) .
The time-averaged X-ray spectrum of the source is described by a multi-color disk model , with the innermost accretion disk temperature of T _ { in } = 1.69 _ { -0.05 } ^ { +0.06 } keV .
During the 2006 January observation , it varied by a factor of 2 in intensity , following a clear correlation of L _ { bol } \propto T _ { in } ^ { 4 } .
It is inferred that the innermost disk radius R _ { in } stayed constant at R _ { in } = 79 _ { -3.9 } ^ { +4.0 } \alpha ^ { 1 / 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 , \sim 9 \alpha ^ { 1 / 2 } solar masses , which would imply that the source is shining at a luminosity of \sim 3 \alpha ^ { 1 / 2 } times the Eddington limit .
These results can be better interpreted by invoking sub-Eddington emission from a rapidly spinning black hole with a mass of 20 – 130 solar masses .