Discovery of a Bright Transient Ultraluminous X-Ray Source, Suzaku J1305-4931 in NGC4945
Abstract
An X-ray source, Suzaku J1305-4931, was discovered in the south-west arm of a nearby Seyfert II galaxy, NGC4945, at 0.5-10keV flux of 2.2×10-12erg 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 3.7Mpc, the bolometric luminosity of the source became Lbol = 4.4 × 1039 α ergs-1, where α = (cos60°/cosi) 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+0.06keV. It varied by a factor of 2 in intensity, following a clear correlation of Lbol ∝ Tin4. The innermost disk radius is inferred to stay constant at Rin = 79-3.9+4.0 α1/2km, 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.
- Publication:
-
Publications of the Astronomical Society of Japan
- Pub Date:
- January 2008
- DOI:
- 10.1093/pasj/60.sp1.S241
- arXiv:
- arXiv:0708.1804
- Bibcode:
- 2008PASJ...60S.241I
- Keywords:
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- accretion;
- accretion disks;
- black hole physics;
- galaxies: individual (NGC 4945);
- X-rays: galaxies;
- Astrophysics
- E-Print:
- 21 pages, 10 figures, Accepted for PASJ 2nd Suzaku special issue