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

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a r X i v : 0 7 0 8 .1 8 0 4 v 1 [ a s t r o - p h ] 1 4 A u g 2 0 0 7 Discovery of a bright transient ultraluminous X-ray source Suzaku J1305-4931 in NGC 4945 Naoki Isobe, 1 Aya Kubota, 1,2 Kazuo Makishima, 1,3 Poshak Gandhi, 1 Richard E. Griffiths, 4 Gulab C. Dewangan, 4 Takeshi Itoh, 3 and Tsunuefumi Mizuno 5 1Cosmic Radiation Laboratory, the Institute of Physical and Chemical Research, 2-1 Hirosawa, Wako, Saitama, 351-0198, Japan isobe@crab.riken.jp 2Department of Electronic Information Systems, Shibaura Institute of Technology, 307 Fukasakum Minuma-ku, Saitama-shi, Saitama, 337-8570, Japan 3Department of Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan 4Department of Physics, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213, USA 5Department of Physics, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan (Received 0 0; accepted 0 0) Abstract 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× 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 1.7×10?14 erg cm?2 s?1 (90% confidence level). At a distance of 3.7 Mpc, the bolometric luminosity of the source becomes Lbol = 4.4× 1039α erg s?1, where α = (cos 60?/ 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 Tin = 1.69 +0.06 ?0.05 keV. During the 2006 January observation, it varied by a factor of 2 in intensity, following a clear correlation of Lbol ∝ T 4in. It is inferred that the innermost disk radius Rin stayed constant at Rin =79 +4.0 ?3.9α 1/2 km, suggest
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