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