Astronomy and Astrophysics – Astrophysics
Scientific paper
2006-08-02
Astrophys.J.650:L75-L78,2006
Astronomy and Astrophysics
Astrophysics
5 pages, 2 figures, submitted to ApJL on May 26th
Scientific paper
10.1086/508613
X-ray spectral state transitions are a key signature of black hole X-ray binaries and follow a well-defined pattern. We examined 12 XMM-Newton observations of the nearby spiral galaxy NGC 1313, which harbors two compact ultraluminous X-ray sources (ULXs), X-1 and X-2, in order to determine if the state transitions in ULXs follow the same pattern. For both sources, the spectra were adequately fitted by an absorbed power-law with the addition of a low temperature (kT=0.1~0.3 keV) disk blackbody component required in 6 of the 12 observations. As the X-ray luminosity of X-1 increases to a maximum at 3x10^40 ergs/s, the power-law photon index softens to 2.5-3.0. This behavior is similar to the canonical spectral state transitions in Galactic black hole binaries, but the source never enters the high/soft or thermal dominant state and instead enters the steep power-law state at high luminosities. X-2 has the opposite behavior and appears to be in the hard state, with a photon index of Gamma=1.7-2.0 at high luminosity, but can soften to Gamma=2.5 at the lower luminosities.
Feng Hua
Kaaret Philip
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