Spectral state transitions of the ultraluminous X-ray sources X-1 and X-2 in NGC 1313.
FENG H. and KAARET P.
Abstract (from CDS):
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 six of the 12 observations. As the X-ray luminosity of X-1 increases to a maximum at 3x1040 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 Γ=1.7-2.0 at high luminosity, but can soften to Γ=2.5 at the lower luminosities.