Spectral State Transitions of the Ultraluminous X-ray Sources X-1 and X-2 in Ngc 1313

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 3 × 10 40 ergs s −1 , 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. 1. INTRODUCTION Ultraluminous X-ray sources (ULXs) are point-like, non-nuclear X-ray sources with luminosities above the Eddington limit of a 10 M ⊙ black hole (2 × 10 39 ergs s −1 , for review see Fabbiano & White 2006). Many ULXs show strong variability and are believed to be black hole binaries. Their X-ray lu-minosities (Colbert & Mushotzky 1999; Kaaret et al. 2001), thermal disk emissions (Kaaret et al. 2003), variation time scales (Strohmayer & Mushotzky 2003; Dewangan et al. 2006) and surrounding emission line nebulae (Kaaret et al. 2004b) suggest that they might have masses of 20–10 3 M ⊙ , falling into the class of intermediate mass black holes (IMBHs). However, if the emission is beamed (King et al. 2001; Körding et al. 2002) or exceeds the Eddington limit (Begelman 2002; Ebisawa et al. 2003), some or all ULXs may be stellar-mass black holes. A comparison of the properties of ULXs with the well-known phenomenology of stellar-mass black hole X-ray binaries should help reveal the nature of ULXs. Spectral state transitions are essential characteristics of black hole binaries (for a review see McClintock & Remillard 2006). Most X-ray binaries have been observed in both the hard and high/soft states. Typically, the intrinsic energy spectrum in the hard state is …