Suzaku Observation of Two Ultraluminous X-Ray Sources in NGC1313

Two ultraluminous X-ray sources (ULXs) in the nearby Sb galaxy NGC 1313,named X-1 and X-2, were observed with Suzaku on 2005 September 15. During theobservation for a net exposure of 28~ks (but over a gross time span of 90~ks),both objects varied in intensity by about 50~%. The 0.4--10 keV X-rayluminosity of X-1 and X-2 was measured as $2.5 \times 10^{40}~{\rm erg~s^{-1}}$and $5.8 \times 10^{39}~{\rm erg~s^{-1}}$, respectively, with the former thehighest ever reported for this ULX. The spectrum of X-1 can be explained by asum of a strong and variable power-law component with a high energy cutoff, anda stable multicolor blackbody with an innermost disk temperature of $\sim 0.2$keV. These results suggest that X-1 was in a ``very high'' state, where thedisk emission is strongly Comptonized. The absorber within NGC 1313 toward X-1is suggested to have a subsolar oxygen abundance. The spectrum of X-2 is bestrepresented, in its fainter phase, by a multicolor blackbody model with theinnermost disk temperature of 1.2--1.3 keV, and becomes flatter as the sourcebecomes brighter. Hence X-2 is interpreted to be in a slim-disk state. Theseresults suggest that the two ULXs have black hole masses of a few tens to a fewhundreds solar masses.Comment: accepted for publication in PAS