EVIDENCE FOR A HIGH ENERGY CURVATURE IN THE XMM-Newton SPECTRUM OF THE ULX

ASCA X-ray spectra of many ULXs were described in terms of optically thick emission from hot (kTin ∼ 1− 2 keV) accretion disks, while recent XMM-Newton and Chandra observations have revealed a cool (kTin ∼ 0.2 keV), soft X-ray excess emission from a number of them. Here we utilize improved calibration and high signal-to-noise XMM-Newton spectra of NGC 1313 X-1 to present evidence for a cool (∼ 0.2 keV) soft excess and a curved or a cutoff power-law (Γ ∼ 1 − 1.5, Ecutoff ∼ 3 − 8 keV). The high energy curvature may also be described by a hot (∼ 1 − 2.5 keV) multicolor disk blackbody. The soft excess emission is unlikely to arise from a cool disk as its blackbody temperature is similar in three XMM-Newton observations, despite a change in the observed flux by a factor of about two. Thus, previous estimates of the black hole mass of 1000M⊙ for NGC 1313 X-1 based on the temperature of the soft excess emission is unlikely to be correct. The power-law cutoff energy is found to decrease from ∼ 8 keV to ∼ 3 keV when the ULX brightened by a factor of about two. The unusual spectral properties of NGC 1313 X-1 are difficult to understand in the framework of the disk/corona models generally adopted for the black hole binaries or active galactic nuclei and may require to invoke super-critical accretion rates. Subject headings: accretion, accretion disks — stars: individual (NGC 1313 X-1) — X-rays: stars