Suzaku observation of two ultraluminous X-ray sources in NGC 1313

A study was made of two ultraluminous X-ray sourccs (ULXs) in the nearby face-on, late-type Sb galaxy NGC 1313 using data from Suzaku, the 5th .Japanese X-ray satellite. Within the 90 ks observation, both sources named X-1 and X-2 exhibited hmniinosity change by about 50 %. The 0.4-10 keV X-ray luminosity was measured to be 2.5 x lo4' erg s-' and 5.8 x erg s-I for X-1 and X-2, respectively, requiring a black hole of 50-20011/1, in order not to exceed the Eddingtion limit. For X-1: the spectrum exhibited a strong power-law component with a high energy cutoff which is thought to arise from strong Comptonization by a disk corona, suggesting the source was in a very high state. Absorption line features with equivalent widths of 40 -80 eV found at 7.0 keV and 7.8 keV in the X-1 spectrum support the presence 'of a highly ionized plasma and a high mass accretion rate on the system. Oxygen abundance of the NGC 1313 circumstellar matter toward X-1 was found to be subsolar, viz. O/H = (5.Of 1.0) x lW4. The spectrum of X-2 in fainter phase is best represented by a multicolor disk blackbody model with zn = 1.2-1.3 keV and becomes flatter as the flux increases; the source is intcrpreted to be in a slim disk state.