Suzaku Observation of Two Ultraluminous X-Ray Sources in NGC 1313
Done, Chris; Isobe, Naoki; Petre, Robert; Haba, Yoshito; Kokubun, Motohide; Makishima, Kazuo; Miyawaki, Ryohei; Dewangan, Gulab C.; Ueda, Yoshihiro; Terashima, Yuichi; Ebisawa, Ken; Takahashi, Hiromitsu; Griffiths, Richard E.; Kubota, Aya; Namiki, Masaaki; Matsushita, Kyoko; Mizuno, Tsunefumi; Tamagawa, Toru; Kotoku, Jun'ichi; Winter, Lisa M.; Miyamoto, Masao; Mushotzky, Ricard F.
Japan, United States, United Kingdom
Abstract
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 the observation for a net exposure of 28 ks (but over a gross time span of 90ks), both objects varied in intensity by about 50%. The 0.4-10keV X-ray luminosities of X-1 and X-2 were measured as 2.5 × 1040 erg s-1 and 5.8 × 1039 erg s-1, respectively, with the former exhibiting the highest ever reported for this ULX. The spectrum of X-1 can be explained by the sum of a strong and variable power-law component with a high-energy cutoff, and a stable multicolor blackbody with an innermost disk temperature of ∼ 0.2 keV. These results suggest that X-1 was in a ``very high'' state, where disk emission is strongly Comptonized. The absorber within NGC 1313 toward X-1 is suggested to have a subsolar oxygen abundance. The spectrum of X-2 is best represented, in its fainter phase, by a multicolor blackbody model with an innermost disk temperature of 1.2-1.3keV, and becomes flatter as the source becomes brighter. Hence, X-2 is interpreted to be in a slim-disk state. These results suggest that the two ULXs have black hole masses of some dozens to a few hundred of solar masses.