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

Tsunefumi Mizuno, Ryohei Miyawaki, Ken Ebisawa, Aya Kubota, Masao Miyamoto, Lisa M. Winter, Yoshihiro Ueda, Naoki Isobe, Gulab C. Dewangan, Chris Done, Richard E. Griffiths, Yoshito Haba, Motohide Kokubun, Jun'ichi Kotoku, Kazuo Makishima, Kyoko Matsushita, Ricard F. Mushotzky, Masaaki Namiki, Robert Petre, Hiromitsu TakahashiToru Tamagawa, Yuichi Terashima

Research output: Contribution to journalArticlepeer-review

32 Citations (Scopus)


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 90 ks), both objects varied in intensity by about 50%. The 0.4-10 keV 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 1.2-1.3 keV, 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.

Original languageEnglish
Pages (from-to)S257-S267
JournalPublications of the Astronomical Society of Japan
Issue number1 SPEC. ISS.
Publication statusPublished - 2007
Externally publishedYes


  • Accretion, accretion disks
  • Black hole physics
  • X-rays: individual (NGC 1313 X-1, NGC 1313 X-2)

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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