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Ultraluminous X-ray sources (ULXs) are promising candidates for intermediate-mass black holes. Here we report about the Suzaku observation of two ULXs, X-1 and X-2, in NGC1313, both showed intensity-correlated spectral changes over a time span of 90 ks. The brighter source, X-1, exhibited the highest luminosity, 2.5 x 10 40 erg/s in 0.4-10 keV, ever recorded from this source. The intensity and spectral variations of X-1 are both ascribed to a strong power-law like component with a mild high energy curvature, while about 10 % of the flux is carried by a stable soft component which can be modeled by a cool disk emission. These properties suggest that the source was in the “very high state”, wherein the disk emission is strongly Comptonized and the optically-thick disk is truncated at a large radii or cooled off. The spectrum of X-2 is best represented, in its fainter phase, by a multicolor blackbody model with the innermost disk temperature of 1.2-1.3 keV, and becomes flatter as the source becomes brighter. Hence X-2 is consistently interpreted to be in a slim disk state. These results suggest that the two ULXs have black hole masses of a few tens to a few hundreds solar masses. Suzaku Observation of Two Ultraluminous X-Ray Sources in NGC1313 Ultraluminous X-ray sources (ULXs): Abstract: •Point-like X-ray sources with Lx>=3x10 39 erg/s •Discovered by Einstein Observatory (Fabbiano 1989). Extensively studied by ASCA, XMM-Newton and Chandra (Makishima et al. 2000, Swartz et al. 2004, Stobbart et al. 2006) •Promising candidate for intermediate-mass black holes . •Key to understand spectral states of black holes. Study of spectral change is important T. Mizuno, H. Takahashi (Hiroshima Univ.), R. Miyawaki, K. Makishima (Tokyo Univ.), M. Miyamato (RIKEN/Tokyo Univ. of Science), K. Ebisawa (ISAS/JAXA), A. Kubota (RIKEN), Y. Ueda (Kyoto Univ.) and NGC1313 team NGC 1313 in X-rays: •Observed by Suzaku on October 15 th •net exposure 28ks (gross span of 90ks) Suzaku Observatoin of X-1 and X-2: XIS data of X-1: ( すすすすすすす 「」 NGC1313 すすすすす すすすすす Ｘ ) 10 -2 10 - 1 10 0 B H B ULX? AGN 10 0 10 2 10 4 10 6 10 8 10 10 M82 IMBH? LLAGN L/L E NLSy1 M/M ◎ XIS0 (0.3-10 keV) X-1 X-2 SN1978K Cal sources 10′ 1 10 100 Energy(keV ) Low State Slim Disk High State Very High State Low State High State Very High State Slim Disk •Nearby face-on Sb galaxy at D=3.7Mpc (Tully 1988) •Hosts three X-ray luminous sources •X-1 and X-2 have been extensively studied in X-rays (in 1993 and 1995 by ASCA, more than 15 times by XMM and 8 times by Chandra. Ideal target for ULXs study •Highest luminosity ever reported (Lx~2.5 x 10 40 erg/s) •Clear time variability by ~50 % (soften in brighter phase) fainter phase brighter phase XMM archival data 2004 June 5 2003 Dec 21, 2004 Jan 8 and 17 2000 Oct 17 and 2005 Feb 7 2003 Aug 23 (Miller et al. 2003; Feng & Kaaret 2006 ) Low-temp MCD Tin~0.2keV variable cutoff PL brighter phase fainter phase •Stable cool disk + highly variable power-law with cutoff •Similar to the very high state in Galactic BHB brighter phase fainter phase XIS data of X-2: •Similar to ASCA/XMM spectra •Clear time variability by ~50 % (soften in fainter phase) fainter phase brighter phase p-free disk model p=0.63, Rin=43 km MCD model Rin=96 km HXD-PIN Summary : •ULXs are promising candidates for intermediate BHs. Missing link to connect stellar-mass BH and AGN a key to understand the BH radiation in a unified picture •NGC1313 in X-rays host two ULXs which have been studied extensively by ASCA, Chandra and XMM-Newton •Suzaku Observation of UGC1313 ULXs (X-1 and X- 2) Intensity-correlated variability in 90ks for both ULXs X-1 (source closer to the nucleus) highest flux ever reported (Lx~2.5 x 10 40 erg/s) stable cool disk + variable power-law with cutoff similar to the very high state in Galactic BHBs M~200M sun to satisfy Eddington limit See also Miyamoto et al. (this meeting) for XMM data of NGC1313 ULXs Data – NXB vs. CXB •No significant detection of hard tail. •Could constrain the spectral model with improved BG models (<=3% systematic errors) Stellar mass BHs 10 40 10 39 10 38 HR-diagram (Ldisk vs. Tin) for ULXs in high state/slim disk L (erg/s) L T ∝ 2 Advection Dominated Disk (Slim Disk) L T ∝ 4 Standard Disk M~200M sun is required to satisfy the Eddington limit •Successful MCD fit to fainter phase data and p-free model fit to brighter phase data •consistently interpreted to be in the slim disk state. XMM archival data 2003 Dec 21, 23, 2006 June5 and 2005 Feb 7 2003 Dec 25 2000 Oct 1, 2004 Jan 8, 16 and Nov 23 (Miller et al. 2003; Feng & Kaaret 2006 ) M~50M sun NGC1313 X-2 ULXs
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