Suzaku Observation of Two Ultraluminous X-Ray Sources in NGC1313

1. Slim Disk 108 1010 106 100 102 104 XIS0 (0.3-10 keV) X-1 SN1978K 10′ X-2 Cal sources Suzaku Observation of Two Ultraluminous X-Ray Sources in NGC1313 (「すざく」によるNGC1313中の大高度Ｘ線源の観測) 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 Abstract: 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 1040 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. Ultraluminous X-ray sources (ULXs): NGC 1313 in X-rays: • Point-like X-ray sources with Lx>=3x1039 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 • 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. L/LE Slim Disk ULX? B H B NLSy1 AGN 100 M82 IMBH? Very High State Very High State 10-1 High State Ideal target for ULXs study LLAGN Low State • Observed by Suzaku on October 15th • net exposure 28ks (gross span of 90ks) High State Low State 10-2 Energy(keV) M/M◎ 1 10 100 Suzaku Observatoin of X-1 and X-2: XIS data of X-1: XIS data of X-2: • Highest luminosity ever reported (Lx~2.5 x 1040 erg/s) • Clear time variability by ~50 %(soften in brighter phase) • Similar to ASCA/XMM spectra • Clear time variability by ~50 % (soften in fainter phase) brighter phase brighter phase Low-temp MCD Tin~0.2keV p-free disk model p=0.63, Rin=43 km variable cutoff PL brighter phase brighter phase fainter phase fainter phase fainter phase fainter phase MCD model Rin=96 km 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 ) 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 ) • Stable cool disk + highly variable power-law with cutoff • Similar to the very high state in Galactic BHB • 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. M~200Msun is required to satisfy the Eddington limit See also Miyamoto et al. (this meeting) for XMM data of NGC1313 ULXs M~50Msun Summary : HR-diagram (Ldisk vs. Tin) for ULXs in high state/slim disk HXD-PIN • 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 1040 erg/s) • stable cool disk + variable power-law with cutoff • similar to the very high state in Galactic BHBs • M~200Msun to satisfy Eddington limit • X-2 (off-nucleus source) • Single MCD for fainter phase and p-free model for brighter phase • common locus on a Ldisk-Tin diagram with ASCA data. Interpreted to be in the slim disk state. M~50Msun. Data – NXB vs. CXB L (erg/s) ULXs 1040 NGC1313 X-2 1039 L∝T4 Standard Disk L∝T2 Advection Dominated Disk (Slim Disk) • No significant detection of hard tail. • Could constrain the spectral model with improved BG models • (<=3% systematic errors) 1038 Stellar mass BHs