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Date and Time of the Query: 2019-06-20 T02:14:52 PDT
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For refcode 2009ApJ...695.1614S:
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2009ApJ...695.1614S The XMM-NEWTON Long Look of NGC 1365: Lack of a High/Soft State in its Ultraluminous X-Ray Sources Soria, R.; Risaliti, G.; Elvis, M.; Fabbiano, G.; Bianchi, S.; Kuncic, Z. Abstract. Based on our long (~300 ks) 2007 XMM-Newton observation of the Seyfert galaxy NGC 1365, we report here on the spectral and timing behavior of two ultraluminous X-ray sources (ULXs), which had previously reached isotropic X-ray luminosities L_X_ ~ 4 x 10^40^ erg s^--1^ (0.3-10 keV band). In 2007, they were in a lower state (L_X_ ~ 5 x 10^39^ and 1.5 x 10^39^ erg s^--1^ for X1 and X2, respectively). Their X-ray spectra were dominated by power laws with photon indices {GAMMA} ~ 1.8 and 1.2, respectively. Thus, their spectra were similar to those at their outburst peaks. Both sources have been seen to vary by a factor of 20 in luminosity over the years, but their spectra are always dominated by a hard power law; unlike most stellar-mass black holes, they have never been found in a canonical high/soft state dominated by a standard disk. The lack of a canonical high/soft state seems to be a common feature of ULXs. We speculate that the different kind of donor star and/or a persistently super-Eddington accretion rate during their outbursts may prevent accretion flows in ULXs from settling into steady standard disks. Key words: black hole physics, X-rays: binaries, X-rays: individual: NGC 1365
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