Mon. Not. R. Astron. Soc., 404, 42-47 (2010/May-1)
In-depth studies of the NGC253 ULXs with XMM-Newton: remarkable variability in ULX1, and evidence for extended coronae.
Abstract (from CDS):
We examined the variability of three ultraluminous X-ray sources (ULXs) in the 2003, 110ks XMM-Newton observation of NGC253. Remarkably, we discovered ULX1 to be three times more variable than ULX2 in the 0.3-10 keV band, even though ULX2 is brighter. Indeed, ULX1 exhibits a power density spectrum that is consistent with the canonical high state or very high/steep power-law states, but not the canonical low state. The 0.3-10 keV emission of ULX1 is predominantly non-thermal, and may be related to the very high state. We also fitted the ULX spectra with disc blackbody, slim disc and convolution Comptonization (simpl⊗diskbb) models. The brightest ULX spectra are usually described by two emission components (disc blackbody + Comptonized component); however, the simpl model results in a single emission component, and may help determine whether the well-known soft excess is a feature of ULX spectra or an artefact of the two-component model. The simpl models were rejected for ULX3 (and also for the black hole + Wolf-Rayet binary IC10 X-1); hence, we infer that the observed soft-excesses are genuine features of ULX emission spectra. We use an extended corona scenario to explain the soft excess seen in all the highest quality ULX spectra, and provide a mechanism for stellar mass black holes to exhibit super-Eddington luminosities while remaining locally sub-Eddington.
© 2010 The Author. Journal compilation © 2010 RAS
black hole physics - galaxies: individual: NGC253 - X-rays: binaries - X-rays: general
Table 1: [B2010b] NGC253 ULXN (Nos 1-3).
ULXs 1, 2, 3 in NGC 253 are respectively [BGK2008] S64, S102, S123 in SIMBAD.
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