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Title: A deep XMM-Newton observation of the ultraluminous X-ray source Holmberg II X-1: the case against a 1000 M solar black hole
Authors: Goad, Michael R.
Roberts, T. P.
Reeves, J. N.
Uttley, P.
First Published: 1-Jan-2006
Publisher: Royal Astronomical Society (RAS)
Citation: Monthly Notices of the Royal Astronomical Society, 2006, 365 (1), pp.191-198
Abstract: We present results from a 112-ks long look by XMM-Newton at the ultraluminous X-ray source (ULX) Holmberg II X-1 (Ho II X-1), long thought to be the one of best candidates for the missing class of intermediate-mass black holes (IMBHs). Our data comprises the first high-quality XMM-Newton/RGS (reflection grating spectrometer) spectrum of an ULX, and an XMM-Newton/EPIC (European Photo Imaging Camera) spectrum with unprecedented signal-to-noise ratio. A detailed timing analysis shows that any variability on time-scales of minutes to hours is very weak (less than a few per cent fractional rms), though larger amplitude variations on much shorter time-scales could be hidden by photon counting statistics. This result suggests that if Ho II X-1 harbours an IMBH, then we are observing this source in a highly unusual and atypical state when compared with the known variability behaviour of other accreting systems of large mass. Moreover, unlike galactic X-ray binaries, our spectral analysis indicates the possible presence of an optically thick low-temperature corona. Taken together our timing and spectral analysis suggests that the compact companion is most likely a high-luminosity analogue of black hole binary systems similar to GRS 1915+105, the galactic microquasar, harbouring a compact object of mass no greater than 100 M⊙.
DOI Link: 10.1111/j.1365-2966.2005.09702.x
Version: Publisher Version
Status: Peer-reviewed
Type: Article
Rights: This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2005 the authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved. Deposited with reference to the publisher’s archiving policy available on the SHERPA/RoMEO website.
Appears in Collections:Published Articles, Dept. of Physics and Astronomy

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