Mon. Not. R. Astron. Soc., 365, 191-198 (2006/January-1)
A deep XMM-Newton observation of the ultraluminous X-ray source Holmberg II X-1: the case against a 1000-M☉black hole.
GOAD M.R., ROBERTS T.P., REEVES J.N. and UTTLEY P.
Abstract (from CDS):
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☉.
2005 The Authors. Journal compilation © 2005 RAS
black hole physics - X-rays: binaries - X-rays: galaxies
Holmberg II X-1 is [KWB2002] 1 in Simbad.
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