X-ray spectral state is not correlated with luminosity in Holmberg II X-1

Abstract

The ultraluminous X-ray source (ULX) Holmberg II X-1 has been observed over 4 months in 2009/2010 by the Swift observatory. The source luminosity varied by a factor of up to 14, reaching a maximum 0.3-10 keV luminosity of ~3.0E40 erg/s. The spectral properties do not vary much over these 4 months, with only a slight monotonic increase of the hardness ratio with the count rate. This means that the erratic flaring activity of the source is not associated with spectral changes, as seen in other ULXs. Conversely, comparison with data obtained by Swift in 2006 shows a completely different picture: while at a luminosity also seen in the 2009/2010 data, the source appears with a hard spectrum. Thus, it appears that, as in Galactic black hole binaries, spectral states in this ULX are not determined only by the X-ray luminosity.

5 pages, 3 figures, ApJL in press

References in corpus (6)

• X-ray Properties of Black-Hole Binaries
• Super-critically accreting stellar-mass black holes as ultraluminous X-ray sources
• XMM-Newton observations of the brightest Ultraluminous X-ray sources
• Masses, Beaming and Eddington Ratios in Ultraluminous X-ray Sources
• X-ray observations of ultraluminous X-ray sources
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• NuSTAR, XMM-Newton and Suzaku Observations of the Ultraluminous X-ray Source Holmberg II X-1
• Discovery of a recurrent spectral evolutionary cycle in the ultra-luminous X-ray sources Holmberg II X-1 and NGC 5204 X-1
• Uncovering Red and Dusty Ultraluminous X-ray Sources with Spitzer
• Evolution of the spectral curvature in the ULX Holmberg II X-1
• Spectral State Transitions of the Ultraluminous X-ray Source IC 342 X-1
• Long-term Spectral Variability of the Ultra-luminous X-ray source Holmberg IX X--1
• Investigating the nature of the ultraluminous X-ray sources in the galaxy NGC 925
• Modelling multiwavelength emission of Ultra-luminous X-ray Sources accreting above the Eddington limit
• Multi-wavelength spectroscopic analysis of the ULX Holmberg II X-1 and its nebula suggests the presence of a heavy black hole accreting from a B-type donor
• Simultaneous X-ray/UV observations of ultraluminous X-ray source Holmberg II X-1 with Indian space mission AstroSat