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

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

2010-11-09

arxiv.org/abs/1011.2231v1

Astrophysical Journal Letters724:L148-L152, 2010

Astronomy and Astrophysics

Astrophysics

High Energy Astrophysical Phenomena

5 pages, 3 figures, ApJL in press

Scientific paper

10.1088/2041-8205/724/2/L148

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.

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