X-ray Spectral State is not Correlated with Luminosity in Holmberg II X-1

The ultraluminous X-ray source (ULX) Holmberg II X-1 has been observed over four 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 ~3.0 X 1040 erg s-1. The spectral properties do not vary much over these fo...

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Bibliographic Details
Published in:Astrophysical journal. Letters 2010-12, Vol.724 (2), p.L148-L152
Main Authors: Grisé, F, Kaaret, P, Feng, H, Kajava, J. J. E, Farrell, S. A
Format: Article
Language:English
Subjects:
ACCRETION DISKS
ASTROPHYSICS, COSMOLOGY AND ASTRONOMY
BLACK HOLES
COSMIC RAY SOURCES
COSMIC X-RAY SOURCES
COUNTING RATES
FLARING
LUMINOSITY
OPTICAL PROPERTIES
PHYSICAL PROPERTIES
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Summary:The ultraluminous X-ray source (ULX) Holmberg II X-1 has been observed over four 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 ~3.0 X 1040 erg s-1. The spectral properties do not vary much over these four 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.
ISSN:2041-8205
2041-8213
DOI:10.1088/2041-8205/724/2/L148