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

Bibcode
2010ApJ...724L.148G
DOI
10.1088/2041-8205/724/2/L148
Grisé, F.; Kaaret, P.; Feng, H.; Kajava, J. J. E.; Farrell, S. A.
Bibliographical reference

The Astrophysical Journal Letters, Volume 724, Issue 2, pp. L148-L152 (2010).

Advertised on:
12
2010
Number of authors
5
IAC number of authors
0
Citations
35
Refereed citations
32
Description
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 × 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.
Type
Refereed
Accretion
Black hole physics

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