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

Fabien Grise, P. Kaaret, H. Feng, J. J.E. Kajava, S. A. Farrell

Research output: Contribution to journalArticle

27 Citations (Scopus)

Abstract

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.

Original languageEnglish (US)
JournalAstrophysical Journal Letters
Volume724
Issue number2 PART 2
DOIs
StatePublished - Dec 1 2010

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luminosity
x rays
Swift observatory
erratic
erg
hardness
observatory

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Grise, Fabien ; Kaaret, P. ; Feng, H. ; Kajava, J. J.E. ; Farrell, S. A. / X-ray spectral state is not correlated with luminosity in Holmberg II X-1. In: Astrophysical Journal Letters. 2010 ; Vol. 724, No. 2 PART 2.
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abstract = "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.",
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X-ray spectral state is not correlated with luminosity in Holmberg II X-1. / Grise, Fabien; Kaaret, P.; Feng, H.; Kajava, J. J.E.; Farrell, S. A.

In: Astrophysical Journal Letters, Vol. 724, No. 2 PART 2, 01.12.2010.

Research output: Contribution to journalArticle

TY - JOUR

T1 - X-ray spectral state is not correlated with luminosity in Holmberg II X-1

AU - Grise, Fabien

AU - Kaaret, P.

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AU - Kajava, J. J.E.

AU - Farrell, S. A.

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N2 - 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.

AB - 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.

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