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

F. Grisé; P. Kaaret; H. Feng; J. J.E. Kajava; S. A. Farrell

doi:10.1088/2041-8205/724/2/L148

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

F. Grisé, P. Kaaret, H. Feng, J. J.E. Kajava, S. A. Farrell

Astronomy & Astrophysics

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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.

Original language	English (US)
Pages (from-to)	L148-L152
Journal	Astrophysical Journal Letters
Volume	724
Issue number	2 PART 2
DOIs	https://doi.org/10.1088/2041-8205/724/2/L148
State	Published - Dec 1 2010

All Science Journal Classification (ASJC) codes

Astronomy and Astrophysics
Space and Planetary Science

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

AU - Grisé, F.

AU - Kaaret, P.

AU - Feng, H.

AU - Kajava, J. J.E.

AU - Farrell, S. A.

PY - 2010/12/1

Y1 - 2010/12/1

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

Abstract

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