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

TL;DR

This study shows that in Holmberg II X-1, the spectral state does not correlate with luminosity, indicating that factors other than brightness influence spectral changes in ULXs, similar to Galactic black hole binaries.

Contribution

It demonstrates that spectral states in Holmberg II X-1 are not solely determined by luminosity, challenging previous assumptions about ULX behavior.

Findings

Spectral properties remain stable despite large luminosity variations.

Spectral states differ at similar luminosities across different observation periods.

Spectral state is not solely driven by X-ray luminosity in this ULX.

Abstract

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.