A Transition to a Low/Soft State in the Ultraluminous Compact X-Ray Source Holmberg II X-1

Abstract

We present three XMM-Newton observations of the ultraluminous compact X-ray source Holmberg II X-1 in its historical brightest and faintest states. The source was in its brightest state in 2002 April with an isotropic X-ray luminosity of ~2 × 1040 ergs s-1 but changed to a peculiar low/soft state in 2002 September in which the X-ray flux dropped by a factor of ~4 and the spectrum softened. In all cases, a soft excess component, which can be described by a simple or multicolor disk blackbody (kT ~ 120-170 eV), is statistically required in addition to a power-law continuum (Γ ~ 2.4-2.9). Both spectral components became weaker and softer in the low/soft state; however, the dramatic variability is seen in the power-law component. This spectral transition is opposite to the "canonical" high/soft-low/hard transitions seen in many Galactic black hole binaries. There is a possible contribution from an optically thin thermal plasma. When this component is taken into account, the spectral transition appears to be normal—a drop of the power-law flux and a slightly softer blackbody component in the low state.