A transition to a low/soft state in the ultraluminous compact X-ray source Holmberg II X-1

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 similar to2 x 10(40) 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 similar to4 and the spectrum softened. In all cases, a soft excess component, which can be described by a simple or multicolor disk blackbody (kT similar to 120-170 eV), is statistically required in addition to a power-law continuum (Gamma similar to 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.