Integral field spectroscopy of the ultraluminous X-ray source Holmberg II X-1

We present optical integral field observations of the H II region containing the ultraluminous X-ray source Holmberg II X-1. We confirm the existence of an X-ray ionized nebula as the counterpart of the source owing to the detection of an extended He II lambda4686 region (21 x 47 pc) at the Chandra ACIS-S position. An extended blue object with a size of 11 x 14 pc is coincident with the X-ray/ He II lambda4686 region, which could indicate that it is either a young stellar complex or a cluster. We have derived an X-ray to optical luminosity ratio of L-X/ L-B greater than or equal to 170, and presumable it is L-X/ L-B similar to 300-400 using the recent HST ACS data. We find a complex velocity dispersion at the position of the ULX. In addition, there is a radial velocity variation in the X-ray ionized region found in the He II emission of +/- 50 km s(-1) on spatial scales of 2 - 3. We believe that the putative black hole not only ionizes the surrounding HII gas, but also perturbs it dynamically ( via jets or the accretion disk wind). The spatial analysis of the public Chandra ACIS-S data reveals a point-like X-ray source and gives marginal indication of an extended component (much less than 15% of the total flux). The XMM-Newton EPIC-PN spectrum of HoII X-1 is best fitted with an absorbed power law in addition to either a thermal thick plasma or a thermal thin plasma or a multi-colour disk black body (MCD). In all cases, the thermal component shows a relatively low temperature ( kT similar to0.14- 0.22 keV). Finally we discuss the optical/X- ray properties of HoII X-1 with regards to the possible nature of the source. The existence of an X-ray ionized nebula coincident with the ULX and the soft X-ray component with a cool accretion disk favours the interpretation as an intermediate-mass black hole (IMBH). However, the complex velocity behaviour at the position of the ULX indicates a dynamical influence of the black hole on the local HII gas.