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

Abstract

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 λ4686 region (21 × 47 pc) at the Chandra ACIS-S position. An extended blue object with a size of 11 × 14 pc is coincident with the X-ray/He II λ4686 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 LX/LB≥170, and presumable it is LX/LB ∼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 (≪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 ∼ 0.14-0.22 keV). Finally we discuss the optical/X-ray properties of HoII X-l 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.