| |
| |
The two main classes of discrete galactic X-ray sources, X-ray binaries and supernova remnants (SNR), have been known and relatively well understood for decades. A third class of galactic X-ray sources was detected with Einstein thanks to its high spatial resolution and its large collecting area (Fabbiano 1989).
These objects have become known as Ultraluminous X-ray sources (ULX) and they have 0.5-10 keV luminosities of 10**(39-41) erg/s, generally higher than black hole binaries such as Cyg X-1 and SMC X-1, but lower than that of active galactic nuclei (AGN).
Assuming Eddington luminosities, this corresponds to accretion onto black holes of between ten and several hundred solar masses suggesting intermediate-mass black holes (IMBHs; see Colbert & Mushotzky 1999, Miller & Colbert 2004 for a review). ULX are not located in the dynamical center of their host galaxies and thus they are not caused by sub-Eddington accretion onto a central AGN-type super-massive black hole.
The identification of the optical counterparts of ULX is essential to determine the nature of these objects. The number of optically identified ULX is still
limited, only a small number of reliable optical counterparts are known to date.
|
|
| |
| |
|
| |
|---|
| |
CFHT archival H-alpha image of the HoII X-1 region. The MPFS FOV and the PMAS mosaic FOV are superposed. The small black and the large red circles give the 90% confidence circles of the Chandra ACIS-S and the ROSAT HRI positions, respectively. The small white circle shows the location of the radio peak at 6, 20, and 90 cm wavelengths (see Tongue & Westpfahl 1995). Long-slit positions (1-5) are indicated with dotted lines.
|
|
| |
Here 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 (4686 A) 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 (4686 A) region, which could indicate that it is either a young stellar complex or a cluster.
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 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 XMM-Newton 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).
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.
|
|
| |
|
|
| |
The co-added PMAS spectrum at the X-ray position (see Figure above) clearly shows an He II (4686) emission line.
|
