Bibcode
Grisé, Fabien; Pakull, Manfred W.; Motch, Christian
Bibliographical reference
Populations of High Energy Sources in Galaxies Proceedings of the 230th Symposium of the International Astronomical Union, Held in Dublin, Ireland 15-19 August 2005, Edited by E. J. A. Meurs; G. Fabbiano. Cambridge: Cambridge University Press, 2006., pp.302-303
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0
2006
Citations
21
Refereed citations
19
Description
We present optical observations of an ultraluminous X-ray source (ULX)
in Holmberg IX, a dwarf galaxy near M81. The ULX has an average X-ray
luminosity of some 1040 erg/s. It is located in a huge (400pc
× 300pc) ionized nebula being much larger than normal supernova
remnants. From the observed emission lines (widths and ratios) we find
that the structure is due to collisional excitation by shocks, rather
than by photoionization.We identify the optical counterpart to be a 22.8
mag blue star (M_V=-5.0) belonging to a small stellar cluster. From
isochrone fitting of our multi-colour photometry we determine a cluster
age of some 60 Myr. We also discovered strong stellar HeIIλ4686
emission (equivalent width of 10 Å) which proves the
identification with the X-ray source, and which suggests the presence of
an X-ray heated accretion disc around the putative black hole.