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The Ultraluminous X-ray Source in Holmberg IX and its Environment

Published online by Cambridge University Press:  12 May 2006

Fabien Grisé ,
Manfred W. Pakull  and
Christian Motch
Fabien Grisé
Affiliation:
Observatoire Astronomique de Strasbourg, 13 rue de l'Université, FRANCE email: [email protected]
Manfred W. Pakull
Affiliation:
Observatoire Astronomique de Strasbourg, 13 rue de l'Université, FRANCE email: [email protected]
Christian Motch
Affiliation:
Observatoire Astronomique de Strasbourg, 13 rue de l'Université, FRANCE email: [email protected]
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Abstract

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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 $10^{40}$ erg/s. It is located in a huge (400pc$\,{\times}\,$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 HeII$\lambda$4686 emission (equivalent width of $\mbox{10 \AA}$) 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.

Keywords

galaxies: individual (IC 342)ISM: supernova remnantsX-rays: galaxiesX-rays: binaries
Type
Contributed Papers
Information
Proceedings of the International Astronomical Union , Volume 1 , Symposium S230: Populations of High Energy Sources in Galaxies , August 2005 , pp. 302 - 303
Copyright
2006 International Astronomical Union