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PhD Stundent: David Cseh Start Date: Jul 2009
End Date: Jun 2012
Responsible Advisor: Stéphane Corbel (CEA Saclay)
Co-Advisors: Rob Fender (Southampton), Luciano Burderi (Cagliari)  Ultraluminous X-ray (ULX) sources are variable, off-nucleus X-ray sources in external galaxies with luminosities greatly exceeding those achievable by Galactic Black Holes (the remnants of massive stars), assuming isotropic emission. The black hole masses required to produce such high luminosities are significantly above those expected from stellar black holes. Thus, assuming isotropic emission, the existence of ULXs would require a new class of "intermediate-mass'' black holes, with masses as high as a few hundreds to thousands solar masses.
An important piece of information for our understanding of ULXs comes from the discovery of huge ionized bubble nebulae around a significant fraction of unobscured ULXs in nearby galaxies, that allowed an independent measure of the (ionizing) X-ray luminosity. For some of these sources, we are also starting to observe associated radio nebulae. The nebulae tend to be quite large, with sizes ranging from 200 to 400 pc. These large nebulae could be interpreted as remnants of an extremely powerful supernova, which was more than 10 times more energetic than a normal supernova. Such ``hypernovae'' may also create Gamma-Ray Bursts, potentially linking ULXs with Gamma-Ray Bursts. An alternative interpretation of the nebulae, which is supported by the recent detection of a radio counterpart, is that they are continuously energized by unseen jets produced by the ULX. The discovery of associated optical/radio nebulae with ULXs has therefore opened up a new window to uncover the nature of ULXs. Indeed, nebulae powered by photo-ionization or jets provide a potential means to measure the total energy output of a ULX in all directions. This capability could potentially enable us to finally answer the question if the X-rays from the ULX are beamed along our line of sight.
In collaboration with University of Iowa (USA) and the University of Southampton (UK), the PhD student will work on radio and optical data in order to characterize the properties of radio nebulae associated with a few nearby ULXs. By using these nebulae as a calorimeter, (s)he will be able to measure the total energy output of ULX independently. In addition, the student will also compare the properties of the ULX nebula with those of Galactic stellar mass black holes and particularly the microquasars. The researcher will make use of a large campaign of multi-wavelength observations, acquired with the various nodes of the ITN, that will help in characterizing both populations of black holes. If ULXs are indeed intermediate mass black holes, this project will help bridge the gap between stellar and supermassive black holes.
The project involves using the most sensitive observatories available to date, i.e., the VLA and ATCA for radio observations, the VLT (optical) and XMM-Newton and Chandra for X- ray observations. At the end of the PhD project, the researcher will have gained the expertise to reduce data at many wavelengths, which is now recognised as being of prime importance for the development of a future independent carrier at a time when many new major facilities are being built, such as LOFAR, ALMA, Simbol-X, JWST, etc...
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