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The Australia Telescope National Facility Colloquium
15:30-16:30 Wed 09 Aug 2006

ATNF Basement Meeting Room

Zdenka Kuncic

(University of Sydney)

Unravelling the Nature of Ultra-Luminous X-ray Sources - Dr Zdenka Kuncic Colloquium

Ultra-Luminous X-ray sources (ULXs) are arguably the most puzzling
sources discovered by X-ray satellites. While it is widely accepted
that their extraordinary X-ray luminosities result from accretion
onto a black hole in a close interacting binary system, the mass of
the black hole remains a highly contentious issue. In particular, it
has been argued that ULXs are powered by accretion onto an
intermediate mass black hole (IMBH). However, formation scenarios for
IMBHs are still highly implausible. Alternatively, ULXs may represent
the extreme end of the X-ray binary population; they could be
extragalactic microblazars or microquasars. In this talk, I will
present new results that demonstrate relativistic beaming can indeed
account for the observed X-ray luminosities, without the need for
invoking unusually high black hole masses. Furthermore, these results
also indicate that, despite the beaming, synchrotron radio emission
from relativistic jets in ULXs is too weak to be detected with
current instrument thresholds. This is consistent with the high rate
of non-detections over detections from radio counterpart searches to

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