出版社：SISSA, Scuola Internazionale Superiore di Studi Avanzati
摘要：It has been suggested that some of the ultra-luminous X-ray sources may be powered by
intermediate-mass black holes (IMBH). According to the recently established black hole activity
plane relationship, the closest IMBHs should be detectable in the radio domain with current instrumentation.
VLBI may be a powerful tool for finding the radio counterparts of ULXs and thus
identifying IMBHs, because it filters out other sources such us HII regions and resolved supernova
remnants. We have observed the most powerful ULX, M82 X-1, with the EVN and derive
an upper limit on its mass of about 500 M⊙. Future high-sensitive VLBI observations may give
tighter constraints on the masses of compact objects in ULXs and may help support or rule out
the IMBH hypothesis.
