Compact radio emission in Ultraluminous X-ray sources
  We present results from our studies of radio emission from selected
Ultraluminous X-ray (ULX) sources, using archival Giant Metrewave Radio
Telescope (GMRT) data and new European VLBI Network (EVN) observations. The
GMRT data are used to find possible faint radio emission from ULX sources
located in late-type galaxies in the Chandra Deep Fields. No detections are
found at 235, 325 and 610 MHz, and upper limits on the radio flux densities at
these frequencies are given. The EVN observations target milliarcsecond-scale
structures in three ULXs with known radio counterparts (N4449- X1, N4088-X1,
and N4861-X2). We confirm an earlier identification of the ULX N4449-X1 with a
supernova remnant and obtain the most accurate estimates of its size and age.
We detect compact radio emission for the ULX N4088-X1, which could harbour an
intermediate mass black hole (IMBH) of 10^5 M\odot accreting at a sub-Eddington
rate. We detect a compact radio component in the ULX N4861-X2, with a
brightness temperature > 10^6 K and an indication for possible extended
emission. If the extended structure is confirmed, this ULX could be an HII
region with a diameter of 8.6 pc and surface brightness temperature \geq 10^5
K. The compact radio emission may be coming from a ~ 10^5 M\odot black hole
accreting at 0.1M_Edd.
