Radio observations of extreme ULXs: Revealing the most powerful ULX radio nebula ever or the jet of an intermediate-mass black hole?

Abstract

The most extreme ultraluminous X-ray sources (ULXs), with LX > 5 × 1040 erg s-1, are amongst the best candidates for hosting intermediate-mass black holes (IMBHs) in the haloes of galaxies. Jet radio emission is expected from a sub-Eddington accreting IMBH in the low/hard (radio bright) state. In a search for such IMBH jet radio emission, we have observed with the Very Large Array (VLA) at 5 GHz a sample of seven extreme ULXs whose X-ray properties indicate they are in the hard state. Assuming they remain in this state, the nondetection of radio emission for six of the target sources allows us to constrain their black hole mass to the IMBH regime, thus ruling out a supermassive black hole nature. For the extreme ULX in the galaxy NGC 2276, we detect extended radio emission formed by two lobes of total flux density 1.43 ± 0.22 mJy and size ̃650 pc. The X-ray counterpart is located between the two lobes, suggesting the presence of a black hole with jet radio emission. The radio luminosity allows us to constrain the black hole mass of this source to the IMBH regime; hence, the extreme ULX in NGC 2276 could be the first detection of extended jet radio emission from an IMBH. The radio emission could also possibly come from a radio nebula powered by the ULX with a minimum total energy of 5.9 × 1052 erg, thus constituting the most powerful and largest ULX radio nebula ever observed. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.