On the radio properties of the intermediate-mass black hole candidate ESO 243-49 HLX-1

Abstract

We present follow-up radio observations of ESO 243-49 HLX-1 from 2012 using the Australia Telescope Compact Array (ATCA) and the Karl G. Jansky Very Large Array (VLA).We report the detection of radio emission at the location of HLX-1 during its hard X-ray state using the ATCA. Assuming that the 'Fundamental Plane' of accreting black holes is applicable, we provide an independent estimate of the black hole mass of MBH ≤ 2.8+7.5-2.1 × 106 MΘ at 90 per cent confidence. However, we argue that the detected radio emission is likely to be Doppler-boosted and our mass estimate is an upper limit. We discuss other possible origins of the radio emission such as being due to a radio nebula, star formation, or later interaction of the flares with the large-scale environment. None of these were found adequate. The VLA observations were carried out during the X-ray outburst. However, no new radio flare was detected, possibly due to a sparse time sampling. The deepest, combined VLA data suggest a variable radio source and we briefly discuss the properties of the previously detected flares and compare them with microquasars and active galactic nuclei.