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

TL;DR

This study reports radio observations of the intermediate-mass black hole candidate ESO 243-49 HLX-1, estimating its mass and analyzing the origin of its radio emission during an X-ray outburst.

Contribution

First radio detection of ESO 243-49 HLX-1 during an X-ray state, providing an independent black hole mass estimate and discussing emission origins.

Findings

Radio emission detected during X-ray hard state

Estimated black hole mass upper limit of ~2.8 million solar masses

Radio emission likely Doppler-boosted, not from star formation or nebula

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 $M_{\rm{BH}}\leq2.8^{+7.5}_{-2.1} \times 10^{6}$ M$_{\odot}$ at 90% 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 suggests a variable radio source and we briefly discuss the properties of the previously detected flares and compare them with microquasars and active galactic nuclei.