X-ray and optical decline of the intermediate mass black hole HLX-1

ABSTRACT HLX-1 is a prominent intermediate-mass black hole (IMBH) candidate, historically exhibiting recur-rent X-ray outbursts with spectral state transitions analogous to those observed in stellar-mass black holes. Here, we present new Hubble Space Telescope , Chandra , and Swift observations from 2018–2022 to characterise the late-time flux decline. HLX-1 has persisted in a low X-ray luminosity state ( L X ≈ a few × 10 39 erg s − 1 ) since the end of its last outburst in 2017. We observe a significant decoupling between the X-ray and optical/UV emission: while the X-rays have faded by at least two orders of magnitude from peak outburst luminosity (in 2010) to the current low state, the optical/UV flux has declined much more slowly over the same time. This results in an X-ray/optical luminosity ratio in-consistent with X-ray reprocessing in a standard accretion disk, as this would require an unphysical reprocessing fraction > 100% at late times. Instead, we find that the optical/UV evolution is well-fit by a cooling, expanding photosphere ( T ≈ 30, 000 K), similar to the late-stage evolution seen in tidal disruption events (TDEs). The redder component of the optical emission is instead consistent with the old stellar population of a massive star cluster (IMBH host). The pre-2017 X-ray bursting phase is consistent with simulations of disk instabilities in TDE evolution: this strengthens the scenario of HLX-1 as an IMBH TDE. Furthermore, our observations resolve the morphology and flux of the mysterious far-UV emitter, seen in projection next to HLX-1, into a ring-like star-forming structure. We re-assess the possibility that HLX-1 and its host star cluster are physically associated with this starburst dwarf, perhaps via a high-speed collision.