Speaker
Description
Intermediate-mass black holes (IMBHs) constitute the missing link between stellar-mass and supermassive black holes (BHs). As such, they are crucial to trace BH growth and its connection with galaxy evolution. However, their presence in the local universe remains elusive: less than a dozen X-ray-bright IMBH candidates have been found so far. Among these, ESO243-49 HLX-1 stands out as one of the most convincing and represents the perfect benchmark to analyse properties, behaviours and host environments of this population.
Located close to the outskirts of the spheroidal galaxy ESO243-49, this variable X-ray source exhibited a total of eight recurrent outbursts between 2008 and 2017 and has since settled in a Low/Hard state. The nature of these outbursts and its optical counterpart, detected from the NIR to the far-UV bands, have long been subject of debate.
In this talk I will present new results from the most recent analysis of HLX-1's optical/UV and X-ray evolution using archival HST, Chandra, and Swift data. The connection between the optical and X-ray emission confirms the constant red component as a host globular cluster or UCD. More importantly, the data challenge the irradiated disk interpretation of the slowly declining blue component, favouring instead emission from an outflow photosphere similar to those seen in tidal disruption events (TDEs). This may also explain the origin of the X-ray oscillations as an inner disk instability in the early years after the TDE. Finally, I will show that our observations resolve a mysterious far-UV source near HLX-1 into a ring-like star-forming structure, and explore the possibility of a physical association with HLX-1 and its host cluster.