Sean Farrell

Interview with Sean Farrell, postdoctoral fellow at the University of Sydney, on the possible intermediate-mass black hole ESO 243-49 HLX-1.

Why is ESO 243-40 HLX-1 a better intermediate-mass black hole candidate than some of the others reported by other researchers?

Amongst the candidate intermediate mass black holes that are ultraluminous X-ray sources (ULXs, such as M82 X-1 and G1 in M31), ESO 243-49 HLX-1 is unique in that its luminosity is a factor of 10 times higher than the next brightest object, and roughly 100-1000 times brighter than the other ULXs. ULXs are X-ray point sources that are located outside the nuclei of galaxies, but have X-ray luminosities that are greater than the theoretical Eddington limit for a stellar mass black hole. The Eddington limit is the result of the pressure balance between matter falling in to the black hole and the radiation coming out, and is a function of the black hole mass. So, bigger black holes are brighter than smaller ones.

Their extreme luminosities were originally interpreted as evidence for intermediate mass black holes, but they can also be explained by the X-rays being beamed towards us, hyper-accretion (where the Eddington limit is temporarily violated), or by slightly more massive stellar mass black holes (up to 100 times the mass of the Sun). By combining these alternative possibilities, we can explain luminosities up to roughly 10⁴¹ ergs per second. [NOTE: That is roughly 30 million times more energy than the Sun produces.] However, HLX-1 has a luminosity of 10⁴² ergs per second, and so cannot be explained via the other possibilities. This is what makes it the strongest candidate intermediate-mass black hole.

What is causing the variability in the X-ray, ultraviolet, and possibly visible-light emissions from this black hole?

The X-ray emission of HLX-1 comes entirely from accretion of matter onto the black hole, and thus the variability in X-rays is driven by the accretion rate and the structure of the accretion disc. We think that HLX-1 might be accreting from a massive companion star in a highly elliptical orbit, where it strips gas from the star every time it passes through the closest approach in the orbit. Most of the ultraviolet emission is also related to the accretion disc, so is also linked to the accretion rate, and at least some of the visible light is also likely to come from the accretion disc (mainly through reprocessing of the X-ray emission in the outer edges of the disc).

However, we currently think that a fair amount of the optical and near-infrared light is coming from a cluster of stars around the black hole, and so should not vary over time. Recent results by some of our competitors claim the detection of variability in the optical light, but so far that is unconfirmed. If it can be confirmed, it would indicate that the star cluster is older than what we currently predict, and that the accretion disc is more dominant than it seems at present.

Please describe the system as it would look from close range: the black hole, accretion disk, companion stars.

The black hole itself would be at the centre of a very hot and very bright accretion disc, which would itself be embedded in cluster of young, hot, massive stars. Both the disc and the stars would appear very blue to the human eye, but the black hole itself would probably appear to be simply a warped region of darkness at the centre of the disc. We currently think that HLX-1 might have trapped a very massive star which is orbiting around the black hole in a highly elliptical orbit, feeding the black hole every roughly 367 days when it passes close to it. During this phase gas would flow from the star to fill up the disc around the black hole. When it moves further away from the black hole accretion would slow and maybe even stop, so the disc would shrink and mostly fade away. Any residual accretion would likely occur via the strong winds of the companion star, but it would be much fainter in optical (and X-ray) light during this phase.

Does this support the idea that supermassive black holes grew from IMBH "seeds?"

It's hard for us to reconcile the detection of a young cluster of stars around HLX-1 with its position outside the plane of its host galaxy. Star formation occurs inside the discs of galaxies, not out in the halo. The only way we could explain recent star formation in the halo was if HLX-1 was the remnant of an accreted dwarf galaxy, which interacted with ESO 243-49 less than 200 million years ago. The interaction would have stripped most of the stars and gas from around the dwarf galaxy, and compressed the remaining gas to trigger star formation. This is an entirely plausible scenario if the dwarf galaxy contained an intermediate mass black hole of about 20,000 times the mass of the Sun. This therefore supports the idea that supermassive black holes may grow from IMBHs in dwarf galaxies that merge with other galaxies to steadily grow into more massive black holes.

What future observations do you plan for this system, and are you watching any similar candidates?

We have another batch of Hubble observations scheduled for the next year [2012-2013], with which we will test whether there is any variability in the optical light and therefore place more stringent constraints on the contributions from the accretion disc and the stellar population. We also have observations planned in radio and optical wavelengths to search for evidence of a past interaction with a dwarf galaxy, and hopefully also unravel the interaction history and the trajectory.

In the meantime, I'm involved in a number of projects that are currently searching for new objects like HLX-1 and others that are studying ULXs in general in an attempt to understand what the lower-luminosity objects really are.