A rogue, middle-mass black hole has been spotted disrupting an orbiting star in the halo of a distant galaxy, and it’s all thanks to the observing powers of the Hubble Space Telescope and Chandra X-ray Observatory. However, exactly what the black hole is doing to the star remains in question as there are conflicting X-ray measurements.
Black holes come in different size classes. At the smaller end of the scale are the stellar-mass black holes born in the ashes of supernova explosions. At the top end of the scale are the supermassive black holes, which can grow to have many millions or billions of times the mass of our sun, lurking in the hearts of galaxies. In between those categories are intermediate-mass black holes (IMBH), which have mass ranging from hundreds up to 100,000 solar masses, or thereabouts.
“They represent a crucial missing link in black hole evolution between stellar mass and supermassive black holes,” Yi-Chi Chang of the National Tsing Hua University in Hsinchu, Taiwan said in a statement.
The problem is that intermediate-mass black holes are hard to find, partly because they tend not to be as active as supermassive black holes or as obvious as a stellar-mass black hole when its progenitor star goes supernova.
However, occasionally, an IMBH will spark to life when it instigates a tidal disruption event. This happens when a star or gas cloud gets too close to the black hole and gravitational tidal forces rip the star or gas cloud apart, producing bursts of X-rays.
“X-ray sources with such extreme luminosity are rare outside galaxy nuclei and can serve as a key probe for identifying elusive IMBHs,” said Chang.
In 2009, Chandra spotted anomalous X-rays originating from a region 40,000 light-years from the center of a giant elliptical galaxy called NGC 6099, which lies 453 million light-years from us. This bright new X-ray source was called HLX-1, and its X-ray spectrum indicated that the source of the X-rays was 5.4 million degrees Fahrenheit (3 million degrees Celsius), a temperature consistent with the violence of a tidal disruption event.
But what followed was unusual. The X-ray emissions reached a peak brightness in 2012 when observed by the European Space Agency’s XMM-Newton X-ray space telescope. When XMM-Newton took another look in 2023, it found the X-ray luminosity had substantially dwindled. In the meantime, the Canada–France Hawaii Telescope had identified an optical counterpart for the X-ray emission, one that was subsequently confirmed by Hubble.
There are two possible explanations for what happened. The first is that Hubble’s spectrum of the object shows a tight, small cluster of stars swarming around the black hole. The black hole might have once been at the core of a dwarf galaxy that was whittled down — unwrapped like a Christmas present — by the gravitational tides of the larger NGC 6099. This process would have stolen away all the dwarf galaxy’s stars to leave behind a free-floating black hole with just a small, tight grouping of stars left to keep it company. But the upshot of this is that the cluster of stars is like a stellar pantry to which the black hole occasionally goes to feast.
It seems certain that a tidal disruption event involving one of these stars is what Chandra and Hubble have witnessed, but was the star completely destroyed? One possibility is that the star is on a highly elliptical orbit, and at its perihelion (closest point to the black hole) some of the star’s mass is ripped away — but the star managed to survive for another day. This would potentially explain the X-ray light curve: The emission from 2009 was as the star was nearing perihelion, while the peak in 2012 was during perihelion, and the latest measurements in 2023 would be when the star was farthest from the black hole and not feeling its effects so much. We might then expect another outburst of X-rays during its next perihelion, whenever that might be.
However, there’s an alternative hypothesis: The star may have been stripped apart a piece at a time, forming a stream of material around the black hole.
When Chandra first detected the X-ray emission from the tidal disruption event, this stream was just beginning to wrap back on itself, the self-intersection giving rise to shock-heating that produced X-rays. Then, the 2012 measurements would have been of a fully-fledged hot accretion disk of gas, the star by now completely ripped apart. The material within this disk would have spiraled into the black hole’s maw, thus depleting the disk, which would explain why it is much less luminous in X-rays in 2023.
Picking out the correct scenario apart will require further surveillance.
“If the IMBH is eating a star, how long does it take to swallow the star’s gas? In 2009, HLX-1 was fairly bright. Then, in 2012, it was about 100 times brighter, and then it went down again,” Roberto Soria of the Italian National Institute for Astrophysics (INAF), who is a co-author of a new study describing the observations of HLX-1, said in the statement. “So now we need to wait and see if it’s flaring multiple times, or if there was a beginning, a peak, and now it’s just going to go down all the way until it disappears.”
Making new observations of an IMBH such as HLX-1 is key to better understanding the role they play in the black hole ecosystem. One model suggests that supermassive black holes might form and grow through the merger of many IMBH, but nobody knows how common intermediate-mass black holes are in the universe.
“So if we are lucky, we’re going to find more free-floating black holes suddenly becoming X-ray bright because of a tidal disruption event,” said Soria. “If we can do a statistical study, this will tell us how many of these IMBHs there are, how often they disrupt a star, [and] how bigger galaxies have grown by assembling smaller galaxies.”
Alas, Chandra, XMM-Newton and Hubble all have small fields of view, meaning that they only see small patches of the sky. Because we don’t know where the next tidal disruption event might take place, the chances of our space telescopes looking in the right place at the right time are slim.
In essence, Chandra got lucky back in 2009.
Fortunately, help is now on hand. The Vera C. Rubin Observatory comes fully online later this year to begin a 10-year all-sky survey, and spotting the flares of tidal disruption events will be a piece of cake for it. Once it finds such an event, Hubble and Chandra will know where to look and can follow up on it. IMBHs have remained mostly hidden for now, but their time in the shadows is coming to an end.
The findings were published on April 11 in The Astrophysical Journal.