Scientists have detected supermassive black holes that are millions or billions as massive as the Sun, as well as black holes that are only five to 30 times as massive as the Sun. But there’s another class of object that has proved trickier to spot—intermediate-mass black holes (IMBHs), which have masses equal to hundreds or thousands of Suns but have never been clearly identified by scientists.
Now, scientists led by Dacheng Lin, an astrophysicist at the University of New Hampshire, have identified “smoking-gun evidence” of an IMBH, making it the strongest evidence of these rare objects yet, according to a study published on Tuesday in The Astrophysical Research Letters.
This mid-sized black hole gave itself away by chowing down on a star, a process that emitted a massive blast of X-ray light that can be seen from Earth. Based on these observations, Lin’s team calculated that the black hole is about 50,000 times as massive as the Sun, and that it is located in a galaxy hundreds of millions of light years from the Milky Way.
“The most promising explanation for the source is that it is an IMBH in an off-center star cluster with the X-ray/optical outburst due to a tidal disruption event (TDE), in which a star having a close encounter with the BH was tidally disrupted and subsequently accreted, producing the multiwavelength flare,” the team said in the paper.
The flare, which has the unwieldy name 3XMM J215022.4−055108, was first detected in 2006 by NASA's Chandra X-ray Observatory and the European Space Agency’s X-ray Multi-Mirror Mission (XMM-Newton). At that time, it was unclear if this burst of X-rays was caused by an extragalactic IMBH or a neutron star—the collapsed corpse of a star—within our own Milky Way.
To pinpoint the flare’s origins, Lin and his colleagues followed up on the older observations using the Hubble Space Telescope in May 2018. Hubble was able to track the location of the X-rays to a star cluster far from the Milky Way, which ruled out the idea that the flare came from a neutron star within our own galaxy. The team was then able to estimate the black hole’s size based on the luminosity and shape of the flare.
There are a handful of other potential detections of mid-sized black holes, but none are as clear-cut as the one observed by Lin and his colleagues. "The main difference is that our object is tearing a star apart, providing strong evidence that it is a massive black hole, instead of a stellar-mass black hole," Lin said in a statement.
"Intermediate-mass black holes are very elusive objects, and so it is critical to carefully consider and rule out alternative explanations for each candidate,” he added. “That is what Hubble has allowed us to do for our candidate."
Scientists know that stellar-mass black holes are formed by the explosions of large stars and that supermassive black holes gain mass until they become the center of large galaxies. But very little is known about the formation and evolution of IMBHs, which is why it is so important to find more of these hidden gravitational gobblers.
“It is very important to search for more objects like this,” said Lin in an email. “A sizable sample is needed to understand how they are form and where they reside as a population. So my main task is to search for more objects like this from future X-ray observations by various observatories.”
Update : This article has been updated to include comments from lead author Dacheng Lin.