Known black holes in the universe are either stellar-size, a handful of times the mass of Sun, or supermassive, weighing millions if not billions of that of a regular star. The lack of in-between objects has perplexed astronomers for a long time, but in the last several years candidates of these “intermediate-mass black holes” (IMBH) have been spotted. And now a team has found the best evidence yet for one.

As reported in The Astrophysical Journal Letters, the international team of astronomers followed up on an X-ray flare from a distant galaxy spotted by NASA’s Chandra telescope and the European XMM-Newton telescope back in 2006. The observations suggested a black hole 50,000 times the mass of the Sun, but the data was not strong enough to exclude alternative explanations. A very hot neutron star within the galaxy, which is located 805 million light-years from Earth, may have had a similar signature.

To clarify its nature, the team followed up the source, known as 3XMM J215022.4?055108, with the Hubble Space Telescope. They discovered that it was not actually located in the galaxy's center, where a black hole would normally sit, but just outside it. The emission emanated from a stellar cluster outside the main galaxy, possibly the stripped core of an ancient small dwarf galaxy that lost its star to the much much heavier nearby galaxy, which is where astronomers expected to find an IMBH, strengthening the case for this being an elusive mid-size black hole. 

"Intermediate-mass black holes are very elusive objects, and so it is critical to carefully consider and rule out alternative explanations for each candidate. That is what Hubble has allowed us to do for our candidate," lead author Dacheng Lin of the University of New Hampshire said in a statement. 

The black hole flared up because it ripped a star apart. This not only allowed for its original detection but, thanks to continuous observations over the years, also allowed for a detailed characterization of the disruption event. The team considers this a crucial difference between this finding and previous candidates for intermediate-mass black holes, like HLX-1.

"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 as people often worry about for previous candidates including HLX-1," Lin said.

There are still many unknowns when it comes to these objects. Their formation and their potential relationship with the other types of black holes is still shrouded in mystery, but hopefully finding one mid-size black hole will open the door to finding others hiding out there.