Astronomers have found a great many black holes, both at the heart of other galaxies and in our own – they've even photographed two of them. However, in every case, this was achieved because while the hole itself was black, it influenced nearby objects, either radiating X-rays as it consumed material or an orbiting partner star. A new suspect, however, is not feeding and has no companion star with which to dance.

When gravitational force distorts space-time, it causes light to bend around it, acting like a lens. This trait has been used for improving our capacity to see distant galaxies and to detect the presence of planets as they cause temporary brightening in more distant stars as they pass in front.

However, when two telescopes scanning the Milky Way looking for these sorts of brightening events (known as microlensing) picked up an example known as OB110462 in 2011, the effect was far too large to be caused by a planet. That's one thing competing papers published in the The Astrophysical Journal agree on. 

PhD student from the University of California, Berkeley Casey Lam and her supervisor Dr Jessica Lu calculate the mass of the object to be between 1.6 and 4.4 times the mass of the Sun in a preprint available on ArXiv.org. A team led by Dr Kailash Sahu of the Space Telescope Science Institute used the same data to reach an estimate of 7.1 solar masses in the other paper. 

Neither could be a planet, and the mass is too concentrated to be a gas cloud. An ordinary star with that much material would be easily bright enough to see, leaving two options – a neutron star or a black hole. 

The dividing line between these two eventual fates for large stars is thought to be 2.2 solar masses. More than that and you have a black hole, any less and neutron pressure prevents complete collapse, leaving a neutron star. Consequently, Lam's estimate range allows both as possibilities, but makes a black hole more likely, while (as we previously reported) Salu's figure unambiguously requires a black hole.

Either way, the discovery would represent the first example of a dense stellar remnant without a companion star. "With microlensing, we're able to probe these lonely, compact objects and weigh them. I think we have opened a new window onto these dark objects, which can't be seen any other way." Lu said in a statement. 

Lam's search turned up four other microlensing events involving masses too small to be black holes, with two of them in a range that could be neutron stars, although heavy white dwarfs are also possible.

Astronomers are keen to get enough samples of solo stellar remnants to estimate their galactic population. This will make testing theories about the proportion of stars that meet this fate possible. It will also open the door to exploring the idea that some black holes with masses like these are not from stellar collapse, but left over from the Big Bang. 

Microlensing surveys of parts of the sky with a high density of stars turn up around 2,000 microlensing events each year. Most, however, involve one star passing in front of another. Studying all of these in the hope of identifying those caused by black holes would be an immense task, so Lam focussed on those that lasted the longest, having concluded these are more likely to involve black holes. OB110462 lasted two years – exceptional in a field where most events last days or weeks.

OB110462 was observed by both the Optical Gravitational Lensing Experiment (OGLE) and Microlensing Observations on Astrophysics (MOA). Moreover, the Hubble Space telescope provided follow-up data that revealed the star lensed by OB110462 still appears off from its true position ten years later thanks to lingering gravitational distortion. Although the error bars for the Hubble data are wide, they still provide enough guidance to estimate OB110462's distance and mass.

A yet-to-be-published third paper uses the conclusions from the first two to argue that while OB110462 is free-floating today, it was once part of a binary pair of stars.