Universe Today
According to stellar evolution models, there should be hundreds of millions of black holes floating around in our galaxy. But so far we’ve only found 70. That’s in large part because they are so hard to find, but a citizen science project from the University of Southampton is actively seeking out these hidden monsters with the help of tens of thousands of amateur astronomers through a project on Zooniverse called Black Hole Hunters.
Since black holes absorb light, astronomers can’t simply look for them the same way they look for stars, or even exoplanets. In some cases, they do emit X-rays and radio waves, but only if they happen to be actively devouring a nearby star or planet. If they are just drifting in the void, alone, they are basically completely invisible to our normal astronomical techniques.
But, like so many other parts of astrophysics, Einstein comes to the rescue. Or more accurately, his theory of general relativity does. As a black hole passes in front of a star, that star's light bends around it, creating an effect typically called “microlensing”. In the wider universe, this can result in some truly spectacular images, which are fittingly named Einstein rings.
YouTube video from the Black Hole Hunters team. Credit - Black Hole Hunters YouTube ChannelMicrolensing, though, typically only happens as a one-off, as a black hole only passes in front of a star once from our perspective. There is, however, an exception to that rule - sometimes black holes are paired up with stars that they aren’t actively feeding on. In these cases, instead of a one-off microlensing event, the lensing periodically repeats as the black hole orbits its companion star, creating an effect known as gravitational self-lensing.
That gravitational self-lensing is what the Black Hole Hunters project is actively looking for. Originally, the Black Hole Hunters project, which was created in 2021, prioritized sorting through photometry data from the Super Wide Angle Search for Planets (SuperWASP) survey. However, this ground-based data suffered from a significant amount of noise and weather gaps. So Dr. Adam McMaster, the creator of the project who was originally a student at The Open University, and who recently moved to the University of Southampton, decided to work on a different data pipeline and relaunch the project.
There were two major changes to the data pipeline - first is that now it is using data from the Transiting Exoplanet Survey Satellite (TESS). TESS, which is space-based rather than ground-based, famously searches for exoplanets. As a consequence of that, it captures the light curves of hundreds of thousands of stars every few minutes - a perfect dataset to search for gravitational self-lensing.
Fraser talks about what would happen if the Milky Way lost its black hole.The second major change, unsurprisingly perhaps, is the introduction of AI. Specifically a machine learning algorithm that classified these stellar light curves into “interesting” and “boring” camps. Doing so with modern AI tools allowed Dr. McMaster and his colleagues to filter through millions of light curves in a matter of days rather than the decades it would have taken to do by hand. However, AI machine learning algorithms can still be tricked by occurrences like sunspots and pulsating variables, and TESS itself suffers from its own glitches sometimes, so the final determination needs a set of human eyes.
Those human eyes don’t have to be trained, though, as Grace Clarke, an undergraduate at the University of Southampton, and a volunteer on the project, mentioned in a press release. Black Hole Hunters requires basic pattern recognition rather than a degree in astronomy. But what it will provide at the end of the survey is interesting areas to point more advanced telescopes at to specifically look for these types of self-lensing black holes.
Admittedly, one of the things that citizen science projects struggle with is volunteer retention, and the Black Hole Hunters project is going through a media blitz in an effort to attract more of them to finish its classification tasks. According to the project’s Zooniverse page, it’s about 43% complete, with almost 23,000 volunteers contributing almost 12,000,000 classifications since October 2021. If you’re interested in contributing to the project, and potentially finding a brand new black hole in our own galaxy, you can check out the project’s Zooniverse page here.
Learn More:
Univeristy of Southampton - Scientists enlist armchair astronomers to find thousands of black holes hiding in our galaxy
UT - Wandering Black Holes May Hold Clues to the Evolution of the Universe
UT - Hubble Discovers the First "Missing" Black Hole in this Famous Star Cluster
