Black holes tend to come in two varieties of stellar size, weighing either up to a few dozen times the mass of the Sun, or millions to billions of times its mass (supermassive). Researchers have now found evidence for an intermediate type. Only a handful of these objects have been discovered and the latest one was caught in the act of ripping a star apart.
The black hole is located in a large galaxy about 740 million light-years from us. Thanks to a series of observations researchers have been able to track a sudden burst of X-ray luminosity, an often telltale sign of a star being ripped apart by a massive black hole. This usually happens near the central supermassive black hole (like in this case), so detecting this event from the edge of a galaxy is definitely something new. The study, published in Nature Astronomy, puts the mass of the object at roughly 50,000 times the mass of the Sun.
"We spotted the source flaring in brightness in two images from 2005 – it appeared far bluer and brighter than it had just a few years previously," co-author Jay Strader, from Michigan State University, said in a statement. "By comparing all the data we determined that the unfortunate star was likely disrupted in October 2003 in our time, and produced a burst of energy that decayed over the following 10 years or so."
Researchers found the object in the European Space Agency’s XMM-Newton catalog. It was then followed up with different instruments such as NASA’s Chandra and Swift, as well as optical telescopes.
Researchers didn’t expect to discover a tidal disruption event (the technical term for a star ripping apart) from an intermediate-mass black hole. And the fact they've seen one suggests there might be a lot more of these black holes than we thought.
"Learning more about these objects and associated phenomena is key to our understanding of black holes," Norbert Schartel, ESA Project Scientist for XMM-Newton, added. "Our models are currently akin to a scenario in which an alien civilisation observes Earth and spots grandparents dropping their grandchildren at pre-school: they might assume that there's something intermediate to fit their model of a human lifespan, but without observing that link, there's no way to know for sure. This finding is incredibly important and shows that the discovery method employed here is a good one to use."
Last year, two intermediate-mass black holes were discovered. One thanks to an orbiting pulsar that gave away its location, the other thanks to a cloud of gas. Both of these were in or in the proximity of the Milky Way.
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