Studying the region closest to a black hole is not easy. By their nature, black holes swallow light, so the only way to do it is indirectly, observing its effects on other things rather than the object itself. Last April we snapped the first-ever photo of one (or at least, its event horizon, the precipe where the last light can be seen before it's swallowed by the black hole) but it took an Earth-sized observatory – the Event Horizon Telescope – to do it. Now, researchers have used X-rays to map the surroundings of a black hole for the first time.
To do this, however, they had to wait for some material to be eaten by the black hole in order to catch a glimpse of just what’s going on.
During the feeding process, the plasma a black hole is gobbling up gets heated to incredible temperatures and starts to emit light in the X-ray part of the electromagnetic spectrum. These X-rays bounce around the dramatically warped region, allowing researchers to use the peculiar signal to reconstruct the geometry of space-time around the supermassive black hole.
In a new paper published in Nature Astronomy, scientists showed how this was applied to the supermassive black hole at the heart of galaxy IRAS 13224–3809. This is one of the most variable X-ray sources in the sky, as its brightness can change by a factor of 50 in just a few hours, which made it an ideal candidate to test this reverberation map method.
“Everyone is familiar with how the echo of their voice sounds different when speaking in a classroom compared to a cathedral – this is simply due to the geometry and materials of the room [the "acoustics"], which causes sound to behave and bounce around differently,” lead author William Alston, from the University of Cambridge, UK, said in a statement.
“In a similar manner, we can watch how echoes of X-ray radiation propagate in the vicinity of a black hole in order to map out the geometry of a region and the state of a clump of matter before it disappears into the singularity. It’s a bit like cosmic echo-location.”
The scientists had been expecting to observe the reverberation light echos which they used to map the region, but they found something else surprising.
Material spiraling into the black hole forms an accretion disk and above it, there is a region of high energy electrons with temperature in the billions of degrees: the black hole corona. Based on the data collected by the European Space Agency’s XMM-Newton satellite, the researchers discovered the corona changes size over a matter of just a few days.
The observation provides a unique picture of what’s happening around the black hole but it also allows for precise measurements of the black hole's properties. The size of the corona depends on the black hole's mass, its spin, and what goes around it. Since the mass and the spin do not change that quickly, the team managed to pin those down. IRAS 13224–3809's supermassive black hole has a mass of about 1.9 million times the mass of the Sun, less than half the mass of Sagittarius A*, the black hole at the center of the Milky Way.
Upcoming missions, such as Athena, will allow for many more of these observations. This will not only help further our understanding of black holes, but will also help to test gravitational theories and how galaxies evolve, given the pivotal role black holes play.