Research explores how destroyed star rains onto black hole

New details about what happens when a black hole tears apart a star have been gathered by a trio of orbiting X-ray telescopes giving scientists, including Leicester astronomers, an excellent opportunity to understand the extreme environment around a black hole.

When a star comes too close to a black hole, the intense gravity of the black hole can rip the star apart. In these events, called “tidal disruptions,” some of the stellar debris is flung outward at high speeds, while the remainder falls into black hole. This causes a distinct X-ray flare that can last for a few years.

NASA’s Chandra X-ray Observatory, Swift Gamma Ray Explorer, and ESA’s XMM-Newton were used to put together different pieces of this astronomical puzzle in a tidal disruption event called ASASSN-14li. The event occurred near a supermassive black hole estimated to weigh a few million times the mass of the Sun. The black hole is located in the center of PGC 043234, a galaxy that lies about 290 million light years from Earth. This makes this event the closest tidal disruption discovered in a decade.

After the star is destroyed, the black hole’s strong gravitational forces pull most of the remains of the star toward it. This infalling debris is heated to millions of degrees and generates a huge amount of X-ray light. Soon after this surge of X-rays, the amount of light decreases as the material falls beyond the event horizon (that is, the point of no return) of the black hole.

Paul O’Brien from the Department of Physics and Astronomy, a member of the Swift team and a co-author on the paper, said: “Black holes only emit light when they interact with material that comes close to them, in this case an entire star. Unfortunately for this star, it got too close and was completely destroyed allowing us to witness the intense power of a black hole.”

Gas often falls towards black holes by spiraling inward in a disk. But how this process starts has remained a mystery. In ASASSN-14li, astronomers were able to witness the formation of such a disk by looking at the X-ray light at different wavelengths (known as the "X-ray spectrum") and how that changed over time.

The researchers determined that the X-rays being produced come from material that is either very close to or is actually in the smallest possible stable orbit around the black hole.

These results appeared in the 22 October issue of the journal Nature.