Study indicates star may have been consumed by black hole

UC Berkeley astronomers believe they have discovered a star that fell into a black hole and was destroyed after a continuous burst of gamma rays was detected by a NASA satellite.

In a study published in Science Express, Joshua Bloom, an assistant professor of astronomy at UC Berkeley and lead author of the paper, said he believed what he and other astronomers witnessed was a collision between a star and a super massive black hole, which are located in the center of galaxies.

“The event that we just saw probably only comes around once per 100 million years per galaxy,” Bloom said. “Our idea is that it should never happen again from this galaxy, but we’re hoping one day we’ll be able to see it again in some other galaxy.”

According to Bloom, he and other astronomers he collaborated with on the study initially did not think anything was unusual about the gamma ray flare that was picked up on March 28 by the Swift satellite until it continued repeating for the next several days.

But upon further observation, Bloom said they realized they were seeing something no other astronomer had seen before.

Previous gamma ray bursts detected by the satellite had frequently been observed as a by-product of supernova explosions — stellar explosions that are extremely luminous and cause a burst of radiation that often briefly outshine an entire galaxy, before fading from view over several weeks or months.

But, the gamma ray bursts that Bloom and others saw should only last for fractions of a second and, if they are extremely powerful, can explode for at most an hour, according to Eliot Quataert, a UC Berkeley professor of astronomy.

As such, peculation began circulating amongst astronomers from UC Berkeley and other universities around the world. From their observations, astronomers collaborating on the study were able to  calculate the amount of energy released by the burst and were able to pinpoint the source of the rays to be at the center of a galaxy 3.8 million light years away.

Andrew Levan, an associate professor at the University of Warwick, said cases of stars falling into a black hole are rare and happen maybe once per galaxy per 100,000 years, but that they have been seen before.

According to Levan, stars orbiting around the galaxy sometimes pass too closely to black holes, which have a much stronger gravitational force field. On occasion, if  the pull is too strong, the star will be sucked into the black hole and will be destroyed.

“It becomes tidally deformed,” he said, comparing it to the effect of the moon’s gravity on Earth’s tides. “The black hole raises tides on the star, that are much stronger, and turns the star from being star-shaped, a sphere, into a banana that gets stretched out.”

Levan added that in this particular case, after the star was destroyed by the black hole, it produced a jet of energy, releasing rays all across the electromagnetic spectrum, including gamma rays, x-rays, and infrared rays.

Brad Cenko, a UC Berkeley postdoctoral fellow and a co-author of the study, said what was so unique about the discovery that Bloom and others made was that the beam of light produced by the explosion was directly in line with Earth and the satellite’s sensors, allowing the image to be captured so clearly.

“That jet just happened to be pointed right along our line of sight,” he said.  “If it had been pointed at a slightly different direction, we probably wouldn’t have seen this bright x-ray and radio emission in particular.”