UNLV astrophysicist key in NASA discovery

University professor part of research team that observed  black-hole engulfing star

Sep 2 ,2011

A UNLV astrophysicist was part of a team of scientists that recently discovered the first evidence of a super-massive black hole swallowing a star.

Professor Bing Zhang was one of 58 authors involved in detailing the observations in the Aug. 25 edition of the journal, Nature.

The article was titled “Onset of a Relativistic Jet from the Tidal Disruption of a Star by a Massive Black Hole,” and the group was led by Penn State University astronomer David Burrows.

Zhang is a member of the NASA Swift team, a group dedicated to observing and making discoveries on gamma ray bursts (GRB). Zhang said that the Swift satellite had observed what they had thought to be a GRB in March, which is how they made the discovery.

“NASA’s Swift satellite was there searching for gamma-ray bursts. One burst triggered the Burst Alert Instrument (BAT) on board the satellite,” Zhang said. “Initially we thought it was another GRB, but soon we realized that it is not a traditional burst, but is something we never observed before.”

He added that unlike normal GRBs, whose X-ray emissions fade quickly, this emission was sustained and registered on the BAT multiple times.

“We immediately thought that it is a completely different beast,” he said.

What they discovered was that they were seeing a relativistic jet — a stream of material jetting out of the black hole at speeds close to the speed of light. Light from the jet traveled 3.9 billion light-years to Earth, and it is a black hole with an estimated 10 million solar masses, according to Zhang.

“Looking at the ‘lightcurve’ – how flux changes with time – we found that the source is gradually fading, suggesting that the accreted materials are depleting,” Zhang said.

“The decay seems to follow the prediction of a star being swallowed by the black hole.”
The jet is magnetized, according to the Nature article. Zhang said that this is the feature needed to tap into the spin energy of a black hole.

The black hole’s enveloping of the star, coupled with the star’s magnetism, allowed the jet to develop.

Following the article, Zhang teamed up with UNLV post-doctoral researcher Wei-Hua Lei to find the spin rate of the black hole.

“I work together with professor Zhang on the central engine model of gamma-ray bursts and other high energy sources, which studies the physical mechanism for powering the relativistic jets related to these astrophysical phenomena,” Lei said in an email interview. “We found that the black hole of this source is spinning rapidly, up to 90 [percent] of the maximum speed a black hole can rotate.”

Zhang said that the discoveries made through this black hole are new ground in the scientific community.

“The reason that people didn’t predict such kinds of events was that it is hard to imagine that a huge black hole would launch something when swallowing a little star,” Zhang said. “After this was seen, an immediate reaction was that the black hole must be spinning rapidly, so that one may tap the spin energy of the black hole through magnetic processes.”

Lei, a lecturer from Huazhong University of Science and Technology (HUST) and a post-doctoral researcher in Prof. Zhang’s group, said that his role was to fit the data with the model.

He added that the discovery is a major achievement for the scientific community.
“The spin is a crucial parameter of a black hole,” Lei said.

WEB REFER: Prof. Zhang and the Swift team’s article in Nature: www.nature.com/nature/journal/v476/n7361/full/nature10374.html