For the first time, astronomers have witnessed the violent death of a star as it happened. But this star didn’t die from natural causes; it died as a result of straying too close to a black hole, whose immense tidal forces tore the unfortunate star to shreds before eating the remains.
The accused supermassive black hole resides in the center of a galaxy some 3.9 billion light-years away in the constellation Draco.
This violent event was noticed on March 29 after NASA’s Swift space telescope detected X-rays being generated in the center of a distant galaxy, a galaxy with a supermassive black hole in its core that has, until now, remained dormant. Astronomers initially assumed that the X-ray signal was the onset of a gamma-ray burst, but they were wrong.
Something was strange about these emissions — a source named Swift J1644+57 — and after follow-up observations by the Japan-led Monitor of All-sky X-ray Image (MAXI) instrument aboard the International Space Station (ISS) and, in a separate study, the National Radio Astronomy Observatory’s Expanded Very Large Array (EVLA) near Socorro, N.M., it quickly became apparent that the emissions were originating from the death throes of a star being eaten.
The two Swift J1644+57 studies appear in the Aug. 25 issue of the journal Nature.
“Incredibly, this source is still producing X-rays and may remain bright enough for Swift to observe into next year,” said David Burrows, professor of astronomy at Penn State University and lead scientist for the mission’s X-Ray Telescope instrument. “It behaves unlike anything we’ve seen before.”
The working theory is that the star in question strayed too close to a supermassive black hole, approximately twice the mass of the four-million-solar-mass supermassive black hole residing in the center of the Milky Way.
As the tidal shear is so powerful close to the black hole’s event horizon, the star’s structure would have warped dramatically. Very quickly, the stellar plasma will have streamed around the spinning black hole, forming a superheated disk of plasma. The plasma closest to the black hole then got pulled into the event horizon, accelerating as it did so.
The rapid acceleration and complex magnetic fields nearest to the black hole then funneled the matter into the event horizon, but some of the matter escaped as jets, blasting into space at relativistic speeds (exceeding 90 percent the speed of light) from the black hole’s spin axis.
It just so happens that one of those jets are pointed directly at the Swift satellite that detected the powerful X-rays generated right at the base of the jet. Intense radio waves were also detected (by the EVLA study) as the relativistic jet slammed into the interstellar medium surrounding the black hole.
This is the first time that X-ray emissions and radio emissions have been linked to a single event in the center of a distant galaxy, leading to the discovery that a star was being killed by a black hole.
“Our observations show that the radio-emitting region is still expanding at more than half the speed of light,” said Edo Berger, an associate professor of astrophysics at Harvard and a coauthor of the radio paper. “By tracking this expansion backward in time, we can confirm that the outflow formed at the same time as the Swift X-ray source.”
Black hole, you are found guilty of murder. (Or is it involuntary manslaughter? After all, I doubt the black hole ate the star intentionally…)
Image (top): Images from Swift’s Ultraviolet/Optical (white, purple) and X-Ray telescopes (yellow and red) were combined to make this view of Swift J1644+57 (NASA/Swift/Stefan Immler). Image (bottom): What happens when a star falls into a black hole (NASA/Goddard Space Flight Center/Swift)