Astronomers see distant eruption as black hole destroys star

For the first time, scientists have captured on camera the birth and growth of a swift jet of material that was released when a star that strayed too near to a supermassive black hole was torn apart by its intense gravity.

Utilizing radio and infrared observatories, such as the Very Long Baseline Array (VLBA) of the National Science Foundation, the scientists monitored the event in two merging galaxies known as Arp 299, which are located around 150 million light-years away from Earth. Important details of the cataclysmic meeting were disclosed when a star more than twice the mass of the Sun was destroyed by a black hole 20 million times more massive than the Sun at the center of one of the galaxies.

Tidal disruption events, or TDEs, are a kind of star death that have only been found in a few number of cases, despite scientific speculation that they could occur more frequently. According to theories, material sucked from the dying star creates a revolving disk around the black hole that shoots material jets outward from its poles at almost the speed of light and emits powerful X-rays and visible light.

"We have never been able to directly observe the formation and evolution of a jet from one of these events before," Astrophysical Institute of Andalusia in Granada, Spain's Miguel Perez-Torres stated.

The first clue was found on January 30, 2005, when scientists in the Canary Islands, using the William Herschel Telescope, detected a brilliant burst of infrared radiation originating from the core of one of the colliding galaxies in Arp 299. From the same area, a new and unique source of radio emission was discovered by the VLBA on July 17, 2005.

Seppo Mattila of the University of Turku in Finland stated, "Over time, the new object stayed bright at infrared and radio wavelengths, but not in visible light and X-rays." He went on, "The most likely explanation is that visible light and X-rays were absorbed by thick interstellar gas and dust near the galaxy's center, which then reradiated the energy as infrared." To track the object's infrared radiation, the researchers employed NASA's Spitzer satellite telescope and the Nordic Optical Telescope on the Canary Islands.

Over the course of over ten years, more studies using the VLBA, the European VLBI Network (EVN), and other radio telescopes revealed that the source of radio emission was spreading in a single direction, as would be predicted for a jet. The material in the jet travelled at an average speed of one-fourth the speed of light, according to the observed expansion. Luckily, radio waves travel through the galaxy to reach Earth rather than being absorbed at its core.

In order to get the resolving power—the capacity to discern minute details—needed to identify the expansion of an object so far away, these studies employed numerous radio telescope antennas spaced thousands of kilometers apart. After years of meticulous data collecting, the scientists were rewarded with proof of a jet.

Supermassive black holes, which have core masses tens of millions to billions of times that of the Sun, are found in the center of most galaxies. Not even light can escape from a black hole due to the intense gravitational attraction of its concentrated mass. Superfast particle jets are propelled outward and a revolving ring of material formed around such supermassive black holes when they are actively absorbing material from their surrounds. This is the phenomena observed in quasars and radio galaxies.

Perez-Torres said, "But supermassive black holes are mostly in a quiet state because they are not actively consuming anything." He said, "We have a rare opportunity to further our understanding of the formation and evolution of jets in the vicinity of these powerful objects through tidal disruption events."

"This specific tidal disruption event may be just the tip of the iceberg of what has up until now been a hidden population," Mattila added, referring to the dust that absorbed any visible light. "We may be able to discover many more, and learn from them, by looking for these events with radio and infrared telescopes," he stated.

Studying such phenomena might help scientists understand the conditions under which galaxies formed billions of years ago since they may have been more frequent in the distant Universe.

The scientists said that they were surprised by the discovery. As part of a quest to find supernova explosions in such colliding pairings of galaxies, the first infrared burst was seen. Many star explosions have occurred in Arp 299, earning it the moniker "supernova factory." At first, this new object was thought to be a supernova explosion. Six years after its discovery, in 2011, the radio-emitting part just started to exhibit an elongation. The expansion was observed to be expanding later on, indicating that the scientists are viewing a jet rather than a supernova.

In the observations of Arp 299, Mattila and Perez-Torres oversaw a group of thirty-six scientists from twenty-six different universities worldwide. Their research was released in the Science journal's online edition on June 14.