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Enduring ‘radio rebound’ powered by jets from gamma-ray burst

Researchers create first-ever movie of cosmic explosion

Astronomers using telescopes across the electromagnetic spectrum and the Atacama Large Millimeter Array (ALMA) studied a cataclysmic stellar explosion known as a gamma-ray burst, or GRB, and found its enduring “afterglow.” The rebound, or reverse shock, triggered by the GRB’s powerful outflows slamming into surrounding debris, lasted thousands of times longer than expected. These observations provide fresh insights into the physics of GRBs, one of the universe’s most energetic explosions.

In the blink of an eye, a massive star more than 2 billion light years away lost a million-year-long fight against gravity and collapsed, triggering a supernova and forming a black hole at its center.

This newborn black hole belched a fleeting yet astonishingly intense flash of gamma rays known as a gamma-ray burst toward Earth, where it was detected by NASA’s Neil Gehrels Swift Observatory on Dec. 19, 2016.

While the gamma rays from the burst disappeared from view a scant seven seconds later, longer wavelengths of light from the explosion – including X-ray, visible light and radio – continued to shine for weeks. This allowed astronomers to study the aftermath of this fantastically energetic event, known as GRB 161219B, with many observatories including the X-Ray Telescope onboard Neil Gehrels Swift Observatory, the United Kingdom Infra-Red Telescope, the Very Large Array and ALMA.

These observations enabled a team of astronomers, including Northwestern University astrophysicists Wen-fai Fong and Raffaella Margutti, to produce ALMA’s first-ever time-lapse movie of a cosmic explosion. The study, led by Tanmoy Laskar, a Jansky Postdoctoral Fellow of the National Radio Astronomy Observatory (NRAO), appears today, July 26, in the Astrophysical Journal.

Tanmoy Laskar, an astronomer at U.C. Berkeley and NRAO Jansky Fellow, sonified the radio data output of an ALMA Observatory observation and tuned it so that it falls within the acoustic spectrum.

The cosmic movie revealed a surprisingly long-lasting “echo” at radio wavelengths, called a reverse shock.

“With our current understanding of GRBs, we would normally expect a reverse shock to last only a few seconds,” Laskar said. “This one lasted a good portion of an entire day.”

A reverse shock occurs when material blasted away from a GRB by its powerful outflows runs into the surrounding gas. This encounter slows down the escaping material, sending a shockwave back down the outflow.

“Having a reverse shock traveling back into the explosion’s outflow is like having a powerful light-bulb illuminating the interiors of the explosion. This is what makes this event unique,” said Margutti, an assistant professor of physics and astronomy in the Weinberg College of Arts and Sciences and a faculty member in Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA).

This remarkable discovery was enabled by extremely rapid responses by observatories at all wavelengths.

“This type of effort is very challenging,” said Fong, also an assistant professor of physics and astronomy in Weinberg College and a faculty member in CIERA. “After 20 years of searches following GRBs, this has only happened a few times, and this is the first with such unprecedented detail.”

“Though more than two billion light-years away, this GRB is actually the nearest such event for which we have measured the detailed properties of the outflow, thanks to the combined power of all these observatories and ALMA in particular,” said Kate Alexander, who will soon join Northwestern as an Einstein Postdoctoral Fellow.  

“Our study allows us to probe the nature of the relativistic outflows in these energetic transients and the engines that launch and feed them. This will play an important role in mapping the physics of the universe to the dawn of the first stars,” Laskar concluded. 

For more information on this discovery, see the press release from NRAO.

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