‘Radio rebound’ observed in aftermath of gamma ray burst

News release from Northwestern University

This frame from an animation illustrates data collected in the aftermath of a gamma ray burst, showing a “radio rebound” shock wave that shed light on the processes at work in such cataclysmic explosions. Image: NRAO Outreach

Astronomers using telescopes across the electromagnetic spectrum and the Atacama Large Millimetre 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 centre.

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.

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 Centre 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.”