The echo of a gamma-ray burst emitted by a newborn black hole detected by the ALMA radio telescope

GRB 161219B and its echo
GRB 161219B and its echo

An article published in the journal “Astrophysical Journal” describes the study of a sort of echo generated by a gamma-ray burst cataloged as GRB 161219B emitted by a newborn black hole. The gamma-ray emissions lasted only seven seconds but emissions at other electromagnetic frequencies lasted even for weeks, which enabled a team of astronomers to use the ALMA radio telescope to study the ones at millimeter wavelengths. They offered other information on the gamma-ray burst and on the characteristics of its powerful jets.

On December 19, 2016, NASA’s Swift Space Observatory detected a gamma-ray burst lasting about 7 seconds. Subsequently, electromagnetic emissions at other frequencies were detected, which included a sort of echo, that allowed prolonged detections during the following weeks. That gamma-ray burst was cataloged as GRB 161219B and was associated with a supernova, cataloged as SN 2016jca, which occurred over 2 billion light years away.

Among the various electromagnetic emissions of that gamma-ray burst’s echo there were the ones at millimeter wavelengths detected thanks to the ALMA (Atacama Large Millimeter/submillimeter Array) radio telescope, inaugurated in March 2013. Tanmoy Laskar of the National Radio Astronomy Observatory, lead author of the study, explained that those wavelengths contain information on how the gamma-ray burst’s jets interact with surrounding gas and dust.

The observations conducted with ALMA allowed to reconstruct the events following the supernova SN 2016jca. The exploded star left a core that formed a black hole that emitted the gamma-ray burst GRB 161219B. Its jets hit the surrounding debris causing a reverse shockwave, an echo of the gamma-ray burst that lasted much longer. Tanmoy Laskar explained that they expected that echo to last no more than a minute while it lasted a good part of a day.

Carole Mundell of the University of Bath, another author of the study, explained that for decades astronomers thought that the reverse shockwave produced a flash of visible light but so far it’s been very difficult to find it despite careful research. The observations made with ALMA show that maybe those searches were conducted in the wrong place and that observations at millimeter wavelengths could be the best hope of identifying what she called cosmic fireworks.

The image (NRAO/AUI/NSF, S. Dagnello) shows an artistic impression of the reverse shockwave as it comes back through the gamma-ray burst’s jets GRB 161219B. The whole sequence of the events that generated that echo was summarized in the animation visible in the short clip (NRAO/AUI/NSF; S. Dangello).

Gamma-ray bursts are extremely energetic phenomena with jets that contain the energy that the Sun emits in billions of years. For this reason they’re visible at distances of billions of light years. Many of them have been detected but GRB 161219B is only the fourth for which evidence of a reverse shockwave was found and that made it particularly interesting.

The materials around the collapsed star were about 3,000 times less dense than the gas surrounding stars, and the observations made with ALMA suggest that low density is critical to generate reverse shockwave emissions. This could explain why that kind of echo is so rare. An instrument such as ALMA, with its sensitivity and the ability to quickly aim its antennas to detect a transient event, provides astronomers with excellent study possibilities.

Events such as supernovae and gamma-ray bursts are generally observed with other instruments, but when there’s an echo that includes emissions at millimetric wavelengths, a study with the ALMA radio telescope can provide further information on extreme phenomena. The energies released by those gamma-ray bursts are enormous and the jets have speeds so high as to include relativistic effects so their study offers the possibility to improve our knowledge of the cosmos in various ways.

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