Gamma ray burst from the youth of the cosmos

GRB afterglow

Afterglow of the gamma ray burst GRB181123B (circle) (Image: Travis Rector / University of Alaska Anchorage), Mahdi Zamani and Davide de Martin)

Gamma ray bursts are among the brightest phenomena in the cosmos. Within a few seconds they release as much radiation as the sun does in its entire lifetime. Now astronomers have discovered the second furthest gamma-ray flash ever observed. The brief burst of gamma rays occurred around ten billion light years away, and thus in the early days of the cosmos. This also sheds new light on the likely originators of these high-energy flashes – the collisions of two neutron stars. The distant lightning now suggests that such mergers already existed in the still young universe.

More than 50 years ago, the gamma-ray bursts were discovered by chance when the Compton Observatory, orbiting in orbit, registered one of these extremely short flashes almost daily. About a third of these events, it is now known, can be traced back to what are known as short bursts of gamma rays. These bursts of short-wave and therefore very high-energy gamma radiation are not caused by supernovae, but rather probably by the collision of two neutron stars. But because these lightning bolts only last a few seconds, locating them and thus also localizing their source is a real challenge. This is only possible if a gamma-ray observatory such as NASA’s Fermi satellite detects the lightning, its message is then noticed in good time by astronomers, who then point other telescopes at the suspected source. On average, there are only seven to eight short bursts of gamma rays each year that can be located.

Ten billion light years away

One of these cases was captured by astronomers headed by Kerry Paterson from Northwestern University in Evanston. On November 23, 2018, NASA’s Swift Observatory sent the report of a gamma-ray burst to the astronomers network – it was Thanksgiving evening in the United States. Nevertheless, the researchers reacted quickly: Within a short time, Paterson and her team overturned the planned observations of the Gemini North telescope on Mauna Kea in Hawaii and directed the telescope to the area of ​​the sky from which the gamma-ray burst, baptized SGRB181123, had come. The lightning itself was long gone, but even short bursts of gamma rays show an afterglow in longer-wave, less energetic radiation areas for a few hours. Other telescopes such as the Gemini South in Chile and the telescopes of the Keck Observatory in Hawaii targeted this point. “We were able to make extensive observations just a few hours after the outbreak,” says Paterson. “The sharp Gemini images allowed us to trace the gamma-ray burst back to a specific galaxy.”

But how far was this galaxy? To find out, the researchers analyzed the galaxy’s light spectrum with several spectrographs in the optical and near-infrared wave range. “After we got the optical spectrum, it was clear that this event was one of the most distant brief bursts of gamma-ray that has ever been recorded,” says Paterson. According to the measurements, the source of the radiation was around ten billion light years away from us. “This makes GRB181123B the furthest short outbreak of gamma rays with a proven optical afterglow and one of the furthest away at all”, state the astronomers. The gamma-ray burst comes from a time when the universe was only 3.8 billion years old – almost a third as old as it is today.

Tip of an iceberg?

“We did not expect to find such a distant short burst of gamma rays because they are extremely rare and faint,” says Paterson’s colleague Wen-fai Fong. “We were therefore pleasantly surprised.” Closer analyzes show that the host galaxy of this gamma-ray burst had a stellar mass of around 17 billion at the time of the eruption and was already showing a decreasing star formation rate. Her most active time was already behind her – although the cosmos as a whole was still young and star formation had only just reached its peak. The fact that short bursts of gamma rays occurred relatively early in cosmic history suggests that there must have been binary systems made up of two neutron stars, as the astronomers explain.

“A gamma-ray flash from this time indicates that this pair of neutron stars must have merged relatively quickly,” explains Fong. The process from the formation of the neutron stars in supernovae through their gradual approach to collision could have taken less than a billion years. “According to our data, such mergers can happen surprisingly quickly,” says Paterson. On the basis of additional model simulations, she and her colleagues also assume that GRB 181123B was not an isolated case or exception. “We could have discovered the tip of an entire iceberg from distant short bursts of gamma rays,” said the researcher.

Source: Kerry Paterson (Northwestern University, Evanston) et al., Astrophysical Journal Letters; accepted

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