Gamma ray eruptions are among the brightest phenomena in the cosmos. Within a few seconds, they release as much radiation as the sun in its entire lifetime. Now astronomers have discovered the second-closest gamma ray outburst ever observed. The short gamma ray eruption occurred about ten billion light-years away, 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 eruption now speaks to the fact that such fusions already existed in the still young universe.
More than 50 years ago, gamma-ray eruptions were discovered by chance, when the Compton Observatory recorded one of these extremely short flashes almost daily. About a third of these events, we know today, are due to the so-called short gamma ray eruptions. These eruptions of short-wave and thus very high-energy gamma radiation are not caused by supernovae, but probably by the collision of two neutron stars.
But because these flashes last only a few seconds, their location and thus the localization of their source is a real challenge. This can only be achieved if a gamma-ray observatory such as NASA’s Fermi satellite detects the flash, then its message is noticed in time by astronomers who in turn point other telescopes towards the suspected source. On average, there are only seven to eight short gamma ray eruptions per year, in which this detection succeeds.
Ten billion light-years away
Astronomers around Kerry Paterson of Northwestern University in Evanston captured one of these cases. On November 23, 2018, NASA’s Swift Observatory sent a gamma-ray eruption message to the astronomer network – it was the evening of Thanksgiving 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 the Mauna Kea in Hawaii and directed the telescope to the celestial area from which the gamma flash christened SGRB181123 had come.
The flash itself was long gone, but even short gamma ray eruptions show a afterglow in longer, lower-energy areas for a few hours. Other telescopes such as the Gemini South in Chile and the telescopes of the Keck Observatory in Hawaii took aim at this site. “We were able to make extensive observations just a few hours after the eruption,” Paterson said. “The sharp Gemini images allowed us to trace the gamma ray eruption to a specific galaxy.”
But how far away was this galaxy? To find out, the researchers analyzed the galaxy’s light spectrum with multiple spectrographs in the optical and near-infrared wave range. “After receiving the optical spectrum, it was clear that this event was one of the most distant short gamma ray eruptions ever recorded,” Paterson says.
According to the measurements, the source of the radiation was about ten billion light-years away from us. “This makes GRB181123B the farthest short burst of gamma beam with proven optical afterglow and one of the most distant ever,” the astronomers note. The gamma flash dates back to a time when the universe was only 3.8 billion years old – almost a third as old as it is today.
Tip of the iceberg?
“We didn’t expect such a distant short gamma ray eruption, because they are extremely rare and low in light,” says Paterson’s colleague Wen-fai Fong. “We were therefore pleasantly surprised.”
Further analysis shows that the host galaxy of this gamma flash at the time of the eruption had a stellar mass of about 17 billion and had an already decreasing star formation rate. It had already had her most active time with it – even though the cosmos as a whole was still young and star formation was only just reaching its peak.
The fact that short gamma-ray eruptions occurred relatively early in cosmic history suggests that there must have been double star systems of two neutron stars at that time, as the astronomers explain.
“A gamma flash from this period indicates that this pair of neutron stars must have merged relatively quickly,” Explains Fong. The process from the formation of neutron stars in supernovae to their gradual approach to collision may have taken less than a billion years. “According to our data, such mergers can be rapid,” Paterson said.
On the basis of additional model simulations, she and her colleagues also assume that GRB 181123B was not an isolated case or an exception. “We may have discovered the tip of an entire iceberg from distant short gamma ray bursts,” the researcher said.