The afterglow of an explosion billions of light-years away has provided astronomers with a window into what the “teenage” years of our universe may have looked like, according to a new study.
Astronomers detected the afterglow of a short gamma ray burst 10 billion light-years away. This is considered a rare event because afterglows of these short bursts are both fast and faint and can disappear hours later.
They have named the burst SGRB181123B, which occurred about 3.8 billion years after the Big Bang. It’s the second most-distant short gamma ray burst, known as an SGRB, ever detected and the most distant to have a visible afterglow.
The study published Tuesday in the Astrophysical Journal Letters.
“This SGRB essentially allows us to explore ‘terra incognita,’ ” said Wen-fai Fong, senior study author and assistant professor of physics and astronomy in Northwestern University’s Weinberg College of Arts and Sciences, in an email.
“They are extremely hard to find (because they are so distant, they are also fainter which makes them even more difficult to catch) and therefore we do not know the true rates of SGRBs during this period of the universe,” Fong said.
“SGRBs originate from the mergers of two neutron stars, and it is of great interest to understand when neutron star mergers happen in our universe, and how long they take to merge,” she said.
Short gamma ray bursts are incredibly energetic and bright, occurring when the dense remains of exploded stars, called neutron stars, collide. The merging of the two neutron stars releases short bursts of gamma rays, which are the most energetic form of light.
On average, astronomers can hope to detect fewer than than 10 short gamma ray bursts in a year that are close enough for them to conduct follow-up observations using telescopes. But their afterglows fade away after a few hours, meaning that it’s rare for astronomers to actually observe them in any detail.
“We believe we are uncovering the tip of the iceberg in terms of distant SGRBs,” said Kerry Paterson, lead study author and postdoctoral associate in Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics, in a statement. “That motivates us to further study past events and intensely examine future ones.”
NASA’s Neil Gehrels Swift Observatory detected the event on Thanksgiving inight n November 2018. A group of astronomers at Northwestern University was able to gain remote access of the Gemini Observatory’s Gemini-North telescope and measure the afterglow of the burst only hours later. The telescope is located on Mauna Kea in Hawaii.
The quest for these bursts is a labor of love that requires teamwork.
Fong was asleep after a Thanksgiving dinner with her family in New York, but Paterson was observing using the W.M. Keck Observatory in Hawaii. Together, they were able to point both Keck and remotely initiate observations through Gemini-North the evening the burst was detected.
“I love gamma-ray bursts because they evolve on human timescales,” Fong said. “From the time you eat lunch to the time you eat dinner on the same day, the GRB afterglow has been evolving, fading in brightness on minute/hour timescales.
“Once they’re detected, you really want to capture every moment that you can with them!” she said. “The human element of this particular burst was unreal to me and a great example of teamwork. If you had to do it all on your own, you would never sleep.”
“We were able to obtain deep observations of the burst mere hours after its discovery,” Paterson said. “The Gemini images were very sharp, allowing us to pinpoint the location to a specific galaxy in the universe.”
Follow-up observations were conducted using the Gemini-South telescope in Chile, the W.M. Keck Observatory in Hawaii and the MMT Observatory in Arizona. The Gemini-South telescope has a near-infrared spectrography which allows it measure in more red wavelengths and to peer into the distant universe.
That’s when the researchers discovered the great distance of the burst’s host galaxy and realized they had detected a very distant short gamma ray burst’s afterglow.
“It is not easy to ask a large telescope to move quickly,” Fong said. “But in the game of GRBs, you will never know what you missed until you react quickly, point the telescopes and take images. This SGRB shows that it was definitely worth pointing!”
This particular burst occurred when the universe was 30% of its current age, which is estimated to be 13.8 billion years old. This adolescent time period of the universe is also referred to the researchers as “cosmic high noon.”
“The farther out into the universe you go, the farther back in time you are looking,” Fong said.
By studying the afterglow of the burst, they could understand what neutron star mergers were like in the young universe. This was the peak of star formation in the history of the universe. Today, it’s “rather quiet,” Fong said.
At the time of this burst, there was a lot of activity occurring in the universe, including rapid star formation and the quick growth of galaxies. In order for this neutron star merger to occur, that means the pair of stars orbiting each other that created them — or binary stars — needed to grow to their massive size and evolve until they died.
“It’s long been unknown how long neutron stars — in particular those that produce SGRBs — take to merge,” Fong said. “Finding an SGRB at this point in the universe’s history suggests that, at a time when the universe was forming lots of stars, the neutron star pair may have merged fairly rapidly.”
Fong said that the fact that they can count the number of short GRBs at these distances on one hand after 16 years of Swift operations, reveals how difficult these detections truly are.
But the researchers are motivated to continue their search for these bursts since they don’t know when they’ll discover something like this, Fong said.