A short gamma ray burst left the most-distant optical afterglow ever detected –incredibly faint and fast signals sometimes lasting mere hours–some 10 billion light years away, 3.8 billion years after the Big Bang. Astronomers suspect that up to one-third of all short gamma ray bursts come from merging neutron stars in globular clusters of old stars blinding whole galaxies with light and destroying millions of worlds. Known as SGRB181123B, it is the second most-distant well-established SGRB ever detected and the most distant event with an optical afterglow.
The appearance of an SGRB at such an early time, report astronomers at the Keck Observatory and Northwestern University could alter theories about their origins, particularly the length of time it takes two neutron stars to merge and produce these powerful explosions, as well as the rate of neutron star mergers in the young universe.
“This was a very exciting object to study,” said Kerry Paterson, a postdoctoral associate at Northwestern University’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and lead author of the study. “Our research now suggests neutron star mergers could occur surprisingly quickly for some systems — with neutron star binaries spiraling together in less than a billion years to create an SGRB.”
“We certainly did not expect to discover a distant SGRB, as they are extremely rare and very faint,” said Wen-fai Fong, an assistant professor of physics and astronomy at Northwestern University and a member of CIERA (Center for Interdisciplinary Exploration and Research in Astrophysics). Paterson is a postdoctoral associate in CIERA. a senior author of the study. “We perform ‘forensics’ with telescopes to understand its local environment, because what its home galaxy looks like can tell us a lot about the underlying physics of these systems.”
Tip of the Iceberg
“We believe we are uncovering the tip of the iceberg in terms of distant SGRBs,” said Paterson.
The gamma-ray light lasts for less than two seconds, while the optical light can last for a matter of hours before fading. Therefore, rapid follow-up of the optical afterglow of these intense flashes of gamma-ray radiation is critical. Within just a few hours after NASA’s Neil Gehrels Swift Observatory detected the object and broadcast a worldwide alert, Paterson’s team quickly pointed the Gemini North and Keck I telescopes toward the location of the SGRB.
Using the Gemini Multi-Object Spectrograph followed by Keck Observatory’s Multi-Object Spectrograph for Infrared Exploration (MOSFIRE) instrument, the researchers were able to measure the very faint afterglow of the object, which is named GRB181123B because it was the second burst discovered on November 23, 2018 — Thanksgiving night.
Repointing the Keck Telescope
“It was unreal,” said Fong, co-author of the study. “I was in New York with my family and had finished having a big Thanksgiving dinner. Just as I had gone to sleep, the alert went off and woke me up. While somewhat of a nuisance, you literally never know when you’ll land a big discovery like this! I immediately triggered the Gemini observations and notified Kerry. Thankfully, she happened to be observing at Keck that night and was able to rearrange her original observing plan and repoint the telescope towards the SGRB.”
“It was such an adrenaline-rush to be at Keck when the SGRB alert went off and personally move the telescope towards the object to capture data mere hours after the burst,” said Paterson.
“Once we obtained the optical spectrum from Keck’s DEIMOS, it was clear this event was one of the most distant SGRBs measured, which further fueled our investigation to determine its precise distance,” said Paterson.
This led the team to collect additional observations with Keck Observatory, along with the Gemini South telescope in Chile and Multi-Mirror Telescope in Arizona. With a distance calculated at a cosmological redshift of 1.754, the data confirmed the object is the most distant high-confidence SGRB with an optical afterglow detection ever found.
“The identification of certain patterns in the spectrum, together with the colors of the galaxy from the three observatories, allowed us to precisely constrain the distance and solidify it as one of the most distant SGRBs to date in 16 years of Swift operations,” said Paterson.
Once the team identified the host galaxy, they were able to determine key properties of the parent stellar population within the galaxy that produced the SGRB.
“Performing ‘forensics’ to understand the local environment of SGRBs and what their home galaxies look like can tell us a lot about the underlying physics of these systems, such as how SGRB progenitors form and how long it takes for them to merge,” said Fong. “We certainly did not expect to discover an extremely distant SGRB, as they are very rare and faint, but we were pleasantly surprised! This motivates us to go after every one that we possibly can.”
Some of the most energetic and brightest explosions in the universe, SGRBs most likely occur when two neutron stars merge. This merger causes a short-lived burst of gamma rays, which is the most energetic form of light. Astronomers typically only detect seven or eight SGRBs each year that are well-localized enough for further observations. And because their afterglows typically last, at most, a few hours before fading into oblivion, they rarely linger long enough for astronomers to get a close look.
Follow-up Using Gemini-South in Chile
With follow-up observations using Gemini-South in Chile, MMT in Arizona and Keck in Hawaii, the team realized SGRB181123B may be more distant than most.
“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.”
“With SGRBs, you won’t detect anything if you get to the sky too late,” Fong added. “But every once in a while, if you react quickly enough, you will land on a really beautiful detection like this.”
Peering Into the ‘Cosmic High Noon’
To uncover the SGRB’s distance from Earth, the team then accessed a near-infrared spectrograph on Gemini-South, which can probe redder wavelengths. By taking a spectrum of the host galaxy, the researchers realized they had serendipitously uncovered a distant SGRB.
After identifying the host galaxy and calculating the distance, Fong, Paterson and their team were able to determine key properties of the parent stellar populations within the galaxy that produced the event. Because SGRB181123B appeared when the universe was only about 30% of its current age — during an epoch known as “cosmic high noon” — it offered a rare opportunity to study the neutron star mergers from when the universe was a “teenager.”
When SGRB181123B occurred, the universe was incredibly busy, with rapidly forming stars and fast-growing galaxies. Massive binary stars need time to be born, evolve and die — finally turning into a pair of neutron stars that eventually merge.
“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.”
Source: “Discovery of the optical afterglow of short GRB 181123B at z = 1.754: Implications for delay time distributions,” was supported by the National Science Foundation (award numbers AST-1814782 and AST-1909358) and NASA (award number HST-GO-15606.001-A).
Image credit at top of page: A gamma ray burst produced this jet that emerged nearly at the speed of light. DESY; INTERNATIONAL GEMINI OBSERVATORY/NOIRLAB/NSF/AURA/K. PATERSON & W. FONG (NORTHWESTERN UNIVERSITY). IMAGE PROCESSING: TRAVIS RECTOR (UNIVERSITY OF ALASKA ANCHORAGE), MAHDI ZAMANI & DAVIDE DE MARTIN