Unusual gamma-ray burst reveals previously undetected hybrid neutron-star merger event

The standard view of gamma-ray bursts as a signature for different types of dying stars might need a rewrite. Recent astronomical observations, supported by theoretical modeling, reveal a new observational fingerprint of neutron-star mergers, which may shed light on the production of heavy elements throughout the universe.

"Astronomers have long believed that gamma-ray bursts fell into two categories: long-duration bursts from imploding stars and short-duration bursts from merging compact stellar objects," said Chris Fryer, an astrophysicist and Laboratory Fellow at Los Alamos National Laboratory. Fryer is coauthor and leader of the modeling team on a paper about the phenomenon published in Nature. "But in a recently observed event, we've found a kilonova along with a long-duration gamma-ray burst, and that has thrown a wrench into this simple picture."

Hypernovae/supernovae are the visible-light, transient objects produced in these explosions from imploding objects, while kilonovae are visible-light transients produced by merging compact objects where at least one is a neutron star. Gamma-ray bursts can accompany both types of transients. Supernovae are produced when a massive star explodes; only a small subset of supernovae arise from the explosion mechanism that produces gamma-ray bursts.

The long and short of gamma-ray bursts
Long-duration GRBs (longer than two seconds) are typically associated with supernovae, while short-duration GRBs (less than two seconds) are commonly associated with neutron-star mergers. These various forms of observable electromagnetic emission are all known as transients. Neutron-star mergers form some of the heaviest elements-those beyond iron on the periodic table.

On Dec. 11, 2021, several observatories and satellites recorded a very bright, 50-second gamma-ray burst and optical, infrared and x-ray emissions associated with the burst. This long burst was relatively nearby-about a billion light years away in a different galaxy than the Milky Way-but its emission characteristics did not fit the profile of long-burst events. Instead, the evidence pointed to a compact-object merger in a theorized but previously unobserved hybrid event that produces a kilonova but emits a long-duration gamma-ray burst.

"Our modeling team at Los Alamos compared the observation to a suite of supernova and kilonova simulations, and we were unable to convincingly match the signal to a supernova model, whereas several kilonova models give a good match of the optical and infrared data points," said Ryan Wollaeger, a coauthor of the paper and member of the Los Alamos modeling team. "There is still more theoretical modeling to do to fully understand this transient, however."

Challenging the standard understanding
"This detection breaks our standard idea of gamma-ray bursts," said Eve Chase, also a coauthor of the paper, a postdoc at Los Alamos and a member of the Los Alamos team. "We can no longer assume that all short-duration bursts come from neutron-star mergers, while long-duration bursts come from supernovae. We now realize that gamma-ray bursts are much harder to classify. This detection pushes our understanding of gamma-ray bursts to the limits."

The observation, dubbed GRB211211A, provides the first direct evidence of a hybrid event. Gravitational-wave observations would confirm the nature of GRB211211A, but unfortunately sensitive gravitational wave detectors like LIGO (Laser Interferometer Gravitational-Wave Observatory) were not operating at the time of this detection.

Although the long-duration burst challenges the accepted understanding of compact-binary-merger models, Fryer said, a merger nonetheless explains all the other observed features of GRB211211A.

Fryer and his Ph.D. advisor Stan Woosley coined and developed in 1999 the widely accepted black-hole accretion-disk paradigm as the simplest explanation for the two classes of gamma-ray-burst events. Under this paradigm, merging compact objects, with their halos of gravitationally attracted material (accretion disks), would produce short-duration gamma-ray bursts. The collapse of massive stars into supernovae, with longer-lived accretion disks, would produce longer bursts. A growing set of observations have supported these two classes and the types of stellar objects associated with them, Fryer said.

GW PhD student plays key role in gamma-ray burst discovery
Washington DC (SPX) Dec 08 - The recent discovery of a long gamma-ray burst (GRB) triggered by the collision of two neutron stars challenged the scientific consensus on the cause of that cosmic phenomenon. Brendan O'Connor, a sixth-year PhD student in the George Washington University Department of Physics, is among the scientists who observed this momentous event and interpreted its significance.

O'Connor served as principal investigator (PI) of an International Gemini Observatory program that studied this unique explosion, known as GRB 211211A, and helped distinguish this particular event from other GRBs.

Long GRBs last up to a minute and were thought to be caused by supernovae, the explosion of a massive star. Short GRBs last less than two seconds and are associated with kilonovae caused by the merger of two dense bodies in outer space, such as neutron stars and black holes. O'Connor gathered critical data on the host galaxy environment of GRB 211211A, which indicated that this particular explosion occurred just one billion light years away, far closer to Earth than most other GRBs.

O'Connor observed that the burst happened far from the center of its host galaxy and in a low-density environment, which is not typical for supernovae. He took into account the distance, environment, and brightness of the explosion and deduced that the burst was caused by the collision of two neutron stars, an event that was previously believed to only cause short GRBs. GRB 211211A is the first long GRB known to have been caused by the merger of two compact objects. It is also the first GRB of its kind to display kilonova emission, which is the signature heavy elements, such as gold and platinum, synthesized during the merger. The discovery of a kilonova provided the smoking-gun evidence that GRB 211211A was produced during a merger.

In addition to being a doctoral student at GW, O'Connor is also a graduate research assistant at NASA's Goddard Space Flight Center.

"GRB 211211A was a very unexpected, but extremely fortunate revelation. We were extremely lucky that the explosion occurred so nearby, as the proximity to Earth allowed us to study the event in exquisite detail. Only with this fantastic dataset were we able to unveil the signatures of a kilonova imposed on top of the gamma-ray burst emission." - Brendan O'Connor, PhD student, George Washington University

"We started with two gamma-ray burst classes thirty years ago, where collapsars produced the long GRBs and mergers the short ones. But in our field we know that paradigms change. Now it appears we have a long GRB from what appears to be a merger of two neutron stars, which is a puzzle we need to solve. Nature just served us a curveball and we need to understand the consequences." - Chryssa Kouveliotou, professor of astrophysics and Department of Physics chair, George Washington University

Research Report:A nearby long gamma-ray burst from a merger of compact objects

Related Links
Los Alamos National Laboratory
Stellar Chemistry, The Universe And All Within It

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Short gamma-ray bursts traced farther into distant universe
Chicago IL (SPX) Nov 23, 2022
A Northwestern University-led team of astronomers has developed the most extensive inventory to date of the galaxies where short gamma-ray bursts (SGRBs) originate. Using several highly sensitive instruments and sophisticated galaxy modeling, the researchers pinpointed the galactic homes of 84 SGRBs and probed the characteristics of 69 of the identified host galaxies. Among their findings, they discovered that about 85% of the studied SGRBs come from young, actively star-forming galaxies. The astr ... read more