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

The usual view of gamma-ray bursts as a signature for several types of dying stars may want a rewrite. Latest astronomical observations, supported by theoretical modeling, reveal a brand new observational fingerprint of neutron-star mergers, which can make clear the manufacturing of heavy components all through the universe.

“Astronomers have lengthy believed that gamma-ray bursts fell into two classes: long-duration bursts from imploding stars and short-duration bursts from merging compact stellar objects,” stated Chris Fryer, an astrophysicist and Laboratory Fellow at Los Alamos Nationwide Laboratory. Fryer is coauthor and chief of the modeling workforce on a paper concerning the phenomenon revealed at the moment in Nature. “However in a just lately noticed occasion, we have discovered a kilonova together with a long-duration gamma-ray burst, and that has thrown a wrench into this easy image.”

Hypernovae/supernovae are the visible-light, transient objects produced in these explosions from imploding objects, whereas kilonovae are visible-light transients produced by merging compact objects the place at the least one is a neutron star. Gamma-ray bursts can accompany each sorts of transients. Supernovae are produced when a large star explodes; solely a small subset of supernovae come up from the explosion mechanism that produces gamma-ray bursts.

The lengthy and wanting gamma-ray bursts

Lengthy-duration GRBs (longer than two seconds) are sometimes related to supernovae, whereas short-duration GRBs (lower than two seconds) are generally related to neutron-star mergers. These numerous types of observable electromagnetic emission are all generally known as transients. Neutron-star mergers type among the heaviest components—these past iron on the periodic table.

On Dec. 11, 2021, a number of observatories and satellites recorded a really vivid, 50-second gamma-ray burst and optical, infrared and X-ray emissions related to the burst. This lengthy burst was comparatively close by—a couple of billion light years away in a special galaxy than the Milky Way—however its emission traits didn’t match the profile of long-burst occasions. As a substitute, the proof pointed to a compact-object merger in a theorized however beforehand unobserved hybrid occasion that produces a kilonova however emits a long-duration gamma-ray burst.

“Our modeling workforce at Los Alamos in contrast the commentary to a collection of supernova and kilonova simulations, and we have been unable to convincingly match the sign to a supernova mannequin, whereas a number of kilonova fashions give a great match of the optical and infrared information factors,” stated Ryan Wollaeger, a coauthor of the paper and member of the Los Alamos modeling workforce. “There’s nonetheless extra theoretical modeling to do to completely perceive this transient, nevertheless.”

Difficult the usual understanding

“This detection breaks our normal thought of gamma-ray bursts,” stated Eve Chase, additionally a coauthor of the paper, a postdoc at Los Alamos and a member of the Los Alamos workforce. “We will not assume that every one short-duration bursts come from neutron-star mergers, whereas long-duration bursts come from supernovae. We now understand that gamma-ray bursts are a lot more durable to categorise. This detection pushes our understanding of gamma-ray bursts to the bounds.”

The commentary, dubbed GRB211211A, offers the primary direct proof of a hybrid occasion. Gravitational-wave observations would verify the character of GRB211211A, however sadly delicate gravitational wave detectors like LIGO (Laser Interferometer Gravitational-Wave Observatory) weren’t working on the time of this detection.

Though the long-duration burst challenges the accepted understanding of compact-binary-merger fashions, Fryer stated, a merger nonetheless explains all the opposite noticed options of GRB211211A.

Fryer and his Ph.D. advisor Stan Woosley coined and developed in 1999 the broadly accepted black-hole accretion-disk paradigm as the only clarification for the 2 courses of gamma-ray-burst occasions. Below this paradigm, merging compact objects, with their halos of gravitationally attracted materials (accretion disks), would produce short-duration gamma-ray bursts. The collapse of huge stars into supernovae, with longer-lived accretion disks, would produce longer bursts. A rising set of observations have supported these two courses and the sorts of stellar objects related to them, Fryer stated.

A world workforce comprising researchers at universities, analysis institutes, NASA and Los Alamos collaborated on the work. Fryer led the modeling workforce, which included Wollaeger and Chase. The Los Alamos workforce has developed supernova and kilonova modeling codes that run on supercomputers. Making use of these codes to the observational data was key to deciphering the observations of GRB211211A.