Gamma-ray bursts (GRBs) come in two varieties, long and short. Long GRBs, which last a couple of seconds to one minute, form when a star at least 10 times the mass of our Sun explodes as a supernova. Short GRBs, which last less than two seconds, occur when two compact objects, like two neutron stars or a neutron star and a black hole, collide to form a kilonova. While observing the aftermath of GRB 211211A, a long GRB event detected in 2021, two independent teams of astronomers found the surprising signs of a neutron-star merger rather than the expected signal of a supernova. This surprising result marks the first time that a kilonova has been associated with a long GRB and challenges the current understanding of these powerful explosions.
When neutron stars merge, they can produce radioactive ejecta that powers a kilonova signal, as this conceptual image shows. Image credit: Dreamstime.
Long-duration GRBs (longer than two seconds) are typically associated with supernovae, while short-duration GRBs (less than two seconds) are commonly associated with kilonovae and neutron-star mergers.
“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 Dr. Chris Fryer, an astrophysicist at Los Alamos National Laboratory.
“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.”
On December 11, 2021, several observatories and satellites — such as the Gemini North Observatory, NASA’s Neil Gehrels Swift Observatory and Fermi Gamma-ray Space Telescope — recorded a very bright, 50-second gamma-ray burst and optical, infrared and X-ray emissions associated with the event.
Dubbed GRB 211211A, this long GRB was relatively nearby — about 350 megaparsecs (1.1 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.
“Astronomers usually investigate short GRBs when hunting for kilonovae,” said Jillian Rastinejad, a Ph.D. student at Northwestern University.
“We were drawn to this longer-duration burst because it was so close that we could study it in detail. Its gamma rays also resembled those of a previous, mysterious supernova-less long GRB.”
This Gemini North image, superimposed on an image taken with the NASA/ESA Hubble Space Telescope, shows the telltale near-infrared afterglow of a kilonova produced by GRB 211211A. Image credit: International Gemini Observatory / NOIRLab / NSF / AURA / M. Zamani / NASA / ESA.
“Our modeling team 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 Dr. Ryan Wollaeger, also from Los Alamos National Laboratory.
“There is still more theoretical modeling to do to fully understand this transient, however.”
“This detection breaks our standard idea of gamma-ray bursts,” said Dr. Eve Chase, also from Los Alamos National Laboratory.
“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 findings appear today in two papers in the journal Nature.
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J. Rastinejad et al. 2022. A kilonova following a long-duration gamma-ray burst at 350 Mpc. Nature, in press; doi: 10.1038/s41586-022-05390-w
E. Troja et al. 2022. A nearby long gamma-ray burst from a merger of compact objects. Nature, in press; doi: 10.1038/s41586-022-05327-3
Source link: https://www.sci.news/astronomy/hybrid-neutron-star-merger-11455.html
