Complex computer models explain origins of unprecedented GRB

GRB211211A, first detected in 2021, provided the first evidence of a long gamma-ray burst produced by a massive kilonova explosion.
Chris Young
An artist's impression of GRB211211A.

In 2021, astronomers observed a massive kilonova explosion that provided compelling new observational evidence.

A study released the following year showed, for the first time, that long gamma-ray bursts (GRBs) can result from the merger of a neutron star, either with another neutron star or with a black hole.

Now, a team of astrophysicists from Northwestern University has used computer simulations to offer a potential explanation for last year's unprecedented finding.

A massive Kilonova explosion

The Northwestern team developed numerical simulations that follow the evolution of a jet in a black hole-neutron merger. Using their model, they found that a post-merger black hole can launch jets of material from the neutron star it has devoured.

Key to this is the mass of the black hole's accretion disk and the strength of its magnetic field. Massive disks with a strong magnetic field lead to the black hole launching a short-duration jet much brighter than ever observed. On the other hand, when the disk has a weaker magnetic field, the black hole launches a jet with the same luminosity and long duration as the one discovered in 2021 — dubbed GRB211211A.

The researchers detailed their methodology and findings in a new paper in the Astrophysical Journal. Their recent discovery explains long GRBs while shedding new light on the behavior of black holes.

"So far, no one else has developed any numerical works or simulations that consistently follow a jet from the compact-object merger to the formation of the jet and its large-scale evolution," Northwestern’s Ore Gottlieb, who co-led the work, explained in a press statement. "The motivation for our work was to do this for the first time. And what we found just so happened to match observations of GRB211211A."

"Neutron-star mergers are a captivating multi-messenger phenomena, which result in both gravitational and electromagnetic waves," added Northwestern's Danat Issa, who co-led the work with Gottlieb. "However, simulating these events poses a challenge due to the vast spatial and temporal scale separations involved as well as the diverse physics operating across these scales. For the first time, we have succeeded in comprehensively modeling the entire sequence of the neutron star merger process."

A massive Kilonova explosion

When astronomers first spotted GRB211211A in December 2021, they first assumed it was a 50-second-long event that occurred after the collapse of a massive star.

However, analysis of the GRB's afterglow provided evidence of an enormous kilonova explosion — an event that only occurs after the merger of a neutron star with another compact object.

Before this discovery, it was believed that only supernovae could generate long GRBs. "GRB 211211A reignited interest in the origin of long-duration GRBs that are not associated with massive stars, but likely originating from compact binary mergers," Gottlieb explained.

Next, the researchers aim to continue improving their models to obtain even more insight into the process that forms these incredibly luminous, short-lived explosions.

"My current efforts are directed towards enhancing the physical accuracy of the simulations," Issa said. "This involves the incorporation of neutrino cooling, a vital component that holds the potential to significantly influence the dynamics of the merger process."

"Furthermore, the inclusion of neutrinos serves as a critical step towards achieving a more accurate assessment of the nuclear composition of the material ejected as a consequence of these mergers. Through this approach, my goal is to provide a more comprehensive and accurate picture of neutron star mergers.”

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