New Interpretation of “Impossible” Gamma-Ray Burst Offered by Scientists

by Amir Hussein
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Gamma-Ray Bursts

Researchers at Northwestern University have created simulations indicating that neutron star mergers, not just massive star collapses, can lead to long gamma-ray bursts. This finding enriches our understanding of black hole physics and contradicts current astrophysical models. Source:

Northwestern University scientists conducted the first extensive numerical simulation aligning with the enigmatic observations of a black hole-neutron star merger.

In 2022, a team from Northwestern University provided new observational evidence suggesting that long gamma-ray bursts (GRBs) could result from a neutron star colliding with a dense object like another neutron star or a black hole, previously considered implausible.

Another Northwestern team now proposes an explanation for the extraordinary and intensely bright burst of light observed.

They created the first numerical simulation tracking a black hole-neutron star merger’s jet evolution over great distances, revealing that the resulting black hole can eject jets from the engulfed neutron star.

The critical factors are the mass of the gas whirlpool (accretion disk) around the black hole and its magnetic field strength. In massive disks with a strong magnetic field, the black hole ejects a short, exceptionally bright jet, surpassing any previous observational brightness. Conversely, a weaker magnetic field in a massive disk results in a jet of similar luminosity and duration to the mysterious GRB (named GRB211211A) observed in 2021 and reported in 2022.

This discovery not only elucidates the origins of long GRBs but also provides insights into black hole nature, magnetic fields, and accretion disks.

The groundbreaking full-scale simulation of a black hole-neutron star merger jet evolution was credited to Ore Gottlieb of Northwestern University.

This study was recently published in the Astrophysical Journal.

Ore Gottlieb from Northwestern explained that no prior numerical simulations consistently traced a jet from a compact-object merger to its formation and large-scale evolution, which their work aimed to do for the first time. Surprisingly, their findings matched the observations of GRB211211A.

Danat Issa from Northwestern, who co-led the project with Gottlieb, described neutron-star mergers as fascinating multi-messenger phenomena producing both gravitational and electromagnetic waves. The comprehensive modeling of this entire merger sequence was a first in this field.

During the study, Gottlieb was a CIERA Fellow at Northwestern’s astrophysics center and is now a Flatiron Research Fellow at the Flatiron Institute’s Center for Computational Astrophysics. Issa, a graduate student in Northwestern’s physics and astronomy department and a CIERA member, is mentored by co-author Alexander Tchekhovskoy, an associate physics and astronomy professor at Northwestern.

The discovery of the GRB211211A kilonova in December 2021, initially believed to be from a massive star collapse, revealed evidence of a rare kilonova following a neutron star merger. This finding, published in Nature in December 2022, challenged the long-held assumption that only supernovae could create long GRBs.

Gottlieb commented that GRB 211211A revived interest in the origin of long-duration GRBs not linked to massive stars, likely stemming from compact binary mergers.

The Hubble Space Telescope captured GRB 211211A’s location, marked in red.

Gottlieb, Issa, and their team endeavored to simulate the entire compact-merger event sequence, from pre-merger to the GRB event’s conclusion. This complex task was previously unmodeled due to its computational demands. They divided the scenario into two simulations, starting with the pre-merger phase and then using its output for the post-merger simulation.

The simulations revealed the merger of compact objects into a larger black hole. This black hole attracted the neutron star’s remains, forming an accretion disk before some debris was ejected as a jet at near-light speed.

The simulations showed that a strong magnetic field in a massive disk leads to a short, incredibly bright GRB, while a weaker magnetic field produces a long-duration jet resembling long GRBs.

Gottlieb noted that “long” is relative, as GRBs under two seconds are short, while those over two seconds are long. Even these brief events pose significant modeling challenges.

Issa highlighted the difficulty in capturing these mergers’ evolution over several seconds using supercomputer simulations.

Gottlieb and Issa plan to refine their models further, with Issa focusing on incorporating neutrino cooling to enhance physical accuracy. This addition aims to provide a more detailed understanding of neutron star mergers.

The study, “Large-scale Evolution of Seconds-long Relativistic Jets from Black Hole–Neutron Star Mergers,” was authored by Ore Gottlieb, Danat Issa, and others, and published on August 31, 2023, in The Astrophysical Journal Letters (DOI: 10.3847/2041-8213/aceeff).

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Frequently Asked Questions (FAQs) about Gamma-Ray Bursts

What is the new discovery about gamma-ray bursts?

Researchers at Northwestern University have found that long gamma-ray bursts, previously thought to be caused only by massive star collapses, can also result from neutron star mergers. This discovery challenges established astrophysical theories and enhances our understanding of black hole physics.

How do neutron star mergers create gamma-ray bursts?

Neutron star mergers can create gamma-ray bursts through the formation of a black hole that ejects jets of material from the engulfed neutron star. The process depends on the mass of the accretion disk and the strength of its magnetic field.

What was unique about the Northwestern University study?

The Northwestern University study was unique in that it presented the first large-scale numerical simulation that matched the observations of a black hole-neutron star merger, providing a potential explanation for previously unexplained gamma-ray bursts.

What implications does this discovery have?

This discovery provides new insights into the nature and physics of black holes, their magnetic fields, and accretion disks. It also expands our understanding of the origins of long gamma-ray bursts, offering a new perspective on these cosmic events.

What was the role of numerical simulations in this research?

Numerical simulations played a crucial role in this research by allowing scientists to track the jet evolution in a black hole-neutron star merger over great distances. This approach helped them understand how the post-merger black hole could launch jets of material from the swallowed neutron star.

More about Gamma-Ray Bursts

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