Last year, Northwestern University researchers reported new observational evidence that long gamma-ray bursts (GRBs) can result from a neutron star merging with another compact object (another neutron star or a black hole); This was a discovery previously thought impossible.

Now another Northwest team offers a potential explanation for what caused the unprecedented and incredibly bright flash of light. The study, titled “Large-Scale Evolution of Second Relativistic Jets from Black Hole-Neutron Star Mergers,” was published Aug. 31 in the journal Astrophysical. Daily.

After developing the first numerical simulations following the evolution of the jet during the long-distance merger of a black hole and a neutron star, astrophysicists discovered that the black hole could eject jets of material from the neutron star absorbed after the merger.

But the key components are the mass of the turbulent gas vortex (or accretion disk) surrounding the black hole and the strength of the disk’s magnetic field.

In massive disks, when the magnetic field is strong, the black hole ejects a short-lived jet much brighter than any observed so far. However, when the massive disk has a weaker magnetic field, the black hole fires a long-lasting jet of the same luminosity as the mysterious GRB (named GRB211211A) observed in 2021 and recorded in 2022.

The new discovery not only helps explain the origin of long gamma-ray bursts, but also provides information about the nature and physics of black holes, their magnetic fields and accretion disks.

“Until now, no one has done a numerical study or simulation that consistently follows a jet from the assembly of a compact object to its formation and large-scale evolution,” said Ore Gottlieb of Northwestern, who led the study. . “The motivation for our work was to do this for the first time. And what we found is consistent with the observations of GRB211211A.”

“Neutron star mergers are a fascinating multi-messenger phenomenon that results in both gravitational and electromagnetic waves,” said Danat Issa of Northwestern, who led the study with Gottlieb. “However, simulating these events is a challenge because of the huge spatial and temporal scales, as well as the diversity of physics operating at these scales. For the first time, we have been able to comprehensively model the entire sequence of the neutron star fusion process.”

During the research, Gottlieb served as a CIERA Fellow at the Northwest Center for Astrophysics Interdisciplinary Research and Research (CIERA); He is currently a Flatiron Research Fellow at the Flatiron Institute’s Center for Computational Astrophysics. Issa is a graduate student in the Department of Physics and Astronomy at the Northwestern Weinberg College of Arts and Sciences and is a member of CIERA. Issa is mentored by Oleksandr Chekhovskyi, co-author of the paper, associate professor in the Weinberg Department of Physics and Astronomy and a member of CIERA.

interesting kilonova

When astronomers first detected GRB211211A in December 2021, they initially assumed the 50-second event was due to the collapse of a massive star. But by studying the late emission of a long burst of gamma rays called afterglow, they found evidence of a kilonova, a rare event that only occurs after a neutron star merges with another compact object.

This discovery (published) nature magazine In December 2022), it overturned the well-established and long-held notion that only supernovae could produce long gamma-ray bursts.

“GRB 211211A has renewed interest in the origin of long-duration gamma-ray bursts that are not associated with massive stars but likely originate from a compact binary merger,” Gottlieb said.

Pre-merger to long GRB

To shed more light on what happens during compact merger events, Gottlieb, Issa, and colleagues sought to model the entire process from pre-merge to the end of the GRB event, namely the shutdown of the jets that shut down the GRB. Because this is an incredibly expensive computational skill, the entire scenario had never been simulated before. Gottlieb and Issa overcame this problem by dividing the script into two simulations.

First, the researchers modeled the pre-merger phase. They then took the result of the first simulation and linked it to the post-merge simulation.

“This reconstruction was not as easy as we hoped, as the space-time used by the two simulations is different,” Chekhovsky said, “but Danat figured it out.” said.

“Combining the two simulations allowed us to make the computations much cheaper,” Gottlieb said. “Physics in the pre-merge stage is very complex because there are two objects. After the pre-merge, everything becomes much simpler because there is only one black hole.”

During the simulation, compact objects first coalesced, forming a larger black hole. The black hole’s strong gravity pulled the destroyed fragments of the neutron star towards it. Before the debris fell into the black hole, some of the debris first spiraled around the black hole in the form of an accretion disk. The resulting disk in the configuration studied was particularly large, one-tenth the mass of our Sun. Then, when the mass fell from the disk into the black hole, it caused the black hole to eject a jet reaching close to the speed of light. Source