The first large-scale numerical simulation of black hole-neutron star merger matches puzzling observations.
In 2022, scientists from Northwestern University presented novel observational data indicating that long gamma-ray bursts (GRBs) might originate from the collision of a neutron star with another dense celestial body, such as another neutron star or a black hole — a finding that was previously believed to be impossible.
Now, another Northwestern team offers a potential explanation for what generated the unprecedented and incredibly luminous burst of light.
After developing the first numerical simulation that follows the jet evolution in a
Full simulation of the large-scale evolution of a jet from a black hole-neutron star merger. Credit: Ore Gottlieb/
From pre-merger to long GRB
To further reveal what occurs during compact-merger events, Gottlieb, Issa and their collaborators sought to simulate the whole process — from before the merger all the way through to the end of the GRB event, when the GRB-producing jets shut off. Because it is such an incredibly computationally expensive feat, the entire scenario had never been modeled before. Gottlieb and Issa overcame that challenge by dividing the scenario into two simulations.
First, the researchers ran a simulation of the pre-merger phase. Then, they took the output from the first simulation and plugged it into the post-merger simulation.
“Because the space-time used by the two simulations is different, this remap was not as straightforward as we had hoped, but Danat figured it out,” Tchekhovskoy said.
“The daisy chaining of the two simulations allowed us to make the computation much less expensive,” Gottlieb said. “The physics is very complicated in the pre-merger stage because there are two objects. It gets much simpler after the pre-merger because there is only one black hole.”
In the simulation, the compact objects first merged to create a more massive black hole. The black hole’s intense gravity pulled the now-destroyed neutron star’s debris toward it. Before the debris fell into the black hole, some of the debris first swirled around the black hole as an accretion disk. In the configuration studied, the emerging disk was particularly massive with one-tenth the mass of our sun. Then, when the mass fell into the black hole from the disk, it powered the black hole to launch a jet that accelerated to near light speed.
Disk properties matter
A surprise emerged as the researchers adjusted the strength of the massive disk’s magnetic field. Whereas a strong magnetic field resulted in a short, incredibly bright GRB, a weak magnetic field generated a jet that matched observations of long GRBs.
“The stronger the magnetic field, the shorter is its lifetime,” Gottlieb said. “Weak magnetic fields produce weaker jets that the newly formed black hole can sustain for a longer time. A key ingredient here is the massive disk that can maintain, together with weak magnetic fields, a GRB consistent with observations and comparable to the luminosity and long duration of GRB211211A. Although we found this specific binary system to give rise to a long GRB, we also expect that other binary mergers that produce massive disks will lead to a similar outcome. It’s simply a question of the post-merger disk mass.”
Of course, “long” is relative in this scenario. GRBs are divided into two classes. GRBs with durations less than two seconds are considered short. If a GRB is two seconds or longer, then it’s considered long. Even events this brief are still exceptionally difficult to model.
“A major portion of this disk material ultimately gets consumed by the black hole, with the whole process lasting mere seconds,” Issa said. “Here lies the main challenge: It is very difficult to capture the evolution of these mergers, using simulations on supercomputers, over a span of several seconds.”
Next up: Neutrinos
Now that Gottlieb and Issa have successfully and comprehensively modeled the full sequence of the merger, they are excited to continue to update and improve their models.
“My current efforts are directed towards enhancing the physical DOI: 10.3847/2041-8213/aceeff
The study was funded by