Unprecedented gamma-ray burst explained by long-lived jet

Final yr, Northwestern College researchers reported new observational evidence that long gamma-ray bursts (GRBs) may end up from the merger of a neutron star with one other compact object (both one other neutron star or black hole)—a discovering that was beforehand believed to be unimaginable.

Now, one other Northwestern workforce gives a possible clarification for what generated the unprecedented and extremely luminous burst of sunshine. The examine, “Massive-scale evolution of seconds-long relativistic jets from black hole-neutron star mergers,” was printed Aug. 31 within the Astrophysical Journal.

After growing the primary numerical simulation that follows the jet evolution in a black hole-neutron star merger out to massive distances, the astrophysicists found that the post-merger black hole can launch jets of fabric from the swallowed neutron star.

However the important thing components are the mass of the violent whirlpool of fuel (or accretion disk) surrounding the black hole and the power of the disk’s magnetic field.

In large disks, when the magnetic subject is robust, the black hole launches a short-duration jet that’s a lot brighter than something ever seen in observations. When the huge disk has a weaker magnetic subject, nevertheless, the black hole launches a jet with the identical luminosity and lengthy period because the mysterious GRB (dubbed GRB211211A) noticed in 2021 and reported in 2022.

Not solely does the brand new discovery assist clarify the origins of lengthy GRBs, it additionally provides perception into the character and physics of black holes, their magnetic fields and accretion disks.

“To date, nobody else has developed any numerical works or simulations that persistently observe a jet from the compact-object merger to the formation of the jet and its large-scale evolution,” mentioned Northwestern’s Ore Gottlieb, who co-led the work. “The motivation for our work was to do that for the primary time. And what we discovered simply so occurred to match observations of GRB211211A.”

“Neutron-star mergers are a fascinating multi-messenger phenomena, which end in each gravitational and electromagnetic waves,” mentioned Northwestern’s Danat Issa, who co-led the work with Gottlieb. “Nevertheless, simulating these occasions poses a problem as a result of huge spatial and temporal scale separations concerned in addition to the various physics working throughout these scales. For the primary time, we now have succeeded in comprehensively modeling the complete sequence of the neutron star merger course of.”

Throughout the analysis, Gottlieb was a CIERA Fellow at Northwestern’s Heart for Interdisciplinary Exploration and Analysis in Astrophysics (CIERA); now he’s a Flatiron Analysis Fellow on the Flatiron Institute’s Heart for Computational Astrophysics. Issa is a graduate scholar within the Division of Physics and Astronomy at Northwestern’s Weinberg Faculty of Arts and Sciences and member of CIERA. Issa is suggested by paper co-author Alexander Tchekhovskoy, an affiliate professor of physics and astronomy at Weinberg and member of CIERA.

Curious kilonova

When astronomers first noticed GRB211211A in December 2021, they initially assumed that the 50-second-long occasion was generated from the collapse of a large star. However, as they examined the lengthy GRB’s late-time emission, known as the afterglow, they uncovered proof of a kilonova, a uncommon occasion that solely happens after the merger of a neutron star with one other compact object.

The finding (printed in Nature in December 2022) upended the long-established, long-accepted perception that solely supernovae may generate lengthy GRBs.

“GRB 211211A reignited curiosity within the origin of long-duration GRBs that aren’t related to large stars, however possible originating from compact binary mergers,” Gottlieb mentioned.

From pre-merger to lengthy GRB

To additional reveal what happens throughout compact-merger occasions, Gottlieb, Issa and their collaborators sought to simulate the entire course of—from earlier than the merger right through to the top of the GRB occasion, when the GRB-producing jets shut off. As a result of it’s such an extremely computationally costly feat, the complete situation had by no means been modeled earlier than. Gottlieb and Issa overcame that problem by dividing the situation into two simulations.

First, the researchers ran a simulation of the pre-merger phase. Then, they took the output from the primary simulation and plugged it into the post-merger simulation.

“As a result of the space-time utilized by the 2 simulations is totally different, this remap was not as simple as we had hoped, however Danat figured it out,” Tchekhovskoy mentioned.

“The daisy chaining of the 2 simulations allowed us to make the computation a lot inexpensive,” Gottlieb mentioned. “The physics could be very difficult within the pre-merger stage as a result of there are two objects. It will get a lot less complicated after the pre-merger as a result of there is just one black hole.”

Within the simulation, the compact objects first merged to create a extra large black hole. The black hole’s intense gravity pulled the now-destroyed neutron star’s particles towards it. Earlier than the particles fell into the black hole, a few of the particles first swirled across the black hole as an accretion disk. Within the configuration studied, the rising disk was notably large 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 close mild velocity.

Disk properties matter

A shock emerged because the researchers adjusted the power of the huge disk’s magnetic subject. Whereas a robust magnetic subject resulted in a brief, extremely brilliant GRB, a weak magnetic subject generated a jet that matched observations of lengthy GRBs.

“The stronger the magnetic subject, the shorter is its lifetime,” Gottlieb mentioned.

“Weak magnetic fields produce weaker jets that the newly shaped black hole can maintain for an extended time. A key ingredient right here is the huge disk that may keep, along with weak magnetic fields, a GRB per observations and akin to the luminosity and lengthy period of GRB211211A. Though we discovered this particular binary system to offer rise to an extended GRB, we additionally count on that different binary mergers that produce large disks will result in an identical consequence. It is merely a query of the post-merger disk mass.”

After all, “lengthy” is relative on this situation. GRBs are divided into two courses. GRBs with durations lower than two seconds are thought-about quick. If a GRB is 2 seconds or longer, then it is thought-about lengthy. Even occasions this transient are nonetheless exceptionally tough to mannequin.

“A serious portion of this disk materials finally will get consumed by the black hole, with the entire course of lasting mere seconds,” Issa mentioned. “Right here lies the principle problem: It is rather tough to seize the evolution of those mergers, utilizing simulations on supercomputers, over a span of a number of seconds.”

Subsequent up: Neutrinos

Now that Gottlieb and Issa have efficiently and comprehensively modeled the complete sequence of the merger, they’re excited to proceed to replace and enhance their fashions.

“My present efforts are directed in direction of enhancing the bodily accuracy of the simulations,” Issa mentioned. “This entails the incorporation of neutrino cooling, an important part that holds the potential to considerably affect the dynamics of the merger course of. Moreover, the inclusion of neutrinos serves as a vital step in direction of attaining a extra correct evaluation of the nuclear composition of the fabric ejected as a consequence of those mergers. By means of this method, my purpose is to supply a extra complete and correct image of neutron star mergers.”