New research using computer simulations and astronomical data finds that some long-lasting gamma-ray bursts (GRBs) are caused by cosmic mergers that form black holes, challenging previous theories that GRBs are caused entirely by the collapse of stars. This breakthrough provides a more complete understanding of the origin of gamma-ray bursts.
Astrophysicists at the Fratillon Institute and colleagues have used state-of-the-art computer simulations to give us a clearer understanding of how black holes generate the most energetic bursts in the universe.
Cutting-edge computer simulations combined with theoretical calculations are helping astronomers better understand the origins of some of the most energetic and mysterious light phenomena in the universe - gamma-ray bursts (GRBs). The new unified model confirms that some long-lasting gamma-ray bursts are produced after cosmic mergers that created baby black holes surrounded by a giant disk of raw matter.
Astronomers previously thought that the black holes that create long GRBs typically form when massive stars collapse. However, new models show that they could also be created when two dense objects merge, such as a pair of neutron stars - the dense, dead remnants of massive stars - or a black hole and a neutron star. The findings explain the recently observed long GRBs, which astronomers have been unable to link to stellar collapse.
The creators of the simulation published their results Nov. 29 in The Astrophysical Journal Letters.
Simulations show how the merger of a black hole and a neutron star creates the powerful jets and winds that create gamma-ray bursts. A new study proposes a framework linking the physics of such mergers to observations of gamma-ray bursts. Research has found that mergers of massive objects such as black holes and neutron stars can produce long-lasting gamma-ray bursts. Source: Ole Gottlieb
"Our findings connect observations to fundamental physics and unify many unanswered questions in the field of gamma-ray bursts," said Ore Gottlieb, lead author of the new study and a researcher at the Center for Computational Astrophysics (CCA) at the Flatiron Institute in New York City. "For the first time, we can know what happens before a black hole forms through observations of GRBs."
GRBs are the brightest and most violent events in the universe. GRBs have dazzled and puzzled astronomers since they were first detected in 1967. Even as decades have passed, the exact mechanism that produces powerful gamma-ray bursts remains uncertain. Over the years, astronomers have noticed two distinct types of gamma-ray bursts - one lasting less than a second and another lasting 10 seconds or more. The researchers ultimately determined that short GRBs originate from jets emitted after the merger of two compact objects, while long GRBs may occur in jets emitted when massive rotating stars collapse. But over the past year, two unusual observations of long GRBs have shown that it's not just collapsing behemoths that cause long GRBs.
Gottlieb and his colleagues conducted state-of-the-art simulations to test how mergers of massive, compact objects could trigger gamma-ray bursts. The new simulations took several months and were performed in part on a supercomputer at the Fratilon Institute. The new simulation starts when the two compact objects are in close orbit and tracks the jets until they move away from the merger site. This approach allows researchers to make fewer assumptions about the physics involved. By combining simulations with constraints from astronomical data, the scientists constructed a unified model of the origin of GRBs.
Researchers determined that the unusual GRB was created after the merger of two compact objects. The merger creates a black hole surrounded by a large accretion disk - a rapidly spinning donut of magnetic leftover material - that can emit long GRBs. This information from the simulations helps astronomers understand not only the objects that produce these gamma-ray bursts, but also what happens before them.
Simulations show how the merger of a black hole and a neutron star creates the powerful jets and winds that create gamma-ray bursts. A new study proposes a framework linking the physics of such mergers to observations of gamma-ray bursts. Research has found that mergers of massive objects such as black holes and neutron stars can produce long-lasting gamma-ray bursts. Source: Ole Gottlieb
Gottlieb said: "If we see a long gamma-ray burst like the one observed in 2022, we now know that it comes from a black hole with a massive disk. Knowing that there is a massive disk, we can now work out the mass ratio of the two parent bodies, because their mass ratio is related to the properties of the disk. For example, the merger of neutron stars with unequal masses will inevitably produce GRBs with longer durations."
Scientists hope to use this unified model to determine which objects produce short GRBs. The model suggests that these bursts could be caused by black holes with smaller accretion disks, or from a type of object known as a supermassive neutron star, an unstable form of star that rapidly collapses to form a black hole but not before emitting a short GRB pulse. The scientists hope that with more GRB observations, they can further refine their simulations to determine the origin of all GRBs. While GRB observations are still relatively rare, astronomers aim to catch more GRBs when the Vera C Rubin Observatory begins observations in early 2025.
"As we make more observations of GRBs of different pulse durations, we will be better able to detect the central engine that powers these extreme events," Gottlieb said.