Bright Gamma Ray Burst Confounds Models of Black Hole Birth
Last October, following one of the brightest flashes of gamma rays ever observed in the sky, telescopes around the world captured a wealth of data from an event that is thought to herald the collapse of a massive star and the birth of a black hole.
But that fire hose of data demonstrated clearly that our understanding of how stars collapse and generate enormous jets of outflowing material accompanied by powerful blasts of X-rays and gamma rays — and likely lots of heavy elements — is woefully inadequate.
“The data are so good that, basically, the models failed — failed deeply,” said Raffaella Margutti, associate professor of astronomy and of physics at the University of California, Berkeley. “That makes sense because the models are not very complicated. Nature is saying, ‘Well, what you’re seeing is probably an outflow that has way more components than what you think it is.'”
Details of the many sets of observations by radio, optical, X-ray and gamma ray telescopes were presented today at a High Energy Astrophysics Division meeting of the American Astronomical Society in Waikoloa, Hawaii, and in papers published in The Astrophysical Journal Letters.
Margutti was among the astronomers who mobilized observatories around the world after the gamma ray burst was detected by two NASA satellites on Oct. 9, 2022. Called GRB 221009A, it lasted over 300 seconds, marking it as a “long-duration” gamma ray burst (GRB) and linking it to the collapse of the core of a massive star into a black hole — this one about 1.9 billion light years from Earth. Core collapse is thought to push material out the poles of the star in highly collimated jets at speeds close to the speed of light. If Earth is in the path of the jet, we see a burst of gamma rays.
“As the jets slam into gas surrounding the dying star, a bright afterglow of light is produced across the entire spectrum,” said Tanmoy Laskar, assistant professor of physics and astronomy at the University of Utah and lead author of the study accepted by ApJ Letters. “The afterglows of GRBs fade quite rapidly, which means we had to be quick and nimble in capturing the light before it disappeared, taking its secrets with it.”
Margutti, Laskar and colleagues quickly triggered observing programs on NASA’s NuSTAR satellite, as well as observations at a slew of other facilities, including the Giant Metrewave Radio Telescope (GMRT) in India, the MeerKAT Array in South Africa, the U.S. National Science Foundation’s Karl G. Jansky Very Large Array (VLA) in New Mexico (USA), the Atacama Large Millimeter Array (ALMA) in Chile, and the Submillimeter Array (SMA) in Hawaii. The multi-wavelength observations collected by the researchers now comprise one of the most detailed data sets for a GRB afterglow to date. While they think the burst is related to the explosion of a massive star, they have not yet found evidence of light from the supernova.
With the NuSTAR observations, the researchers measured the shape of the X-ray spectrum with exquisite precision, allowing an estimation of how particles are accelerated by the explosion’s shock wave and spiral around magnetic fields roiled by the explosion.
“NuSTAR observations were essential for this study because they helped us pin down the strength of the magnetic field in the afterglow,” Margutti said. Knowing the magnetic field strength is important, because without it, the true energy of the explosion cannot be easily estimated.
From their analysis, the astronomers found that the energy of the jet was typical of most GRBs, even though from Earth it appeared about 70 times brighter than any previous GRB.
“We think that what makes this GRB bright, more than a high intrinsic energy, is instead the particularly narrow angle into which that energy is channeled,” said Kate Alexander, assistant professor of astronomy at the University of Arizona and a co-author of the study.
Upon analyzing and combining the data from all these telescopes, they found that the radio measurements were brighter than expected based on the X-ray and visible light alone. This did not fit the signature of a reverse shock — a hypothesized situation in which a shock wave propagates backward through the jet and generates radio emissions — but indicated something more complicated happened as the jet punched through material surrounding the collapsing star.
“Either we don’t understand reverse shocks, or we’ve found a completely new emission component,” Laskar said.
“We think that there is still a very fast-moving jet that is generating the X-rays and visible light in this afterglow,” added Margutti. “But our modeling suggests that something else entirely is creating the radio light.”
Observations of jets from colliding neutron stars, for example, show that jets are accompanied by turbulence around the narrow jet core that looks from a distance like a sheath of material.
“We know that jets launched by neutron star mergers develop wings of less collimated material around a very narrow core,” Margutti said. “It is natural to expect that a similar effect will happen to a jet that has to pierce through a significantly larger amount of material — for example, a massive star, as in the case of GRB 221009A. So, we do expect a jet with a very narrow core that dominates the high-energy emission, surrounded by a sheath of material.”
Whatever the cause, the data imply that a decades-old theory of GRB jets needs to be revisited, Laskar said.
Margutti emphasized that this stellar collapse has more to tell astronomers. The afterglow is still detectable and is likely to be visible for years. She and colleagues are planning observations with the James Webb Space Telescope, the Hubble Space Telescope and many ground-based telescopes to follow the changing light from GRB 221009A. And at some point, when the jets from the stellar explosion have traveled far enough from the black hole to be visible, they hope to get a picture of the jets using radio interferometers, such as the hemisphere-spanning Very Long Baseline Array.