The largest collapsing star sparks ever gamma-ray burst

In October 2022, an international team of researchers, including Northwestern University astrophysicists, observed. The brightest gamma-ray burst (GRB) ever recorded, GRB 221009A.

Now, a Northwestern-led team has identified the event responsible for the historic explosion — called BOAT („Brightest of All Time”) — the collapse and subsequent explosion of a massive star. The team discovered the explosion, or supernova, using NASA's James Webb Space Telescope (JWST).

While this discovery solves one mystery, another deepens.

The researchers speculated that the newly discovered supernova may contain evidence of heavier elements such as platinum and gold. However, an extensive search did not find a signature with such elements. The origin of the heavy elements in the universe continues to be one of the greatest open questions in astronomy.

The study will be published Friday (April 12) in the journal Nature Astronomy.

„When we confirmed that a GRB was formed by the collapse of a massive star, it gave us the opportunity to test a hypothesis about how some of the heavier elements in the universe form,” Northwestern said. Peter Blanchard, who led the study. „We don't see signatures of these heavy elements, which suggests that very energetic GRBs like BOAT don't produce these elements. That doesn't mean all GRBs don't produce them, but it's an important piece of information about where these heavy elements come from. Future observations with JWST, BOAT determines whether its 'normal' relatives produce these elements.”

Blanchard is a graduate student at Northwestern Center for Interdisciplinary Studies and Research in Astrophysics (CIERA), where he studies superluminous supernovae and GRBs. The study includes co-authors from the Center for Astrophysics | Harvard & Smithsonian; University of Utah; Penn State; University of California, Berkeley; Radboud University in the Netherlands; Space Telescope Science Institute; University of Arizona/Stewart Laboratory; University of California, Santa Barbara; Columbia University; Flatiron Company; University of Greifswald and University of Guelph.

The birth of the boat

When its light burst onto Earth on October 9, 2022, the craft was so bright that it overwhelmed most of the world's gamma-ray detectors. The powerful explosion occurred about 2.4 billion light-years from Earth, in the direction of the Sagitta galaxy, and lasted a few hundred seconds. As astronomers scrambled to spot the origin of this incredibly bright phenomenon, they were immediately struck with a sense of awe.

„As far as we can detect GRBs, there is no doubt that this GRB is the brightest we've ever seen by a factor of 10 or more.” Wen-fei FangAssociate Professor of Physics and Astronomy at Northwestern Weinberg College of Arts and Sciences and member of CIERA, said at the time.

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„This event produced some of the highest energy photons ever recorded by satellites designed to detect gamma rays,” Blanchard said. „This is an event that only Earth can see Once in 10,000 years. We are lucky to live in a time where we have the technology to detect these explosions occurring throughout the universe. It is very exciting to observe a rare astronomical phenomenon like BOAT and understand the physics behind this exceptional phenomenon.”

A 'normal' supernova

Instead of observing the event immediately, Blanchard, his close collaborator Ashley Villar of Harvard University, and their team wanted to observe the GRB later in life. About six months after the GRB was initially detected, Blanchard used JWST to study its aftermath.

„The GRP was so bright that it obscured any possible supernova signature in the first weeks and months of the explosion,” Blanchard said. „In these cases, the so-called afterglow of a GRB was like the headlights of a car coming straight at you, preventing you from seeing the car. So we had to wait for it to fade significantly to give us a chance. See the supernova.”

He used JWST's near-infrared spectrometer to observe the object's light at infrared wavelengths. That's when he saw the characteristic signature of elements like calcium and oxygen commonly found in supernovae. Surprisingly, it's not exceptionally bright – more like an incredibly bright GRB.

„It's not much brighter than previous supernovae,” Blanchard said. „This seems very normal in the context of other supernovae associated with less energetic GRBs. You would expect the same collapsing star to produce a more energetic and brighter GRB, and it would produce a more energetic and brighter supernova. But that's not the case. We have this very luminous GRB, but A normal supernova.”

Missing: Heavy elements

After confirming the existence of a supernova—for the first time—Blanchard and his collaborators looked for evidence of heavier elements within it. Currently, astrophysicists have an incomplete picture of all the mechanisms in the universe that can create elements heavier than iron.

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The primary mechanism for producing heavy elements is the rapid neutron capture process, which requires high concentrations of neutrons. So far, astrophysicists have confirmed the production of heavy elements by this process in the merger of two neutron stars, which was detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2017. These elusive things. There are simply too many heavy elements in the universe and too few neutron-star mergers.

„There may be another source,” Blanchard said. „Binary neutron stars take a very long time to merge. The two stars in the binary system must first explode to leave the neutron stars. Then, the two neutron stars can take billions and billions of years to slowly approach each other. The approach finally merges. But observations of very old stars show that most binary neutron stars „They show that parts of the universe were enriched with heavy metals before the stars merged. This points us to an alternative path.”

Astrophysicists have hypothesized that the collapse of a fast-spinning, massive star could also produce heavier elements—using infrared spectra obtained by JWST, the exact type of star BOAT produced, Blanchard studied the inner layers of the supernova. Heavy components must be built.

„The exploding material of the star is opaque at early times, so you can only see the outer layers,” Blanchard said. „But as it expands and cools, it becomes transparent. You can see photons coming from the supernova's inner layer.”

„Also, different elements absorb and emit photons at different wavelengths, depending on their atomic structure, giving each element a unique spectral signature,” Blanchard explained. „So looking at the spectrum of an object can tell what elements are present. When we examined BOAT's spectrum, we found no signature of heavy elements suggesting that extreme events like GRB 221009A are not primary sources. This is important information as we proceed. Try to follow where the heavy elements are forming. .”

Why so bright?

To tease out the supernova's light from the bright afterglow that preceded it, the researchers combined the JWST data with observations from the Atacama Large Millimeter/Submillimeter Array (ALMA) in Chile.

„Even months after the burst was discovered, the afterglow was bright enough to contribute a lot of light to the JWST spectra,” he said. Thanmoy Laskar, assistant professor of physics and astronomy at the University of Utah and co-author of the study. „Combining the data from the two telescopes allowed us to measure how bright the afterglow was during our JWST observations and allowed us to carefully extract the supernova's spectrum.”

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Although astrophysicists have yet to figure out how a „normal” supernova and a record-breaking GRP were produced by the same collapsed star, Lasker said it could be related. Shape and structure of relativistic jets. As they spin rapidly, massive stars collapse into black holes, creating jets of material that are launched at rates close to the speed of light. If these jets are narrow, they create a more focused – and brighter – beam.

„It's like focusing the beam of an incandescent lamp on a narrow column, as opposed to a broad beam that washes over an entire wall,” Lasker said. „In fact, this is one of the shortest jets ever observed for a gamma-ray burst, giving us a clue as to why the afterglow appeared bright. There may be other factors responsible for this, a question researchers will study for years to come.”

Additional clues may come from future studies of the galaxy where the shuttle occurred. „In addition to BOAT's spectrum, we also obtained the spectrum of its 'host' galaxy,” Blanchard said. „The spectrum shows signs of intense star formation, indicating that the birth environment of the original star may have been different from previous events.”

Team member Yijia Li, a graduate student at Penn State, found that BOAT's host galaxy has the lowest metallicity of all GRB host galaxies. „This is another unique feature of the boat that will help explain its characteristics,” Li said.

The study, „JWST detection of a supernova associated with GRB 221009A without an R-process signature,” was supported by NASA (Award No. JWST-GO-2784) and the National Science Foundation (Award Nos. AST-2108676 and AST-2002577). . This work is based on observations made by the NASA/ESA/CSA James Webb Space Telescope.

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