A team of astronomers including George Washington University physics Ph.D. student Eliza Neights recorded an extraordinary cosmic outburst this July which likely heralds a new kind of stellar explosion. With a flood of data from sources including NASA satellites, the team observed a gamma-ray burst (GRB), the most powerful class of cosmic explosions. But while most GRBs are over in a minute, this one continued for days.
Scientists say the best explanation for the outburst is that a black hole consumed a star, but they disagree on exactly how it happened. The possibilities include a black hole weighing a few thousand times the sun’s mass shredding a star that passed too close or a much smaller black hole falling into and consuming its stellar companion.
“The initial wave of gamma rays lasted at least seven hours, nearly twice the duration of the longest GRB seen previously, and we detected other unusual properties,” said Neights, a student at the Columbian College of Arts and Sciences who partners with NASA’s Goddard Space Flight Center. “This is certainly an outburst unlike any other we’ve seen in the past 50 years.”
Neights and other astronomers shared their results in October at the American Astronomical Society’s High Energy Astrophysics Division meeting in St. Louis, Missouri.
An exceptional burst
Detected about once a day, GRBs can appear anywhere in the sky with no warning. They are very distant events, with the closest-known example erupting more than 100 million light-years away.
The record-setting duration of the July burst, named GRB 250702B, places it in a class by itself. Of the roughly 15,000 GRBs observed since the phenomenon was first recognized in 1973, none are as long and only a half dozen even come close.
Most bursts last from a few milliseconds to a few minutes and are known to form in two ways, either by a merger of two city-sized neutron stars or the collapse of a massive star once its core runs out of fuel. Each produces a new black hole. Some of the matter falling toward the black hole becomes channeled into tight jets of particles that stream out at almost the speed of light, creating gamma rays as they go. But neither of these types of bursts can readily create jets able to fire for days, which is why 250702B poses a unique puzzle.
Seeing the light
The Gamma-ray Burst Monitor on NASA’s Fermi Gamma-ray Space Telescope discovered the burst and triggered multiple times over the course of three hours. It was also detected by the Burst Alert Telescope on NASA’s Neil Gehrels Swift Observatory, the Russian Konus instrument on NASA’s Wind mission, the Gamma-Ray and Neutron Spectrometer on Psyche—a NASA spacecraft currently en route to asteroid 16 Psyche—and Japan’s Monitor of All-sky X-ray Image instrument on the International Space Station.
“The burst went on for so long that no high-energy monitor in space was equipped to fully observe it,” said Eric Burns, an astrophysicist at Louisiana State University and a member of Neights’ team studying the burst’s gamma-ray glow. “Only through the combined power of instruments on multiple spacecraft could we understand this event.”
The Wide-field X-ray Telescope on China’s Einstein Probe also detected the burst in X-rays and showed that a signal was present the previous day. The first precise location came early July 3 when Swift’s X-Ray Telescope imaged the burst in the constellation Scutum, near the crowded, dusty plane of our Milky Way galaxy. Given this location and the previous X-ray detection, astronomers wondered if this event might be a different type of outburst from somewhere within our own galaxy.
Images from some of the largest telescopes on the planet, including those at the Keck and Gemini observatories on Hawaii and the European Southern Observatory’s VLT (Very Large Telescope) in Chile, hinted that there was a galaxy at the spot, so astronomers turned to NASA’s Hubble Space Telescope for a clearer view.
“It’s definitely a galaxy, proving it was a distant and powerful explosion, but it is a strange looking one,” said Andrew Levan, an astrophysics professor at Radboud University in the Netherlands who led the VLT and Hubble study. “The Hubble data could either show two galaxies merging or one galaxy with a dark band of dust splitting the core into two pieces.”
More recent images captured by the NIRcam instrument on NASA’s James Webb Space Telescope strongly support Levan’s interpretation. “The resolution of Webb is unbelievable. We can see so clearly that the burst shined through this dust lane spilling across the galaxy,” said Huei Sears, a postdoctoral researcher at Rutgers University who led the NIRcam observations. “It’s fantastic to see the GRB host in such detail.”
In late August, a team led by Benjamin Gompertz at the University of Birmingham in the U.K. used Webb and VLT to determine the galaxy’s distance and other properties. “The burst was remarkably powerful, erupting with the equivalent energy emitted by a thousand suns shining for 10 billion years,” Gompertz said. “Amazingly, the galaxy is so far away that light from this explosion began racing outward about 8 billion years ago, long before our sun and solar system had even begun to form.”
A comprehensive study of the X-ray light following the main burst used observations from Swift, NASA’s Chandra X-Ray Observatory and the agency’s NuSTAR (Nuclear Spectroscopic Telescope Array) mission. Swift and NuSTAR data revealed rapid flares occurring up to two days after the burst’s discovery.
“The continued accretion of matter by the black hole powered an outflow that produced these flares, but the process continued far longer than is possible in standard GRB models,” said study lead Brendan O’Connor, M.S. ’20, M.Phil. ’20, a McWilliams Postdoctoral Fellow at Carnegie Mellon University. “The late X-ray flares show us that the blast’s power source refused to shut off, which means the black hole kept feeding for at least a few days after the initial eruption.”
Conflicting evidence
Fermi and Swift data indicate a typical, if unusually long, GRB. Spectroscopic Webb observations did not find a supernova explosion, which typically follows a stellar collapse GRB, although it may have been obscured by dust and distance. The Einstein Probe saw X-rays a day before the burst, while NuSTAR tracked X-ray flares up to two days after it. Neither is typical for GRBs.
In addition, a detailed study led by Jonathan Carney, a graduate student at the University of North Carolina, Chapel Hill, shows that the host galaxy is very different from the typically small galaxies that host most stellar collapse GRBs. “This galaxy turns out to be surprisingly large, with more than twice the mass of our own galaxy,” he said.
In either of the two most discussed scenarios, the black hole will have eaten the star in about a day.
The first scenario invokes an intermediate-mass black hole, one with a few thousand solar masses and an event horizon—the point of no return—a few times larger than Earth. A star wanders too close, becomes stretched along its orbit by gravitational forces and is rapidly consumed by the black hole. This describes what astronomers call a tidal disruption event, but one caused by a rarely observed “middleweight” black hole, with a mass much greater than those born in a stellar collapse and much smaller than the behemoths found in the centers of big galaxies.
The gamma-ray team favors a different scenario because, if this burst is like others, the black hole’s mass must be more similar to our sun’s. Their model envisions a black hole weighing about three times the sun—with an event horizon just 11 miles across—orbiting and merging with a companion star. The star is of similar mass to the black hole but is much smaller than the sun. That’s because its hydrogen atmosphere has mostly been stripped away, down to its dense helium core, forming an object astronomers call a helium star.
In both cases, matter from the star first flows toward the black hole and collects into a vast disk, from which the gas makes its final plunge. At some point in this process, the system begins to shine brightly in X-rays. Then, as the black hole rapidly consumes the star’s matter, gamma-ray jets blast outward.
Notably, the helium star merger model makes a unique prediction. At some point, the black hole is totally immersed within the main body of the star, feasting on it from within. The energy it releases explodes the star and powers a supernova.
Unfortunately, this explosion occurred behind enormous amounts of dust, meaning even the Webb telescope is not powerful enough to see the expected supernova. While smoking-gun evidence to explain what happened in July will have to wait for future events, 250702B has already provided new insight into the longest GRBs, thanks in large part to the constant cosmic monitoring of NASA’s observatories and instruments as part of the agency’s quest to explore and understand the universe.
The Neights-led gamma-ray study appeared in the “Monthly Notices of the Royal Astronomical Society.”