Gamma-Ray Burst's Engine Stayed Active for a Month, Astronomers Find

A team of astronomers led by the University of Birmingham has identified a gamma-ray burst whose central engine kept flaring for nearly a month after the initial explosion, far longer than any such burst observed before. The new analysis, posted Sept. 18 to the physics preprint server arXiv and not yet peer-reviewed, examines archival data on an event cataloged as GRB 220706A, which was first detected by NASA's Swift Observatory in July 2022 but whose full significance has only now become clear through years of patient follow-up observation.

The burst's designation reflects the date it was caught: July 6, 2022, when Swift's Burst Alert Telescope triggered on a flash of gamma rays from a galaxy roughly 7 billion light-years away. What makes it remarkable, according to the new study led by Birmingham astrophysicist Benjamin Gompertz, is what happened afterward: X-rays kept flaring from the explosion's source for far longer than theory says they should.

The numbers

Gamma-ray bursts are usually brief by cosmic standards, their initial high-energy flash lasting seconds to a few minutes before fading. GRB 220706A's prompt emission, recorded by Swift, lasted about 50 seconds. But its "central engine," the compact object thought to power the explosion, refused to switch off. X-ray flaring continued until roughly 51 days after the initial trigger in observed time. Correcting for the burst's redshift of 0.8577, that works out to 27 days in the explosion's own rest frame, which the authors describe as the longest central engine activity ever recorded in a gamma-ray burst, surpassing the previous record-holder by about three weeks.

The team also found evidence of an unusually bright supernova emerging from the afterglow roughly 17 days after the burst, with a peak brightness that, after accounting for dust obscuration along the line of sight, rivals some of the most luminous stellar explosions ever linked to a gamma-ray burst. The paper, credited to Gompertz and 32 co-authors across more than a dozen institutions, draws on data from Swift, the NICER and Chandra X-ray telescopes, the Very Large Array and the NOEMA millimeter observatory, and several ground-based optical telescopes.

How we got here

Most long gamma-ray bursts are thought to mark the collapse of a massive, rapidly spinning star that has already shed its outer layers, briefly powering a jet of particles that we see as a burst of gamma rays. That standard picture predicts an engine that burns itself out within minutes to hours. A smaller subset of bursts, dubbed "ultra-long," already strained that picture by lasting thousands of seconds, and researchers have debated for more than a decade whether they come from a genuinely different kind of progenitor star, a different power source, or simply the extreme tail of an otherwise ordinary distribution.

GRB 220706A pushes that debate further than any burst studied so far. In the new paper, Gompertz and his co-authors lay out the comparison plainly, noting just how far outside precedent the new burst falls.

"This is ≈20.6 rest-frame days later than the flares seen in GRB 210204A, which were previously claimed to be the most delayed ever observed," the authors write.

The researchers weigh several explanations for what could keep an engine active so long. A magnetar, an extraordinarily strongly magnetized neutron star, has successfully explained one previous ultra-long burst paired with a bright supernova, but the authors say that scenario looks "fine tuned" for this event. A black hole steadily fed by material falling back from a collapsing supergiant star, with an envelope roughly 10 times the mass of the sun and a radius of about 10 trillion centimeters, is their preferred alternative, though they do not rule out that the optical signal they interpret as a supernova could instead be something else entirely, such as a star torn apart by the black hole's gravity.

Who is affected

The direct audience for the result is the small international community that studies gamma-ray bursts and the extreme physics of collapsing stars and black hole formation, including scientists who run NASA's Swift and NICER missions and the ground- and space-based observatories that supplied follow-up data. The event is already logged in independent tracking resources such as the IceCube collaboration's public GRB catalog, which compiles basic parameters for bursts detected across multiple observatories and is used by physicists searching for associated neutrinos or gravitational waves.

More broadly, the finding matters to anyone trying to understand how the most massive stars die. If engines like this one really can stay active for weeks, models of jet formation, black hole accretion and the supernovae that accompany these bursts may need revising, with implications for how astronomers interpret future bursts seen by Swift's successors and by gravitational-wave detectors looking for related signals.

Reaction and what happens next

Because the paper has only just been posted and has not yet completed peer review, independent reaction from scientists outside the collaboration is still limited. The authors themselves frame the result as a direct challenge to existing models rather than a settled explanation, repeatedly flagging in the paper's discussion section that each of the mechanisms they consider, a magnetar, sustained black hole accretion or a disguised tidal disruption event, has difficulty accounting for every feature of what was observed.

Gompertz, who holds posts in the School of Physics and Astronomy at the University of Birmingham and the university's Institute for Gravitational Wave Astronomy, and his collaborators say the case will likely only be resolved with continued multi-wavelength monitoring of similar bursts and further theoretical modeling of how long a stellar corpse can keep feeding a central engine. The original detection details, including the burst's coordinates and early X-ray and optical follow-up, remain on file in NASA's Gamma-ray Coordinates Network archive, the real-time alert system astronomers used to coordinate observations in the hours after the 2022 trigger. The new paper is now under consideration for publication in a peer-reviewed journal, a process that typically takes several months and could still alter some of its conclusions.