A Faint X-ray Flash Exposes a Dying Star's Missing Jet —a surprising discovery for the international team, which includes researchers from LMU.
A rare cosmic explosion has given astronomers an unprecedented look at a massive star in its final moments, revealing a missing link between ordinary stellar explosions, so-called supernovae, and gamma-ray bursts, the brightest and most powerful phenomena in the Universe.

The event, named EP260321a, was first detected by the Einstein Probe satellite as a brief flash of X-rays from a galaxy about 500 million light-years away. Scientists interpret the signal as a “shock breakout,” the moment when the shock wave from a star’s collapsing interior bursts through its surface and releases the first light of a supernova.

Shock breakouts are thought to occur in every massive star’s death, but they’re notoriously hard to catch. They last only a short time and shine brightest in X-rays. In the past two decades, astronomers have confidently identified just one other clear X-ray shock breakout, making EP260321a an exceptionally rare find.

The X-ray flash set off a worldwide observing campaign. Among the first to catch it was LMU’s 2.1-meter Fraunhofer Telescope at Wendelstein Observatory, which spotted a rapidly brightening supernova later named SN 2026gzf. Observations of how its light evolved showed it belonged to the class known as broad-lined Type Ic supernovae. This class of supernovae typically has material shooting out in a jet at nearly the speed of light, producing gamma-ray bursts.

“Stars die on a daily basis somewhere in the Universe. But it’s rare that something unusual happens close enough for our observations to reveal fundamentally new insights. EP260321a rang an alarm bell right away - an X-ray flash but no gamma-ray alert? A supernova in a nearby galaxy, embedded in a blue knot that had already been getting brighter for years? That’s not what is expected, and it set off a chase around the globe. Fortunately, we were well prepared for exactly this kind of opportunity with LMU’s observatories,” says LMU astrophysicist Daniel Gruen, who led the observations with Wendelstein and with the Hobby-Eberly Telescope in Texas.

Not a typical dying massive star

As it turned out, SN 2026gzf was not your typical dying massive star. Researchers were surprised to find no evidence of a gamma-ray burst or relativistic jet following the explosion. This is even more unexpected because the explosion itself was not weak at all. In fact, its characteristics match well with other supernovae that did produce gamma-ray bursts.

Brendan O’Connor, an astronomer and McWilliams Fellow at Carnegie Mellon University (CMU) and lead author of one of the papers presenting the analysis, published in The Astrophysical Journal Letters, combined the telescope data with observations from NASA’s Chandra X-ray Observatory (CXO) and the NRAO’s Karl G. Jansky Very Large Array (VLA) radio observatory to reveal the full nature of the event.

“SN 2026gzf looks remarkably similar to other energetic supernovae that have been linked to gamma-ray bursts before. Yet multi-wavelength follow-up observations using the most sensitive facilities found no evidence of a jet or an afterglow that is typically seen in those events,” says O’Connor. “One possibility is that a jet was initially present, but ‘choked,’ either by the surface of the star or by material surrounding it.”

Another piece of the puzzle came from the spectra, the fingerprint of light emitted by the supernova and the stars in the galaxy around it. Using LMU’s share of the Hobby-Eberly Telescope, the team obtained an integral field spectrum, an image in which every pixel splits its light into a rainbow, revealing detailed information about its source. “What we found in the spectrum is that the star that exploded had been born from a cloud of pristine gas - mostly hydrogen, with only about 15% of the metals our Sun and local environment contain,” explains Malte Busmann, a graduate student at LMU and co-author of the study, adding, “Well those metals are being added right now. Just four days after the shock breakout, we’re already catching the first glimpses of metals forged by the dying star being flung outward in all directions.”

The site of the explosion is indeed one of the least metal-enriched environments known for this type of supernova. Low-metallicity environments are often thought to help massive stars retain the rotation needed to launch fast jets, yet SN 2026gzf produced no typical gamma-ray-burst jet. The Hobby-Eberly Telescope observations therefore show that low metallicity alone cannot determine whether a dying star successfully produces a gamma-ray burst.

"When a massive star forms from such almost pristine gas, we expect it to eject roughly half of its original mass before it eventually explodes. So this fits with the interpretation of the jet being choked by material surrounding the star" says Joachim Puls, expert on massive stars and their outflows at LMU.

With an exceptionally faint X-ray shock breakout, combined with the absence of the fast-moving jets that typically power gamma-ray bursts, EP260321a/SN 2026gzf acts as a unique bridge between ordinary, non-relativistic supernova shock breakouts and the far more extreme, relativistic explosions that generate gamma-ray bursts.

This discovery establishes that energetic broad-lined Type Ic supernovae do not always produce a gamma-ray burst, a bright relativistic shock breakout, and suggests that massive stars can die through a wider range of pathways than previously recognized.

A glimpse into the future of time-domain astronomy

Together with spectra obtained with CMU’s share in the Southern African Large Telescope and the Dark Energy Spectroscopic Instrument, and images taken by the Dark Energy Camera and Rubin Observatory that LMU participates in, and the Zwicky Transient Facility, a high-fidelity picture of the event emerged. Additional late-time observations have been approved with the James Webb Space Telescope to further reveal the inner workings of the explosion, its geometry, and ejecta composition.

“This sequence of observations offers a glimpse into the future of time-domain astronomy. By coordinating large, repeated sky surveys with smaller, dedicated telescopes, we can learn so much more from the surprises the Universe has in store for us,” says Xander Hall, a graduate student at CMU and second author of the study.