In an unprecedented celestial event, astronomers have detected the longest-lasting gamma-ray burst ever recorded, a violent eruption that persisted for over seven hours. The extraordinarily lengthy burst, originating from a galaxy 8 billion light-years away, is believed to be the result of a novel and exotic cosmic scenario: a black hole consuming its companion star from within, leading to a cataclysmic explosion that challenges existing models of stellar death.
The event, designated GRB 250702B, was first observed on July 2, 2025, by a network of space-based telescopes. Unlike typical long-duration gamma-ray bursts, which fade within minutes, this cosmic blast continued to emit high-energy radiation for thousands of seconds, repeatedly fading and rebrightening. This unusual behavior has led researchers to propose a new mechanism for generating these powerful cosmic flashes, involving the merger of a black hole with a bloated helium star in a close binary system, providing a rare glimpse into one of the universe’s most extreme phenomena.
An Unprecedented Cosmic Explosion
Gamma-ray bursts (GRBs) are the most luminous explosions in the universe, typically associated with the collapse of massive stars or the merger of compact objects like neutron stars. They are usually classified into two categories: short bursts, lasting less than two seconds, and long bursts, which can last up to a few minutes. GRB 250702B shatters this classification, with a staggering duration of approximately seven hours, or 25,000 seconds. This duration is significantly longer than the previous record-holder, GRB 111209A, making it a truly exceptional event that required a new explanation.
The signal from this distant cataclysm traveled for 8 billion years before reaching Earth. Its discovery has provided astrophysicists with a unique dataset to study the lifecycle of massive stars and the dynamics of black hole interactions. The prolonged and fluctuating nature of the gamma-ray emissions suggests a sustained energy source unlike any observed before, pointing away from the standard “collapsar” model where a rapidly spinning, massive star collapses into a black hole.
A Network of Celestial Observers
The detection and subsequent analysis of GRB 250702B were made possible by a collaboration of international observatories. The initial trigger came from NASA’s Fermi Gamma-ray Burst Monitor (Fermi-GBM), which first registered the hours-long stream of high-energy photons. This was complemented by data from the Chinese-European Einstein Probe’s Wide-field X-ray Telescope and the Russian Konus-Wind gamma-ray spectrometer, which together confirmed the event’s extraordinary duration and characteristics.
Follow-up observations were crucial to pinpointing the source. The European Very Large Telescope in Chile detected a faint, near-infrared afterglow, while the Hubble Space Telescope provided high-resolution images that located the burst’s origin. These observations placed GRB 250702B on the outskirts of a very distant and faint galaxy, helping to rule out other potential sources and solidify the association with a stellar death event.
The Helium Merger Model
A New Progenitor System
To explain the unique signature of GRB 250702B, scientists have proposed what is known as the “helium merger model.” This theory posits a binary system containing a black hole and a massive companion star. Over time, as the star exhausts its primary hydrogen fuel, it expands significantly, becoming a bloated helium star. This expansion can alter the orbital dynamics of the system, causing the black hole to be drawn into the star’s outer layers.
Once inside the star, the black hole begins a destructive process of consuming the stellar material from the core outwards. This “inside-out” consumption releases enormous amounts of energy over an extended period, accounting for the burst’s record-breaking length. The process culminates when the black hole’s accretion disk launches a powerful jet of particles, which tears through the remainder of the star and erupts into space as a gamma-ray burst.
Unusual Precursor Signals
Further strengthening this hypothesis was the detection of soft X-rays from the same source approximately one day before the main gamma-ray burst began. This precursor emission, registered by the Einstein Probe, is unprecedented in GRB observations. It likely corresponds to the initial interaction between the black hole and the star’s outer envelope, providing the first tangible evidence of the moments leading up to the star’s final destruction. This early warning signal offers a new window into the physics of such extreme binary systems.
Redefining Stellar Demise
The discovery of GRB 250702B and its proposed explanation represent a significant shift in the understanding of how gamma-ray bursts can be formed. For decades, long GRBs were almost exclusively linked to the collapsar model. The confirmation of a merger-induced explosion in a binary system provides the first strong evidence for a new and very rare progenitor channel. This helps explain outlier events that do not fit neatly into existing categories.
This event forces a re-evaluation of stellar evolution in binary systems and the ultimate fate of massive stars that co-orbit black holes. The slow, methodical destruction of a star from its core provides a new theoretical framework for astronomers to consider when observing transient cosmic phenomena. The rarity of such an alignment might explain why an event of this magnitude and duration has never been seen before, but its discovery suggests that other similarly unusual bursts may be found as observational technology improves.