KALAMAZOO, Mich. — Astronomers have unveiled a groundbreaking discovery: the longest gamma-ray burst ever recorded, an event that revealed a dramatic encounter between a black hole and a star. This unprecedented cosmic phenomenon, labeled GRB 250702B, lasted nearly seven hours, fundamentally challenging existing models of stellar explosions.
Detected on July 2, 2025, by NASA’s Fermi Gamma-ray Burst Monitor, GRB 250702B emitted signals for approximately 25,000 seconds, surpassing the previous record held by GRB 111209A, which had a duration of about 15,000 seconds. This significant finding stems from collaborative work among a team of over 50 scientists, who presented their discoveries in a recent research paper available on the arXiv preprint server.
Gamma-ray bursts are among the universe’s most energetic explosions, typically lasting from mere fractions of a second to a few minutes. Generally categorized into two types—short bursts linked to neutron star collisions and longer emissions associated with the collapse of massive stars—GRB 250702B defies conventional classification. Its massive energy output raised expectations for a short-lived event, yet it persisted longer than anticipated, displaying characteristics unlike those of any previous gamma-ray bursts.
The research team reported strong evidence indicating this burst’s unconventional nature, including a hard spectrum, rapid variability, and high total energy. These features suggest phenomena usually caused by ultrarelativistic jets powered by rapidly spinning stellar remnants. Yet, the extraordinary duration of GRB 250702B contradicts all established theories that explain gamma-ray bursts, prompting the researchers to investigate further.
Standard models, such as collapsars, were quickly ruled out because collapsing stars cannot maintain the necessary rotation for the observed length of time. Similarly, scenarios involving neutron star mergers or magnetar flares were dismissed, as they lack the capacity for prolonged emissions. Other possibilities, including white dwarf mergers, did not align with the delayed peak energy observed in GRB 250702B.
Notably, even the theory of a supermassive black hole tearing apart a star proved inadequate. While such events can unfold over several days, they typically take place in galactic centers and exhibit slower variability than what was documented. Observations confirmed that this gamma-ray burst originated from a star-forming region distinct from its galaxy’s center.
Faced with the limitations of existing models, the research team proposed a novel explanation known as the helium merger model. This scenario involves a binary star system where one star has collapsed into a black hole, while its companion star expands and eventually engulfs it. As the black hole spirals inward, it interacts with the star’s layers and ultimately reaches its dense core.
During this process, the intense forces at play generate an accretion disk around the black hole, producing powerful magnetic fields capable of launching the ultrarelativistic jets that create gamma rays. The interactions within this disk also lead to vigorous stellar winds, culminating in a supernova event that propels the gamma-ray burst into the universe, akin to collapsar mechanisms.
This discovery not only opens a new chapter in the understanding of stellar deaths but also enhances connections within astrophysical studies. The helium merger model may bridge diverse fields, linking gamma-ray bursts with gravitational wave sources and unique supernovae types.
The findings surrounding GRB 250702B signify a remarkable leap in our understanding of extreme cosmic phenomena and remind researchers that the universe still has many secrets waiting to be uncovered.