Discovery of a Gamma-Ray Burst from a Black Hole Falling into a Star

Gamma-ray bursts (GRBs) are the most powerful explosions in the Universe. They are known to occur as a result of the core collapse of rapidly rotating massive stars and neutron star mergers; however, observations of the recent ultra-long GRB 250702B suggest that it originated from a black hole consuming a helium star. While GRBs typically have prompt gamma-ray emission lasting up to several minutes, GRB 250702B is the longest-duration GRB ever observed with a ~25,000 s gamma-ray duration. Observations with Fermi-GBM, Konus-Wind, Swift-BAT, Psyche-GRNS, and MAXI reveal a hard spectrum, subsecond variability, and high total energy release. These properties are typical of GRBs and only known to arise from an ultrarelativistic jet powered by a rapidly spinning, stellar-mass central engine. However, the extreme duration makes GRB 250702B incompatible with all confirmed GRB progenitors. It is instead naturally explained by a helium merger, where a black hole merges with a stripped star and proceeds to consume and explode it from within. In this presentation, I will discuss the gamma-ray observations of this extraordinary event, the helium merger explanation, and potential for observing similar events with the upcoming gamma-ray space telescope COSI.