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

Gamma-ray bursts (GRBs) are the most luminous explosions in the Universe and are known to occur as a result of core collapse of rapidly rotating massive stars and neutron star mergers. They are detected as brief flashes of gamma-rays, with their prompt emission typically lasting between a fraction of a second and several minutes. GRB 250702B was first detected when the Fermi Gamma-ray Burst Monitor triggered multiple times over a ~3 hour period, and it was later found to be the longest GRB ever observed with a ~25,000 s gamma-ray duration. Observations with five GRB monitors reveal a hard spectrum, subsecond variability, and high total energy, which are only known to arise from an ultrarelativistic jet powered by a rapidly spinning, stellar-mass central engine. These properties and the extreme duration are together incompatible with all confirmed GRB progenitors. GRB 250702B is naturally explained with the helium merger model, where a black hole falls into 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 and the helium merger explanation.