Gamma-ray bursts (GRBs) are brief and extremely powerful cosmic explosions. They are thought to result from the collapse of massive stars or the merging of neutron stars in distant galaxies. They commence with an initial, very bright flash, called the prompt emission, with a duration ranging from a fraction of a second to hundreds of seconds. The prompt emission is accompanied by the so-called afterglow, a less brighter but longer-lasting emission over a broad range of wavelengths that fades with time. The first GRB detected by the MAGIC telescopes, known as GRB 190114C, reveals for the

All stellar-mass black holes have hitherto been identified by X-rays emitted from gas that is accreting onto the black hole from a companion star. These systems are all binaries with a black-hole mass that is less than 30 times that of the Sun. Theory predicts, however, that X-ray-emitting systems form a minority of the total population of star–black-hole binaries. When the black hole is not accreting gas, it can be found through radial-velocity measurements of the motion of the companion star. We report here radial-velocity measurements taken over two years of the Galactic B-type star, LB-1