A sudden flare from a low-mass black hole in the binary system V404 Cygni has offered astrophysicists a unique opportunity to make precise measurements of the black hole’s magnetic field, finding it weaker than expected. The results appear in the journal Science. In V404 Cygni, a star slightly smaller than the Sun orbits a black hole 10 times its mass in only 6.5 days. The close orbit and strong gravity of the black hole produce tidal forces that...
