Last week, NuSTAR observed M82 X-2, the archetypal pulsating ultraluminous X-ray source. Discovered by NuSTAR in 2014, this class of objects provided the first undeniable evidence of super-Eddington accretion—accretion at such an extreme rate that the pressure from the radiation generated is enough to power massive winds of material blown off the puffed-up accretion disk. These neutron stars consume their companion star at an incredible rate, radiating at up to 500 times their Eddington limit, the point at which this radiation pressure is as powerful as the force of gravity on the material being accreted. Over 12 years of observations, NuSTAR has uncovered a rich phenomenology in M82 X-2—including spin accelerations, "glitches", and orbital decay—bringing invaluable information on super-Eddington physics. These new observations will test our interpretation of this orbital decay against alternative explanations. NuSTAR is the only instrument capable of this work, as a nearby bright ULX’s low-energy X-ray emission drowns the pulsations in the energy range used by other X-ray observatories, but not in the high-energy band observed by NuSTAR.

Author: Matteo Bachetti (Senior researcher, INAF-Osservatorio Astronomico di Cagliari, Italy)