Timing (Pulsating) ULX-ray sources – lessons learned from M82

M82 X-2 is the archetypal pulsating ultraluminous X-ray sources – X-ray pulsars whose flux can be hundreds of times their Eddington limit. Since 2014, we have been conducting a thorough observation campaign of this pulsar, tracking the acceleration of its rotation over time, and now, tracking its orbital evolution. This work can in principle be applied to other ULXs as well, to try to understand their age, their mechanism of mass transfer and, ultimately, their origin and their fate.