The ultraluminous X-ray pulsar NGC 5907 ULX-1 and its nebula

Ultraluminous X-ray pulsars are accretion-powered pulsars whose X-ray
luminosity, if isotropic, largely exceeds their Eddington limit. Only a handful
are known to date and NGC 5907 ULX-1, distant 17 Mpc from us, is the most luminous of them,
at 1e41 erg/s 500 times its Eddington limit. We constrained through
pulsar timing its orbit and spin behavior, estimating a 5d orbital period and a spin-up
time <40 yr. We detected an extended X-ray feature surrounding the source: likely a young and
powerful bubble driven by the wind from the accreting pulsar system. The nebula
has been powered at ~1e41 erg/s for ~70 kyr proving that a highly super-Eddington
regime can be sustained for much longer than the spin-up time of the pulsar.
I will present the current status of the observations and understanding of this system
and the prospects to solve the still open problems it raises.