NGC 5474 X-1: the neutron star ULX in an old stellar cluster

We report the discovery of a cyclotron resonant scattering feature (CRSF) in the ultraluminous X-ray source (ULX) NGC5474 X-1 that makes it the third ULX with a CRSF. Cyclotron lines arise in strong magnetic fields, which requires the object to be a neutron star. The line energy of 7.5 keV corresponds to the magnetic field strength ∼ 1012 G. The source shows a hard X-ray spectrum with Γ ∼ 1 and strong X-ray variability which is also typical for neutron star ULXs. In the optical range, the object is interesting by the fact that it is projected onto an old stellar cluster (or located inside) with an age of about 1 Gyr.