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

Abstract

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 requiresthe object to be a neutron star. The line energy of 7.5 keV corresponds to the magnetic field strength ∼ 10 12 G. The sourceshows a hard X-ray spectrum with Γ ∼ 1 and strong X-ray variability which is also typical for neutron star ULXs. In theoptical range, the object is interesting by the fact that it is projected onto an old stellar cluster (or located inside) with anage of about 1 Gyr.