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

Abstract

We present the optical and X-ray study of the ultraluminous X-ray source (ULX) NGC 5474 X-1. The X-ray spectrum taken during the bright state of the source (LX ∼ 2 × 1040 egs/s) shows signatures of a broad absorption line at ≃ 8 keV which may be a cyclotron resonant scattering feature. This implies that this system may host a neutron star with a magnetic field ∼1012 G. The first observation of this area with the Hubble Space Telescope (HST) carried out 14 months later revealed that the source was bright in the optical range as well. The subsequent observations have shown that the source faded in both ranges (more than 2.8m in the U band and by a factor of 50-100 in X-rays) and has never become bright again. Deeper HST observations made it possible to impose constraints on the donor star spectral class and mass (< 7 M⊙), as well as to identify a stellar cluster of about 1 Gyr, the centre of which is located at a projected distance of ≃ 2 pc from NGC 5474 X-1. The ULX can be a member of this old cluster, however, the presence of stars with ages of ∼10 Myr within 300 pc around the ULX does not completely exclude the possibility that it is just an accidental projection.