Transient Ultraluminous X-ray Source NGC5474 X-1

We present an optical and X-ray study of the ultraluminous X-ray source NGC5474 X-1. From the analysis of the Chandra and Swift data we found that the source X-ray flux changed by more than 2 orders of magnitude within 3.5 years. In the bright state with a peak luminosity of $1.3\times 10^{40}$ erg/s the source showed a very hard spectrum with $\Gamma \lesssim 1$. We have found no evidence for coherent pulsations. The first observation of this area with the Hubble Space Telescope (HST) carried out in the F336W filter (U band) 14 months after the X-ray flux was observed to be maximal, has revealed that the source was bright in the optical range as well. Later the source faded in both the ranges, the optical variability amplitude reached 2.8 mag. Subsequent deeper HST observations made it possible to identify a stellar cluster of about 1 Gyr in the object place, which claims to be the parent for NGC5474 X-1 However, the presence of stars with ages of $\sim10$ Myr within 300 pc around the ULX does not completely exclude the possibility that NGC5474 X-1 is a relatively young object. Comparing the source properties with other ULXs, we noticed the similarity to both the known neutron-star ULXs and the transient ULXs that are thought to be accreting black holes.