Evidence for Fe K_α line and soft X-ray lag in NGC 7456 ultraluminous X-ray source-1

Abstract

We report the first detection of an Fe K_α line and soft X-ray lag in the ultraluminous X-ray (ULX) source NGC 7456 ULX-1. The XMM-Newton spectra show the presence of the 6.4 keV Fe line at 2.6σ confidence and an upper limit on the FWHM of 32900 km s^-1. Assuming that the line arises by reflection from a Keplerian disk, it must originate beyond 85r_g from the compact object. As a result of Fourier timing analysis, we found that the soft X-ray photons lag behind the hard X-ray photons with a ~1300 s delay. The covariance spectra indicate that the hard spectral component is responsible for the correlated variability and the soft X-ray lag. This is the second ULX in which an Fe K_α line is found, the fifth with a soft X-ray lag, and the first with both features detected.