Evidence for Fe Kα Line and Soft X-ray Lag in NGC 7456 Ultraluminous X-ray Source-1

ABSTRACT We report the first detection of an Fe Kα 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 85rg 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 Kα line is found, the fifth with a soft X-ray lag, and the first with both features detected.