Evidence for Fe K${}_{\alpha }$ line and soft X-ray lag in NGC 7456 ULX-1

Abstract

We report the first detection of a Fe K${}_{\alpha }$ line and soft X-ray lag in the ultraluminous X-ray source (ULX) NGC 7456 ULX-1. The XMM-Newton spectra show the presence of the 6.4 keV Fe line at 2.6$\sigma$ 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 $85r_{\mathrm{g}}$ 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 $\sim$1300 s delay. The covariance spectra indicate that the hard spectral component is responsible for the correlated variability and the soft X-ray lag. This is the second ULX in which a Fe K${}_{\alpha }$ line is found, the fifth with a soft X-ray lag, and the first with both features detected.

5 pages, 8 figures, accepted for publication in MNRAS Letters

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