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

TL;DR

This paper reports the first detection of both a Fe K$_{eta}$ line and a soft X-ray lag in the ULX NGC 7456 ULX-1, providing new insights into the accretion processes and geometry of this source.

Contribution

It presents the first combined detection of a Fe K$_{eta}$ line and soft X-ray lag in a ULX, advancing understanding of ULX accretion physics.

Findings

Detection of Fe K$_{eta}$ line at 6.4 keV with 2.6$\sigma$ confidence.

Observation of a ~1300 s soft X-ray lag behind hard X-ray photons.

Hard spectral component drives the correlated variability.

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 $85 r_{\rm g}$ 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.