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

We report the first detection of an Fe K-alpha 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 2.6 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(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 similar to 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-alpha line is found, the fifth with a soft X-ray lag, and the first with both features detected.

Automated summary

This study reports the first detection of an Fe K-alpha line and soft X-ray lag in the ultraluminous X-ray (ULX) source NGC 7456 ULX-1. The XMM-Newton spectra revealed the Fe line presence and provided an upper limit on its full width at half maximum (FWHM). Fourier timing analysis showed a delay of approximately 1300 s between soft and hard X-ray photons, with the hard spectral component responsible for the correlated variability and soft X-ray lag. This is the second ULX to exhibit an Fe K-alpha line, the fifth with a soft X-ray lag, and the first to have both features detected simultaneously.