Journal
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume 505, Issue 1, Pages L106-L111Publisher
OXFORD UNIV PRESS
DOI: 10.1093/mnrasl/slab061
Keywords
X-rays: binaries; X-rays: individual NGC 7456 ULX-1
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Funding
- Polish National Science Center [2015/17/B/ST9/03422, 2015/18/M/ST9/00541, 2016/23/B/ST9/03123, 2018/31/G/ST9/03224]
- Ramony Cajal Fellowship [RYC2018-025950-I]
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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.
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.
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