The high-energy cyclotron line in 2S 1417-624 discovered with Insight-HXMT during the 2018 outburst

Abstract: We report a detailed timing and spectral analysis of the X-ray pulsar 2S~1417-624 using the data from Insight-HXMT during the 2018 outburst. The pulse profiles are highly variable with respect to both unabsorbed flux and energy. A double-peaked pulse profile from the low flux evolved to a multi-peaked shape in the high-flux state. The pulse fraction is negatively correlated to the source flux in the range of $\sim$(1--6)$\ \times \ 10^{-9}$ erg cm$^{-2}$ s$^{-1}$, consistent with \textit{Rossi} X-ray Timing Explorer (RXTE) studies during the 2009 giant outburst. The energy-resolved pulse profiles around the peak outburst showed a four-peak shape in the low-energy bands and gradually evolved to triple peaks at higher energies. The continuum spectrum is well described by typical phenomenological models, such as the cut-off power law and the power law with high-energy cut-off models. Notably, we discovered high-energy cyclotron resonant scattering features (CRSFs) for the first time, which are around 100 keV with a statistical significance of $\sim$7$\sigma$ near the peak luminosity of the outburst. This CRSF line is significantly detected with different continuum models and provides very robust evidence for its presence. Furthermore, pulse-phase-resolved spectroscopy confirmed the presence of the line, whose energy varied from 97 to 107 keV over the pulse phase and appeared to have a maximum value at the narrow peak phase of the profiles.