Change in spin-down rate and detection of emission line in HMXB 4U 2206+54 with AstroSat observation

This work presents timing and spectral analysis of 4U 2206+54 using data obtained from LAXPC instrument onboard India's AstroSat mission. This source was observed with AstroSat in September–October 2016. We report detection of 5648 (4) s pulsations at MJD 57669 in the latter observation of 4U 2206+54. The pulse profile is sinusoidal and the inherent shape is independent of energy up to 30 keV. The pulse fraction increases with energy from ∼0.5 to ∼0.8%. We report an updated spin-down rate of 2.95(14)×10-7 s s-1. This is about 0.40 times smaller than the previously reported long term value. The energy spectrum is best modeled with an absorbed power-law with high energy exponential cut-off. We have detected presence of broad emission line in 4U 2206+54 at an energy of 7 keV with equivalent width of ∼0.4 keV.