AstroSat and Insight-HXMT Observations of the Long-period X-Ray Pulsar 4U 2206+54

Abstract

We report the timing and spectral studies of the accreting X-ray pulsar 4U 2206+54 using AstroSat and Insight-HXMT observations taken in 2016 and 2020 respectively. X-ray pulsations from the system are detected by both missions. The AstroSat discovered a significant periodic signal at ∼5619 s in 2016, and Insight-HXMT found a pulsation period at ∼5291 s in 2020. A comparison of its spin-period evolution with the present spin-period estimates shows that the neutron star in 4U 2206+54 now has recently been undergoing a spin-up episode after attaining to its slow pulsations of 5750 s around 2015 from its prolonged spin-down phase. The present average spin-up rate of the pulsar is found to be at ∼1.2 × 10−13 Hz s−1. The phase-averaged spectra of the pulsar in 1–60 keV could be explained with a high-energy cutoff power-law continuum model; no evident line features are found with AstroSat. The application of Comptonization models such as comptt and compmag to the phase-averaged spectra of 4U 2206+54 reveals a hotter source photon region near the pulsar with an emission size extending to ∼2–2.8 km. Using the quasi-spherical settling accretion theory, we explain the present spin-up and the possibility of the strong magnetic field of the pulsar.