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 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 $\sim 5619$ s in 2016 and hxmt found a pulsation period at $\sim 5291$ 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 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 $\sim1.2\times10^{-13}$ 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 reveal a hotter source photon region near the pulsar with an emission size extending to $\sim 2-2.8$~km. Using the quasi-spherical settling accretion theory, we explain the present spin-up and the possibility of strong magnetic field of the pulsar.