2012MNRAS.423.3352P

We summarize the analysis of observations made by the Rossi X-ray Timing Explorer and the Swift/Burst Alert Telescope at the time of the recent flaring activity of the transient X-ray pulsar 2S 1553-542. The binary system is characterized by the projected semimajor axis a_x sin i of 168±18 light-seconds, the binary orbital period P_orb of 29.56±0.53 d and the X-ray mass function f_x(M) of 5.3 ±1.2 M_☉. We also found P_spin= 9.2829±0.0003 s with {dot}P_spin= -2.54 ± 0.83x10^–9s/s s/s, i.e. a spin-up rate of ({dot}P_spin/ P_spin) = -8.62x10^–9/yr. The spin period is found to decrease more slowly than that of most other X-ray pulsars. As the flare decays with time, the structure and X-ray counts in the pulse profile showed considerable variability. However, no significant structural variability is seen in single-peaked pulse profiles in different energy bands. Pulsed fraction decreases with the decay of the flare but its value at different energy bands showed a discontinuity. Using X-ray spectroscopy at different orbital cycles, we have detected the presence of the Fe Kα emission line in the spectra with a minimum significance of 3σ. Apart from a consistent increase in blackbody temperature, other spectral parameters are found to be stable throughout orbital cycles. From detailed spectral and timing analysis, we concluded that the X-ray flare is spectrally hard and we confirmed the nature of the compact object as a transient, accretion-powered, spin-up X-ray pulsar with a Be-type companion.