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Keywords:

  • accretion, accretion discs;
  • stars: emission-line, Be;
  • stars: individual: 2S 1553−542;
  • stars: neutron;
  • X-rays: binaries;
  • X-rays: stars

ABSTRACT

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 ax sin  i of 168 ± 18 light-seconds, the binary orbital period Porb of 29.56 ± 0.53 d and the X-ray mass function fx(M) of 5.3 ± 1.2 M. We also found Pspin= 9.2829 ± 0.0003 s with inline image s s−1, i.e. a spin-up rate of inline image yr−1. 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.