Timing and spectral changes of the Be X-ray transient EXO 0531-6609.2 through high and low state

We report on a spectral and timing analysis of BeppoSAX data of the 13.6 s period transient X-ray pulsar EXO 053109-6609.2. Observations were carried out in March 1997 and October 1998, catching the source during a high and a low emission state, respectively. Correspondingly, the X-ray luminosity is found at a level of 4.2 × 1037 erg s−1 and 1.5 × 1036 erg s−1 in the two states. In the high state the X-ray emission in the energy range 1–100 keV is well fitted by an absorbed power-law with photon index Γ ∼ 1.7 plus a blackbody component with a characteristic temperature of ∼3.5 keV. Moreover, we find evidence of iron emission at ∼6.8 keV, a typical feature in this class of sources but never seen before in the EXO 053109-6609.2 spectrum. In the low state an absorbed power-law with Γ ∼ 0.4 is sufficient to fit the 1–10 keV data. During BeppoSAX observations EXO 053109-6609.2 display variations of the pulse profile with the X-ray flux: it showed single peaked and double peaked profiles in the low and high state, respectively. Based on these two observations we infer a spin-up period derivative of −(1.14 ± 0.08) × 10−10 ss−1. By comparing these with other period measurements reported in the literature we find an alternating spin-up and spin-down behaviour that correlates well with the X-ray luminosity.