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

¹ University of Rome 2, Via della Ricerca Scientifica 1, 00133 Roma, Italy
² INAF – Astronomical Observatory of Rome, Via Frascati 33, 00040 Monteporzio Catone, Italy
³ Affiliated with ICRA
⁴ Astronomical Institute “Anton Pannekoek", University of Amsterdam and Center for High-Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
⁵ Dipartimento di Scienze Fisiche ed Astronomiche, Universita' di Palermo, via Archirafi 36, 90123 Palermo, Italy

Corresponding author: N. Rea, rea@mporzio.astro.it

Received: 14 November 2003
Accepted: 10 March 2004

Abstract

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 and 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 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 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)  . 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.