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Issue A&A
Volume 421, Number 1, July I 2004
Page(s) 235 - 239
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20035685



A&A 421, 235-239 (2004)
DOI: 10.1051/0004-6361:20035685

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

N. Rea1, 2, G. L. Israel2, 3, T. Di Salvo4, 5, L. Burderi2 and G. Cocozza1

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

(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 $ 4.2 \times 10^{37}~\rm erg\,s^{-1}$ and $1.5\times 10^{36}~\rm erg\,s^{-1}$ 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 $ \Gamma\sim 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 $\Gamma\sim 0.4$ 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  $\pm$ 0.08)  $\times$ $10^{-10}~{\rm ss}^{-1}$. 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.


Key words: accretion, accretion disks -- X-rays: stars -- stars: neutron -- X-rays: binaries

Offprint request: N. Rea, rea@mporzio.astro.it

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