XMM-Newton observations of the Small Magellanic Cloud: Long term evolution of frequently observed Be/X-ray binaries^*

Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany e-mail: peger@mpe.mpg.de

Received: 15 August 2008
Accepted: 9 September 2008

Abstract

Aims. To follow the long term evolution of Be/X-ray binaries in the Small Magellanic Cloud (SMC) we analyzed twenty-two XMM-Newton  observations towards the calibration target 1E 0102.2-7218  and seven observations towards the neighboring region around the eclipsing binary HD 5980.

Methods. The data from the European Photon Imaging Cameras (EPIC) were used to produce images, X-ray spectra and light curves in different energy bands. We performed a detailed spectral and temporal analysis on the data of all known Be/X-ray binary pulsars that have been detected with a sufficiently high number of counts. Fast Fourier Transformation (FFT) power spectra were produced to search for X-ray pulsations and to determine their significance. To derive accurate values for the pulse periods we utilized the Bayesian method. To identify or confirm the optical counterparts we derived the X-ray positions of the pulsars using maximum likelihood source detection techniques and correlated them with optical catalogues.

Results. There are ten known Be/X-ray binaries located in the observed fields. In addition to the previously published results for three of these sources, we present long term X-ray light curves and pulse period histories for the following six pulsars: CXOU J005736.2-721934 (565 s), RX J0057.8-7207 (152 s), RX J0059.3-7223 (201 s), CXOU J010102.7-720658 (304 s), RX J0101.3-7211 (455 s), SAX J0103.2-7209 (345 s) and RX J0103.6-7201 (1323 s). We do not agree with the identification of the ROSAT source RX J0105.1-7211 = XMMU J010509.7-721146 as the 3.34 s ASCA pulsar AX J0105-722 and suggest that RX J0105.1-7211 is more likely a background AGN. We find soft excess emission around 1 keV in the X-ray spectra of two pulsars (SAX J0103.2-7209 and RX J0103.6-7201). The spin history in combination with the long term X-ray light-curve indicate a type II outburst of CXOU J010102.7-720658 at the end of 2005 and a reversal in the accretion disc torque in SAX J0103.2-7209 in May 2002. An X-ray ephemeris for the binary orbit was only available in the literature for CXOU J005736.2-721934 and is in good agreement with the observed XMM-Newton light-curve.