XMM-Newton observations of the Small Magellanic Cloud: the 280 s Be/X-ray binary pulsar AX J0058-7203 in outburst

Abstract

Context. The Be/X-ray binary pulsar AX J0058-720, discovered by ASCA, was in the field of view of the EPIC instruments during five XMM-Newton observations in the direction of the Small Magellanic Cloud (SMC). Aims. We used the EPIC data of the X-ray pulsar in order to investigate its long-term temporal and spectral behaviour between 2000 and 2005. Methods. X-ray spectra covering 0.3-10 keV and light curves in different energy bands were extracted from the EPIC PN and MOS data. Results. The X-ray spectra are well modelled by a simple absorbed power-law without significant contribution from a soft component. The photon index of 0.7 indicates a relatively hard spectrum. During two observations in November 2005, separated by two days, the 0.2-10 keV luminosity reached a maximum of 2.6 $\times$ 10 36 erg s -1 indicating an X-ray outburst. Twelve days later the source intensity had decreased by more than a factor of two. During an observation in November 2001 the source was not detected with a $2\sigma$ upper limit of 2.8 $\times$ 10 33 erg s -1 . Timing analysis of the observations of AX J0058-720 in 2005 reveals a pulse period of 286.3388 ± 0.0012 s (or 286.8029 ± 0.0012 s due to cycle count ambiguity). The average spin-down of ~0.75 s per year between 1997 and 2005 suggests a moderate activity episode with low average accretion rate, insufficient to spin-up the neutron star. Conclusions. The X-ray properties of AX J0058-720 are typical for a Be/X-ray binary in the SMC. The source shows transient behaviour with more than a factor 900 intensity increases during outburst.