XMM-Newton observations of the Small Magellanic Cloud: X-ray outburst of the 6.85 s pulsar XTE J0103-728

Abstract

Context: A bright X-ray transient was seen during an XMM-Newton observation in the direction of the Small Magellanic Cloud (SMC) in October 2006.
Aims: The data from the European Photon Imaging Cameras (EPIC) allow us to accurately locate the source and to investigate its temporal and spectral behaviour.
Methods: We extracted X-ray spectra covering 0.2-10 keV and pulse profiles in different energy bands from the EPIC data.
Results: The detection of 6.85 s pulsations in the EPIC-PN data unambiguously identifies the transient with XTE J0103-728, discovered as 6.85 s pulsar by the Rossi X-ray Timing Explorer. The X-ray light curve during the XMM-Newton observation shows flaring activity of the source with intensity changes by a factor of two within 10 min. Modelling of pulse-phase averaged spectra with a simple absorbed powerlaw indicates systematic residuals, which can be accounted for by a second emission component. From a model implying blackbody emission due to reprocessing by optically-thick material in the inner accretion disk, we estimate the inner disk radius to ~160 km. The photon index of the powerlaw of ~0.4 indicates a relatively hard spectrum. The 0.2-10 keV luminosity was 2 × 10³⁷ erg s^-1 with a contribution of ~3% from the disk-blackbody component. A likely origin for the excess emission is reprocessing of hard X-rays from the neutron star by optically-thick material near the inner edge of an accretion disk. From a timing analysis we determine the pulse period to 6.85401(1) s, indicating an average spin-down of ~0.0017 s per year since the discovery of XTE J0103-728 in May 2003.
Conclusions: The X-ray properties and the identification with a Be star confirm XTE J0103-728 as Be/X-ray binary transient in the SMC.

Based on observations with

XMM-Newton, an ESA Science Mission with instruments and contributions

directly funded by ESA Member states and the USA (NASA).