XMM-Newton observations of the Small Magellanic Cloud: RX J0105.9-7203, a 726 s Be/X-ray binary pulsar^*

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

Received: 10 March 2008
Accepted: 15 April 2008

Abstract

Aims. To investigate the Be/X-ray binary candidate RX J0105.9-7203 in the Small Magellanic Cloud (SMC), we analysed twenty-two archival XMM-Newton  observations around the calibration target 1E 0102.2-7218.

Methods. We performed spectral and temporal analyses on the EPIC data of fifteen observations. We disregarded the data of one observation due to high background. The data of the remaining six observations were used to derive upper limits for the flux, because the source intensity was below the detection limit.

Results. We detected X-ray pulsations from RX J0105.9-7203 with long periods ranging from 723 ± 2 s to 731 ± 1 s in almost all observations where the source luminosity was above 8.0  10³⁴ erg s^-1. The pulse profiles show more complex features in earlier observations whereas only a broad main pulse is seen at later times. The X-ray spectrum is well modelled by a power-law with photon index 0.71 ± 0.13 and a moderate absorption with N_H = (4.6 ± 3.0)  10²¹ cm^-2. However, during one observation the source was extremely high absorbed: N_H = (2.6 ± 1.7)  10²³ cm^-2. The longterm X-ray lightcurve (covering 7.5 years) shows variations by at least a factor of 2. A long period of relative brightness (~1300 days) until Nov. 2003 was followed by an interval when RX J0105.9-7203 was very faint or not detected at all. Timing analysis of OGLE data of the optical counterpart revealed peaks in the FFT power-spectrum at 2.35 and 3.2 days.