Discovery of 1323 s pulsations from RX J0103.6-7201: The longest period X-ray pulsar in the SMC

Abstract

XMM-Newton archival observations of the Be/X-ray binary candidate revealed pulsations with a period of ~1323 s. This makes the X-ray pulsar with the longest period known in the Small Magellanic Cloud (SMC). More than 150 X-ray observations of by ROSAT, Chandra and XMM-Newton show flux variations by a factor of 50 on time scales of days to years. Using the accurate positions obtained from ACIS-I images, the optical counterpart is identified as a V = 14.6 mag emission line star. EPIC spectra of above 1 keV are consistent with an absorbed power-law with column density between (6-9) × 10²¹ cm^-2, except during one observation when an extraordinary high value of 1.1 × 10²³ cm^-2 was measured which strongly attenuated the power-law emission below 3 keV. A soft excess between 0.5 and 1.0 keV is evident in the spectra which becomes best visible in the highly absorbed spectrum. The soft component can be reproduced by a thermal plasma emission model with its luminosity strongly correlated with the total intrinsic source luminosity. Including results from three other SMC Be/X-ray binaries extends the linear correlation over three orders of magnitude in source intensity, strongly suggesting that the same mechanism is responsible for the generation of the soft emission.