The orbital solution and spectral classification of the high-mass X-ray binary IGR J01054-7253 in the Small Magellanic Cloud

We present X-ray and optical data on the Be/X-ray binary (BeXRB) pulsar IGR J 01054-7253= SXP11.5 in the Small Magellanic Cloud (SMC). Rossi X-ray Timing Explorer (RXTE) observations of this source in a large X-ray outburst reveal an 11.483 ± 0.002 s pulse period and show both the accretion-driven spin-up of the neutron star and the motion of the neutron star around the companion through Doppler shifting of the spin period. Model fits to these data suggest an orbital period of 36.3 ± 0.4 d and ? of (4.7 ± 0.3) × 10^-10 seconds s^-1. We present an orbital solution for this system, making it one of the best-described BeXRB systems in the SMC. The observed pulse period, spin-up and X-ray luminosity of SXP11.5 in this outburst are found to agree with the predictions of neutron star accretion theory. Timing analysis of the long-term optical light curve reveals a periodicity of 36.70 ± 0.03 d, in agreement with the orbital period found from the model fit to the X-ray data. Using blue-end spectroscopic observations we determine the spectral type of the counterpart to be O9.5-B0 IV-V. This luminosity class is supported by the observed V-band magnitude. Using optical and near-infrared photometry and spectroscopy, we study the circumstellar environment of the counterpart in the months after the X-ray outburst.