Swift J0513.4-6547 = LXP 27.2: a new Be/X-ray binary system in the Large Magellanic Cloud

An exceptionally bright new X-ray source in the Large Magellanic Cloud was discovered by the Swift/BAT telescope on MJD 54923 (2009 April 2), and shown to have a pulse period of 27 s using follow-up observations by RXTE/PCA (Krimm et al. 2009). We report here on detailed timing observations taken over the following weeks using Fermi/Gamma Ray Burst Monitor (GBM) which reveal an excellent orbital solution and indicate that the source flux is peaked at ∼10³⁸ erg s^-1. In addition, we report on follow-up optical observations (spectroscopic and photometric) which permit a classification of the mass donor star as B1Ve, and furthermore reveal a strong optical modulation at a period consistent with the binary period found from the Fermi/GBM data - 27.4 d. The dynamical mass estimate for the Be star is not in agreement with that expected for a B1V star - a mismatch similar to this has been previously reported for the few other Magellanic Cloud stars that also have dynamical mass estimates. In addition, the neutron star magnetic field determination from the X-ray data (∼10¹³ G) adds further evidence to the possibility that many such stars in high-mass X-ray binary systems may have magnetic fields greater than previously expected (∼10¹² G).