2001ApJ...553L.165I

Dedicated monitoring of the transient X-ray pulsar XTE J1543-568 during the first year after its discovery has revealed the unambiguous detection of a binary orbit. The orbital period is 75.56±0.25 days, and the projected semimajor axis is 353±8 lt-s. The mass function and position in the pulse period versus orbital period diagram are consistent with XTE J1543-568 being a Be X-ray binary. The eccentricity of less than 0.03 (2 σ) is among the lowest for the 12 Be X-ray binaries whose orbits have now been measured. This confirms the suspicion that small kick velocities of neutron stars during supernovae are more common than expected. The distance is estimated to be larger than 10 kpc and the luminosity at least 10³⁷ ergs.s^–1.