Discovery of an X-ray pulsar in the low-mass X-ray binary 2A 1822-371

We report the discovery of 0.59 s X-ray pulsations from the low-mass X-ray binary, 5.57 hr dipping and eclipsing accretion disk corona source 2A 1822-371. Pulse arrival time analysis indicates a circular orbit with e < 0.03 (95% confidence) and an a sin i for the neutron star of 1.006(5) lt-s, implying a mass function of (2.03 +/- 0.03) x 10(-2) M-.. The barycentric pulse period was 0.59325(2) s in 1996.270 and 0.59308615(5) s in 1998.205, indicating an average spin-up with (P) over dot/P = (-1.52 +/- 0.02) x 10(-4) yr(-1). For a magnetic field strength of (1-5) x 10(12) G as derived from the X-ray spectrum, the implied intrinsic X-ray luminosity is similar to (2-4) x 10(37) ergs s(-1). The pulse amplitude is low but increases steeply as a function of energy from a sinusoidal amplitude of 0.25% in 2-5.4 keV to similar to3% above 20 keV. We discuss the constraints on the masses of the companion star and the fact that several aspects of the energy spectrum are in qualitative accordance with that of a strongly magnetized neutron star.