Pulse Period History and Cyclotron Resonance Feature of the X-Ray Pulsar 1E 2259+586

The results of two follow-up Ginga observations of 1E 2259+586 are presented. A secular spin-down trend was found to hold for more than 10 yr. However, the spin-down rate decreased as the X-ray flux increased for the first time in 1990. This supports a scenario that 1E 2259+586 is a binary X-ray pulsar in a nearly equilibrium state of spin-up and spin-down torque, and not an isolated white dwarf. In the X-ray spectrum, absorption features were found at 5--10 keV. The shape of the absorption features changed between the two observations and between different pulse phases. This absorption structure is likely to be due to cyclotron resonant scattering in a relatively weak magnetic field.