X-RAY PULSAR 1E 2259+586 - A MERGED WHITE-DWARF WITH A 7 SECOND ROTATION PERIOD

The enigmatic X-ray pulsar 1E 2259+586, located at the center of the supernova remnant G109.1-1.0, may be recent merger of two white dwarfs, with a present rotation period of P = 6.98 s. The required mass is higher than 1.32 M⊙, making 1E 2259+586 the most massive white dwarf known. The surface gravity is well over 109 cm s-2, allowing the effective temperature to be as high as 2 × 106 K, and opening up a possibility that the observed soft X-rays come from the white dwarf photosphere. The rate of rotational energy loss implied by the observed period increase is |Ėrot| ≈ 1036 ergs s-1, a factor of a few more than the observed X-ray luminosity. If the interstellar extinction toward 1E 2259+586 is 4 mag, then the optical counterpart should be fainter than 26 mag. Observational and theoretical tests of this scenario are discussed.