BEPPOSAX OBSERVATION OF 4U-1626-67 - DISCOVERY OF AN ABSORPTION CYCLOTRON-RESONANCE FEATURE

We report on an observation of the low-mass X-ray binary 4U 1626-67 pe rformed during the BeppoSAX science verification phase. An absorption feature at similar to 37 keV, which is attributable to electron cyclot ron resonance, has been discovered in its pulse-averaged spectrum. The inferred neutron star magnetic field strength is 3.2(1 + z) x 10(12) G, where z is the gravitational redshift. The feature is deep and narr ow and is resolved in both the broadband fit and in the ratio of obser ved counts to those seen from the Crab pulsar. The cyclotron resonance energy is in good agreement with the empirical relation between cyclo tron energy and high-energy cutoff, while its width is in agreement wi th the expected Doppler broadening of thermal electrons at the cyclotr on resonance frequency. The broadband 0.1-200 keV spectrum is well fit by a two-component model: a 0.27 +/- 0.02 keV blackbody and a power l aw with a photon index of 0.89 +/- 0.02. This is the first broadband o bservation made after the change from spin-up to spin-down that occurr ed in mid-1990: it confirms the harder spectrum with respect to those observed in the 2-10 keV range.