BeppoSAX Observation of 4U 1626–67: Discovery of an AbsorptionCyclotron Resonance Feature

We report on an observation of the low-mass X-ray binary 4U 1626-67 performed during the BeppoSAX science verification phase. An absorption feature at ~37 keV, which is attributable to electron cyclotron resonance, has been discovered in its pulse-averaged spectrum. The inferred neutron star magnetic field strength is 3.2(1+z)×1012 G, where z is the gravitational redshift. The feature is deep and narrow and is resolved in both the broadband fit and in the ratio of observed counts to those seen from the Crab pulsar. The cyclotron resonance energy is in good agreement with the empirical relation between cyclotron energy and high-energy cutoff, while its width is in agreement with the expected Doppler broadening of thermal electrons at the cyclotron 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 law with a photon index of 0.89±0.02. This is the first broadband observation made after the change from spin-up to spin-down that occurred in mid-1990: it confirms the harder spectrum with respect to those observed in the 2-10 keV range.