Bepposax Observation of 4u1626–67: Discovery of an Absorption Cyclotron Resonance Feature

We report on an observation of the low-mass X–ray binary 4U1626–67 performed during the BeppoSAX Science Verification Phase. An absorption feature at ∼ 37 keV, 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) × 10 12 G, where z is the gravitational redshift. The feature is deep and narrow and is resolved in both the broad-band fit and in the ratio of observed counts to those seen from the Crab. 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 broad-band 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 broad-band 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.