A BeppoSAX observation of the massive X-ray binary 4U 1700-37

A 0.5–200 keV BeppoSAX spectrum of the nonpulsating high-mass X-ray binary 4U 1700-37 is presented. The spectrum is well characterized by the standard accreting pulsar model of an absorbed power-law with a photon index of 1.07±0.02 0.03, sharply modified by an exponential cutoff above 5.9 ± 0.2 keV. The e-folding energy of the cutoff is 23.9 ± 0.5 keV. A soft bremsstrahlung component with a temperature of 0.2±0.1 keV is also required, together with a narrow iron line at 6.5 keV. Both continuum components are absorbed by a column of (5.1 ± 0.2) × 1022 atom cm−2. There is some evidence for the presence of a broad cyclotron absorption feature at ∼37 keV, although we cannot exclude the possibility that this is due to an incorrect modeling of the continuum, or instrumental effects. The hypothesis that the compact object is a neutron star rather than a black hole seems most likely.