The broad-band spectrum of OAO1657-415 with BeppoSAX: in search of cyclotron lines

We report on a 30 ks observation of the high-mass X-ray binary pulsar OAO16 57-415 performed by BeppoSAX on September 1998. The wide band spectrum is w ell fit by both a cutoff power law, or a power law modified by a high energ y cutoff, plus a fluorescence Iron line at 6.5 keV. The two models are stat istically equivalent. The inclusion of a cyclotron resonance feature at sim ilar to 36 keV - corresponding to a magnetic held strength 3.2 (1 + z) x 10 (12) G, where z is the gravitational redshift - improves significantly the chi(2) for the cutoff model but only marginally for the power law plus high energy cutoff model. The statistical significance of our data is not adequ ate to discriminate between the two models, and even the Normalized Crab Ra tio technique, successfully used to pinpoint cyclotron features in the spec tra of other X-ray pulsars, is not conclusive in answering the question whe ther the feature is real or it is an artifact due to an improper modeling o f the continuum used to fit the data.