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Resonant absorption cyclotron features are a key diagnostic tool to directlymeasure the strength of the magnetic field of accreting neutron stars. However,typical values for cyclotron features lie in the high-energy part of thespectrum between 20 keV and 50 keV, where detection is often damped by the lowstatistics from single pointed observations. We show that long-term monitoringcampaign performed with Swift-BAT of persistently, but faint, accretinghigh-mass X-ray binaries is able to reveal in their spectra the presence ofcyclotron features. We extracted the average Swift-BAT 15-150 keV spectrum fromthe 54 months long Swift-BAT survey of the high-mass X-ray source IGRJ16493-4348. To constrain the broadband spectrum we used soft X-ray spectrafrom Swift-XRT and Suzaku pointed observations. We model the spectra using aset of phenomenological models usually adopted to describe the energy spectrumof accreting high-mass X-ray binaries; irrespective of the models we used, wefound significant improvements in the spectral fits adding to the models abroad (10 keV width) absorption feature, with best-fitting energy estimatebetween 30 and 33 keV, that we interpret as evidence for a resonant cyclotronabsorption feature. We also discuss instrumental issues related to the use ofSwift-BAT for this kind of studies and the statistical method to weight theconfidence level of this detection. Correcting for the gravitational redshiftof a 1.4 M$_{\sun}$ neutron star, the inferred surface magnetic field is Bsurf3.7 x 10^{12} Gauss. The spectral parameters of IGR J16493-4348 fit well withempirical correlations observed when the whole sample of high-mass binarieswith detected cyclotron features is considered.

Evidence for a resonant cyclotron line in IGR J16493-4348 from the Swift-BAT hard X-ray survey