A NuSTAR observation of the eclipsing binary system OAO 1657-415: The revival of the cyclotron line

Context. OAO 1657-415 is an accreting X-ray pulsar with a high-mass companion that has been observed by several telescopes over the years, in different orbital phases. Back in 1999, observations performed with Beppo-SAX lead to the detection of a cyclotron-resonant-scattering feature, which has not been found again with any other instrument. A recent NuSTAR X-ray observation performed during the brightest phase of the source allows us to perform sensitive searches for cyclotron-resonant-scattering features in the hard X-ray spectrum of the source. Aims. We aim to characterise the source by means of temporal and spectral X-ray analysis, and to confidently search for the presence of cyclotron-resonant-scattering features. Methods. The observation was divided into four time intervals in order to characterise each one. Several timing analysis tools were used to obtain the pulse of the neutron star, and the light curves folded into the time intervals. The NuSTAR spectrum in the energy range 3-79 keV was used, which was modelled with a power-law continuum emission model with a high-energy cutoff. Results. We identify the pulsations associated with the source in the full observation, and find these to be shifted due to the orbital Doppler effect. We show evidence that a cyclotron line at 35.6 +/- 2.5 keV is present in the spectrum. We use this energy to estimate the dipolar magnetic field at the pulsar surface to be 4.0 +/- 0.2 x 10(12) G. We further estimate a lower limit on the distance to OAO 1657-415 of similar or equal to 1 kpc. We also find a possible positive correlation between the luminosity and the energy associated with the cyclotron line. Conclusions. We conclude that the cyclotron line at 35.6 +/- 2.5 keV is the same as that detected by Beppo-SAX. Our detection has a significance of similar to 3.4 sigma.