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  ×  1012 G. We further estimate a lower limit on the distance to OAO 1657-415 of ≃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 ∼ 3.4σ.

We analysed a dedicated NuSTAR observation of the neutron-star low-mass X-ray binary Z-source GX 13+1 to study the timing and spectral properties of the source. From the colour-colour diagram, we conclude that during that observation the source transitioned from the normal branch to the flaring branch. We fitted the spectra of the source in each branch with a model consisting of an accretion disc, a Comptonised blackbody, relativistic reflection (relxillNS), and photo-ionised absorption (warmabs). Thanks to the combination of the large effective area and good energy resolution of NuSTAR at high energies, we found evidence of relativistic reflection in both the Fe K line profile, and the Compton hump present in the 10–25 keV energy range. The inner disc radius is Rin ≲ 9.6 rg, which allowed us to further constrain the magnetic field strength to B ≲ 1.8 × 108 G. We also found evidence for the presence of a hot wind leading to photo-ionised absorption of Fe and Ni, with a Ni overabundance of ∼6 times solar. From the spectral fits, we find that the distance between the ionising source and the slab of ionised absorbing material is ∼4 − 40 × 105 km. We also found that the width of the boundary layer extends ∼3 km above the surface of a neutron star, which yielded a neutron-star radius RNS ≲ 16 km. The scenario inferred from the spectral modelling becomes self-consistent only for high electron densities in the accretion disk, ne ∼ 1022 − 1023 cm−3, as expected for a Shakura-Sunyaev disc, and significantly above the densities provided by relxillNS models. These results have implications for our understanding of the physical conditions in GX 13+1.