Constraints on cyclotron features and accretion regime in the high-mass X-ray binary 4U 1700–37 from NuSTAR

Abstract

4U 1700–37 is a wind-fed high-mass X-ray binary comprising a compact object, most likely a neutron star, accreting from the O6.5 Iaf${}^{+}$ supergiant HD 153919. Coherent pulsations have not been firmly detected and the magnetic field strength remains uncertain, with previous NuSTAR studies reporting only marginal candidate cyclotron resonant scattering features (CRSFs). We analyses all available NuSTAR observations of 4U 1700–37 to characterize its hard X-ray timing and broadband spectral properties, test the robustness of candidate CRSFs against different continuum models and epochs, and constrain the magnetic field and accretion regime of the compact object. We perform a homogeneous timing and spectral analysis of two NuSTAR observations, modeling time-averaged and intensity-resolved spectra with several continua commonly used for accreting pulsars, and use spectral simulations to quantify the significance and model dependence of putative CRSFs. No coherent pulsations are detected and we constrain the pulsed fraction to be <1.5%. The spectra are well described by an absorbed blackbody plus cut-off power-law continuum, but show shallow absorption-like residuals around  ∼ 20 keV and at higher energies ( ∼ 40–50 keV). These features improve the fits and can reach high formal significances for some continua, yet do not constitute firm, model-independent CRSF detections in our baseline analysis. Intensity-resolved spectroscopy hints at shifts of the apparent line centroid with flux. Interpreted as cyclotron features, these results favour a neutron-star magnetic field B ∼ (1.7–4.4) × 10¹² G and a quasi-spherical subsonic accretion regime with an equilibrium spin period P_eq ∼ 1.9 ks. Even without a secure CRSF detection, the homogeneous multi-epoch analysis provides quantitative constraints on the magnetic field and accretion physics of 4U 1700–37 and helps reconcile previously discrepant line-energy measurements.