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

TL;DR

This study uses NuSTAR data to analyze the X-ray properties of 4U 1700-37, constraining its magnetic field and accretion regime, and assessing the presence of cyclotron features.

Contribution

It provides the first detailed spectral and timing analysis of 4U 1700-37 with NuSTAR, constraining magnetic field strength and accretion mode, and evaluating cyclotron feature significance.

Findings

No definitive cyclotron line detection.

Magnetic field estimated at 1.7–4.4 × 10^{12} G.

Supports quasi-spherical subsonic accretion regime.

Abstract

4U 1700-37 is a wind-fed high-mass X-ray binary hosting a compact object, likely a neutron star, accreting from O6.5 Iaf+ supergiant HD 153919. Coherent pulsations not firmly detected; magnetic field strength remains uncertain. We analyze NuSTAR observations to characterize hard X-ray timing and spectral properties, test robustness of candidate cyclotron features, and constrain magnetic field and accretion regime. We perform timing and spectral analysis of two observations, modeling spectra with continua used for accreting pulsars, and use simulations to assess significance of features. No coherent pulsations detected; pulsed fraction constrained below 1.5\%. Spectra are well described by absorbed blackbody plus cutoff power-law continuum, showing residuals around 20 keV and 40--50 keV. Features improve fits but do not constitute firm cyclotron detections. Intensity-resolved spectroscopy suggests possible shifts of apparent line centroid. Results favor neutron-star magnetic field of 1.7--4.4 $\times 10^{12}$ G and quasi-spherical subsonic accretion regime with equilibrium spin period $\sim 1.9$ ks. Analysis provides quantitative constraints on magnetic field and accretion physics, helping reconcile discrepant line-energy measurements.