2020MNRAS.493.3045B

We present a spectral and timing study of the high-mass X-ray binary transient source 4U 1700-37 using NuSTAR and ASTROSAT/LAXPC. The source is observed in two different flux states. A combined spectral analysis of NuSTAR's focal plane modules A and B shows the possible hint of a cyclotron line feature at ∼16 keV. The line feature is consistently present in different continuum models with at least 3σ confidence level. We do not detect the presence of a previously reported 39 keV cyclotron line in the combined spectra. A ∼16 keV cyclotron feature would suggest that the compact object is a neutron star with a magnetic field strength ∼2.1 x 10¹² G in the emission region. We also find the presence of a rare Ni Kα emission line around 7.6 keV in the NuSTAR spectrum. We searched the NuSTAR and ASTROSAT data for coherent or quasi-periodic oscillation signals but found no evidence in the frequency range 0.1 mHz to 10³ Hz.