NuSTAR discovery of a cyclotron line in GRO J1750-27

GRO J1750-27, discovered during an outburst in 1995 with CGRO-BATSE, is one of the farthest known Galactic Be/X-ray binary systems. This relatively poorly studied system recently went into an outburst in September 2021. The source was observed during the latest outburst using the NuSTAR telescope during the rising phase of the outburst. We estimate the spin period of the source to be 4.45 s using which we produced energy-resolved pulse profiles between 3 and 65 keV. We find that the profile is double-peaked at low energies (<18 keV), while evolving into a single peak at higher energies (>18 keV). The broad-band spectrum of this source was fitted well with a high energy cutoff power-law model and we report the discovery of a cyclotron resonant scattering feature (CRSF) in this source at 43 keV, indicating a magnetic field strength of 3.7 x1012 G. Our estimate of the magnetic field strength, using the cyclotron line, is consistent with the estimates made earlier using the accretion torque model from measurements of spin-up rates and fluxes during the previous outbursts.