Discovery of a cyclotron line in GRO J1750-27 using NuSTAR

The Be/X-ray binary, GRO J1750-27, is one of the farthest known galactic X-ray pulsars at a distance between 14 - 22 kpc and was discovered in 1995 with the CGRO-BATSE during a major outburst. The source has shown significant activity only three more times since then i.e, in 2008, 2014 and most recently in 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 ∼ 4.45s using which we produced energy-resolved pulse profiles between 3 and 65 keV. The pulse profile is double-peaked at low energies (<18 keV), evolving into a single peak at higher energies (>18 keV). The broadband spectrum of this source was described with a power-law modified by an exponential cut off and we report the discovery of a cyclotron resonant scattering feature (CRSF) in this source at ∼ 43 keV indicating a surface magnetic field strength of 3.7 x 10^12 G. This value is consistent with the estimate obtained by applying the accretion torque model using the spin up rate and pulsed flux found using the Swift/BAT telescope during the 2014-2015 outburst.