NuSTAR discovery of a cyclotron line in GRO J1750-27

TL;DR

This paper reports the discovery of a cyclotron line at 43 keV in the X-ray binary GRO J1750-27 using NuSTAR, revealing its magnetic field strength and analyzing pulse profiles during an outburst.

Contribution

First detection of a cyclotron resonant scattering feature in GRO J1750-27, providing direct measurement of its magnetic field strength.

Findings

Cyclotron line detected at 43 keV

Pulse profile transitions from double to single peak with energy

Magnetic field estimated at 3.7 x 10^12 G

Abstract

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 broadband 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 $3.7 \times 10^{12}$ 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.