Discovery of cyclotron and narrow Fe K$_{\alpha}$ lines in HMXB GRO J1750-27

TL;DR

This study reports the discovery of cyclotron lines and Fe K$_{ ext{alpha}}$ emission in the X-ray pulsar GRO J1750-27, providing insights into its magnetic field and distance through spectral analysis of NuSTAR data.

Contribution

First detection of cyclotron resonant scattering features and Fe K$_{ ext{alpha}}$ line in GRO J1750-27 using NuSTAR observations, revealing the neutron star's magnetic field and distance.

Findings

Detected Fe K$_{ ext{alpha}}$ line at 6.4 keV.

Identified two cyclotron lines at 36 and 42 keV.

Estimated magnetic field strength of approximately 3.7-4.4 x 10^{12} G.

Abstract

We report on timing and spectral analysis of transient Be X-ray pulsar GRO J1750-27 by using the Nuclear Spectroscopic Telescope Array (NuSTAR) observation from September 2021. This is the fourth outburst of the system since 1995. The NuSTAR observation was performed during the rising phase of the outburst. Pulsations at a period of 4.450710(1) s were observed in the 3-60 keV energy range. The average pulse profile comprised of a broad peak with a weak secondary peak which evolved with energy. We did not find any appreciable variation in the X-ray emission during this observation. The broad-band phase-averaged spectrum is described by a blackbody, a powerlaw or Comptonization component. We report discovery of Fe K$_{\alpha}$ line at 6.4 keV along with presence of two cyclotron resonant scattering features around 36 and 42 keV. These lines indicate a magnetic field strength of $3.7_{-0.3}^{+0.1} \times 10^{12}$ and $4.4 \pm 0.1 \times 10^{12}$ G for the neutron star. We have estimated a source distance of $\sim$ 13.6-16.4 kpc based on the accretion-disc torque models.