Insight-HXMT firm detection of the highest energy fundamental cyclotron resonance scattering feature in the spectrum of GRO J1008-57

TL;DR

This paper reports the detection of the highest energy fundamental cyclotron resonance scattering feature in the spectrum of the accreting pulsar GRO J1008-57 using Insight-HXMT, marking a new record in pulsar observations.

Contribution

The study provides the first robust detection of a cyclotron line at 90.32 keV in GRO J1008-57, significantly higher than previous observations, establishing a new energy record for such features.

Findings

Detected pulsations with a 93.283 s period.

Identified a cyclotron line at 90.32 keV with high significance.

Confirmed the robustness of the line detection across models.

Abstract

We report on the observation of the accreting pulsar GRO J1008-57 performed by Insight-HXMT at the peak of the source's 2017 outburst. Pulsations are detected with a spin period of 93.283(1) s. The pulse profile shows double peaks at soft X-rays, and only one peak above 20 keV. The spectrum is well described by the phenomenological models of X-ray pulsars. A cyclotron resonant scattering feature is detected with very high statistical significance at a centroid energy of $E_{\rm cyc}=90.32_{-0.28}^{+0.32}$ keV, for the reference continuum and line models, HIGHECUT and GABS respectively. Detection is very robust with respect to different continuum models. The line energy is significantly higher than what suggested from previous observations, which provided very marginal evidence for the line. This establishes a new record for the centroid energy of a fundamental cyclotron resonant scattering feature observed in accreting pulsars. We also discuss the accretion regime of the source during the Insight-HXMT observation.