Discovery of a pulse-phase-transient cyclotron line in the X-ray pulsar GROJ2058+42

TL;DR

This paper reports the discovery of phase-dependent cyclotron absorption lines in the X-ray spectrum of the pulsar GROJ2058+42, revealing a neutron star magnetic field of about 10^12 G during a 2019 outburst.

Contribution

First detection of phase-specific cyclotron lines in GROJ2058+42, indicating complex magnetic field interactions in transient X-ray pulsars.

Findings

Cyclotron lines detected at ~10, ~20, and ~30 keV.

Absorption features are phase-dependent, appearing only in 10% of the pulsar's rotation.

Magnetic field estimated at ~10^12 G.

Abstract

We report the discovery of absorption features in the X-ray spectrum of the transient X-ray pulsar GROJ2058+42. The features are detected around $\sim10$, $\sim20$ and $\sim30$ keV in both NuSTAR observations carried out during the source type II outburst in spring 2019. The most intriguing property is that the deficit of photons around these energies is registered only in the narrow phase interval covering around 10% of the pulsar spin period. We interpret these absorption lines as a cyclotron resonant scattering line (fundamental) and two higher harmonics. The measured energy allow us to estimate the magnetic field strength of the neutron star as $\sim10^{12}$ G.