Changes in the nature of the spectral continuum and the stability of the cyclotron line in the X-ray pulsar GRO J2058+42

TL;DR

This study examines how the spectral and timing properties of the X-ray pulsar GRO J2058+42 change at different luminosities, revealing shifts in pulse profiles, spectral shape, and cyclotron line features, and proposing a model for the spectral formation.

Contribution

It provides new insights into the spectral and timing variability of GRO J2058+42 at low luminosity, including the necessity of a two-component spectral model and the stability of the cyclotron line.

Findings

Pulse profile and fraction change with luminosity.

Spectral shape varies from typical pulsar to two-component model.

Cyclotron line at ~10 keV remains consistent across states.

Abstract

The results of the broadband spectral and timing study of the transient X-ray pulsar GRO J2058+42 in a wide energy range at a low luminosity $L_{x} \simeq 2.5\times 10^{36}$ erg s$^{-1}$ are reported. The data revealed that the pulse profile and pulse fraction of the source are significantly changed in comparison with previous NuSTAR observations, when the source was ten times brighter. The cyclotron absorption line at $\sim10$ keV in the narrow phase interval is consistent with the high state observations. Spectral analysis showed that at high luminosities $L_{x}\simeq (2.7-3.2)\times 10^{37}$ erg s$^{-1}$ the spectrum has a shape typical of accreting pulsars, while when the luminosity drops by about an order of magnitude, to $2.5\times 10^{36}$ erg s$^{-1}$ a two-component model is necessary to its describing. This behavior fits into a model in which the low-energy part of the spectrum is formed in a hot spot, and the high-energy part is formed as a result of resonant Compton scattering by incident matter in an accretion channel above the surface of a neutron star.