A curious case of the accretion-powered X-ray pulsar GX 1+4

TL;DR

This study provides detailed spectral and timing analysis of the accretion-powered X-ray pulsar GX 1+4, revealing its high spin period, complex pulse profiles, and emission mechanisms, including a blackbody component and magnetic field estimates.

Contribution

It introduces a comprehensive spectral and timing analysis of GX 1+4 during an intermediate state, highlighting the emission components and magnetic field estimation, which are novel insights.

Findings

Spin period of 176.778 s, one of the highest since discovery.

Detection of a sharp peak in pulse profiles below 25 keV.

Estimated magnetic field strength of (5-10)×10^12 G.

Abstract

We present detailed spectral and timing studies using a NuSTAR observation of GX 1+4 in October 2015 during an intermediate intensity state. The measured spin period of 176.778 s is found to be one of the highest values since its discovery. In contrast to a broad sinusoidal-like pulse profile, a peculiar sharp peak is observed in profiles below ~25 keV. The profiles at higher energies are found to be significantly phase-shifted compared to the soft X-ray profiles. Broadband energy spectra of GX 1+4, obtained from NuSTAR and Swift observations, are described with various continuum models. Among these, a two component model consisting of a bremsstrahlung and a blackbody component is found to best-fit the phase-averaged and phase-resolved spectra. Physical models are also used to investigate the emission mechanism in the pulsar, which allows us to estimate the magnetic field strength to be in $\sim$(5-10)$\times$10$^{12}$ G range. Phase-resolved spectroscopy of NuSTAR observation shows a strong blackbody emission component in a narrow pulse phase range. This component is interpreted as the origin of the peculiar peak in the pulse profiles below $\le$25 keV. The size of emitting region is calculated to be $\sim$400 m. The bremsstrahlung component is found to dominate in hard X-rays and explains the nature of simple profiles at high energies.