Spin period evolution of GX 1+4

TL;DR

This study compiles and analyzes decades of X-ray data from multiple observatories to understand the long-term spin period evolution of the accreting pulsar GX 1+4, revealing a steady spin-down with flux-dependent fluctuations.

Contribution

It provides a comprehensive analysis of GX 1+4's spin evolution using new and archival data, offering insights into accretion processes affecting pulsar spin behavior.

Findings

The pulsar's spin period increased by about 50% over three decades.

Long-term spin-down is consistent with a constant frequency decrease.

Short-term flux fluctuations correlate with anti-correlation in spin changes.

Abstract

We aim both to complement the existing data on the spin history of the peculiar accreting X-ray pulsar GX 1+4 with more past and current data from BeppoSAX, INTEGRAL, and Fermi and to interpret the evolution in the framework of accretion theory. We used source light curves obtained from BeppoSAX/WFC and INTEGRAL/ISGRI to derive pulse periods using an epoch-folding analysis. Fermi/GBM data were analyzed by fitting a constant plus a Fourier expansion to background-subtracted rates, and maximizing the Y2 statistic. We completed the sample with hard X-ray light curves from Swift/BAT. The data were checked for correlations between flux and changes of the pulsar spin on different timescales. The spin-down of the pulsar continues with a constant change in frequency, i.e., an apparently accelerating change in the period. Over the past three decades, the pulse period has increased by about ~50%. Short-term fluctuations on top of this long-term trend do show anti-correlation with the source flux. Possible explanations of the observed long-term frequency and its dependence on flux are discussed.