X-ray spectral and flux variability of the microquasar GRS 1758-258 on timescales from weeks to years

TL;DR

This study analyzes 12 years of X-ray observations of the black hole binary GRS 1758-258, revealing its spectral state changes, flux variability, and long-term periodicities, with implications for accretion disk models.

Contribution

It provides a comprehensive long-term spectral and timing analysis of GRS 1758-258, highlighting its hysteresis behavior and potential disk warping effects during state transitions.

Findings

GRS 1758-258 predominantly in hard state with seven soft state episodes

Flux decline above 3 keV during soft states, unlike typical transients

Detected long-term periodicities drifting between 18 and 20 days

Abstract

We present the spectral and timing evolution of the persistent black hole X-ray binary GRS 1758-258 based on almost 12 years of observations using the Rossi X-ray Timing Explorer Proportional Counter Array. While the source was predominantly found in the hard state during this time, it entered the thermally dominated soft state seven times. In the soft state GRS 1758-258 shows a strong decline in flux above 3 keV rather than the pivoting flux around 10 keV more commonly shown by black hole transients. In its 3-20 keV hardness intensity diagram, GRS 1758-258 shows a hysteresis of hard and soft state fluxes typical for transient sources in outburst. The RXTE-PCA and RXTE-ASM long-term light curves do not show any orbital modulations in the range of 2 to 30 d. However, in the dynamic power spectra significant peaks drift between 18.47d and 18.04d for the PCA data, while less significant signatures between 19d and 20d are seen for the ASM data as well as for the Swift/BAT data. We discuss different models for the hysteresis behavior during state transitions as well as possibilities for the origin of the long term variation in the context of a warped accretion disk.