High Energy Jet Emission from GRS 1758-258 & 1E 1740.7-2942 with INTEGRAL?

TL;DR

This study analyzes the spectral and temporal evolution of two persistent black hole binaries, GRS 1758-258 and 1E 1740.7-2942, using INTEGRAL data over nearly two decades, revealing complex spectral components and correlations that inform their emission mechanisms.

Contribution

The paper provides a detailed long-term spectral analysis of the sources, testing different Comptonization models and examining correlations to distinguish emission processes.

Findings

Detection of spectral deviations above 200 keV indicating a hard tail.

Strong correlation of count rates below 300 keV suggests a common physical origin.

Disfavoring of the hybrid Comptonization scenario based on high-energy correlation analysis.

Abstract

GRS 1758-258 and 1E 1740.7-2942 are two long-known persistent black hole binaries in the Galactic Center region. Using INTEGRAL's extensive monitoring of the Galactic Center and Bulge, we studied their temporal and spectral evolutions in the 30-610 keV energy range from March 2003 through April 2022 with the IBIS/ISGRI gamma-ray telescope. Our analyses found that the sources typically had Comptonized spectra, though not always with the same parameters. The spectral states with more than 8 Ms of observation time show deviations from a Comptonized spectrum above ~200 keV or a "hard tail" that extends up to at least 600 keV. The origin of this component remains debated with the most popular scenarios being synchrotron emission from the jet or Comptonization in a hybrid thermal/non-thermal plasma. Anyway, the GRS 1758-258 and 1E 1740.7-2942 spectra are acceptably described by CompTT+po (jet) and Eqpair (hybrid Comptonization) scenarios. To differentiate between the two scenarios, we calculated the Spearman correlation coefficient comparing 30-50 keV count rates with those in higher energy bands (50-100, 100-300, and 300-600 keV). The count rates below 300 keV are strongly correlated, indicating those photons arise from the same physical process. Above 300 keV the count rates are either anti-correlated or not correlated with the 30-50 keV count rates for GRS 1758-258, which suggests that the photons originate from a different physical process. For 1E 1740.7-2942, the level of correlation is unclear due to scatter in the data points. However, the 300-600 keV count rates are consistent with a constant value. This disfavors the hybrid Comptonization scenario for both sources.