The X-ray Spectral Evolution of Galactic Black Hole X-ray Binaries Toward Quiescence

TL;DR

This study tracks the X-ray spectral evolution of three black hole X-ray binaries as they transition into quiescence, revealing spectral softening and an upper limit for the quiescent state, with implications for accretion models.

Contribution

First detailed spectral evolution analysis of multiple BHXBs during decay into quiescence, establishing spectral properties and limits across different systems.

Findings

Quiescent BHXBs have softer spectra than low-hard state BHXBs.

Spectral properties show no dependence on binary orbital parameters.

Photon index plateaus at ~2.08 in quiescence, setting an observational upper limit.

Abstract

Most transient black hole X-ray binaries (BHXBs) spend the bulk of their time in a quiescent state, where they accrete matter from their companion star at highly sub-Eddington luminosities (we define quiescence here as a normalized Eddington ratio l_x = L_{0.5-10 keV}}/L_{Edd} < 1e-5). Here, we present Chandra X-ray imaging spectroscopy for three BHXB systems (H 1743-322, MAXI J1659-152, and XTE J1752-223) as they fade into quiescence following an outburst. Multiple X-ray observations were taken within one month of each other, allowing us to track each individual system's X-ray spectral evolution during its decay. We compare these three systems to other BHXB systems. We confirm that quiescent BHXBs have softer X-ray spectra than low-hard state BHXBs, and that quiescent BHXB spectral properties show no dependence on the binary system's orbital parameters. However, the observed anti-correlation between X-ray photon index and l_x in the low-hard state does not continue once a BHXB enters quiescence. Instead, the photon index plateaus to an average value of 2.08+/-0.07 by the time l_x reaches approximately 1e-5. l_x~1e-5 is thus an observationally-motivated upper limit for the beginning of the quiescent spectral state. Our results are discussed in the context of different accretion flow models, and across the black hole mass scale.