Utilising over 500 (~daily) archival observations of the black hole X-ray binary GX 339-4 with the Rossi X-ray Timing Explorer, we have been able to map out quantitatively how accretion on to a black hole evolves.
Black hole X-ray binaries typically spend the majority of their lifetimes in a `quiescent’ state punctuated by intense (peak L~10^39 erg/s) outbursts, which can last for months or even years. During these dramatic outbursts the temporal and spectral X-ray properties undergo marked changes, tracing out distinct common patterns. The X-ray spectrum of a BHXRB is dominated by two principal components: soft X-rays from the accretion disc, and hard X-rays from the “corona”, which is thought to be a cloud of hot electrons Compton up-scattering the disc photons. A portion of these coronal photons will irradiate the accretion disc and be reprocessed, leading to a third component known as the “reflection spectrum”. The relative strength of the reflection and corona will be a strong function of the accretion disc and corona geometry. For example, if the inner radius of the accretion disc becomes truncated, then the reflection will weaken as less corona photons are able to be intercepted. At the same time the observed flux from the corona will remain constant.
The figure above compares how the flux of the two components evolves during an outburst in unprecedented detail. We can reveal that during the canonical low/hard state (1 & 4), where outbursts begin and end, the reflection is much weaker than the corona, but gradually increases in relative strength as the two components increase in flux. This indicates that the inner part of the accretion disc must be substantially truncated, and then gradually moves closer to the black hole as the source luminosity rises. In stark contrast, the reflection is typically brighter than the corona during the soft state (2), and becomes increasingly dominant as the source flux decays. Independent evidence suggests that the accretion disc inner radius is constant during this soft state, thus the corona must gradually collapse as the soft state progresses in order to allow more corona photons to be reflected. These same trends are observed over three full outbursts, therefore it appears that the accretion geometry evolves in a common and predictable pattern.