Venturing beyond the ISCO: detecting X-ray emission from the plunging regions around black holes

Abstract

We explore how X-ray reverberation around black holes may reveal the presence of the innermost stable circular orbit (ISCO), predicted by General Relativity, and probe the dynamics of the plunging region between the ISCO and the event horizon. Being able to directly detect the presence of the ISCO and probe the dynamics of material plunging through the event horizon represents a unique test of general relativity in the strong field regime. X-ray reverberation off of the accretion disc and material in the plunging region is modelled using general relativistic ray tracing simulations. Xray reverberation from the plunging region has a minimal effect on the time-averaged X-ray spectrum and the overall lag-energy spectrum, but is manifested in the lag in the highest frequency Fourier components, above 0.01 c 3 (GM ) −1 (scaled for the mass of the black hole) in the 2-4 keV energy band for a non-spinning black hole or the 1-2 keV energy band for a maximally spinning black hole. The plunging region is distinguished from disc emission not just by the energy shifts characteristic of plunging orbits, but by the rapid increase in ionisation of material through the plunging region. Detection requires measurement of time lags to an accuracy of 20 per cent at these frequencies. Improving accuracy to 12 per cent will enable constraints to be placed on the dynamics of material in the plunging region and distinguish plunging orbits from material remaining on stable circular orbits, confirming the existence of the ISCO, a prime discovery space for future X-ray missions.

Testing General Relativity with NuSTAR Data of Galactic Black Holes

“…In such a case, the Comptonized photons from the corona should generate a reflection 9 component. However, the gas is highly ionized in the plunging region (Reynolds & Begelman 1997;Wilkins et al 2020). The impact of such radiation on the estimate of the Johannsen deformation parameter α 13 was studied in , where it was shown that for fast-rotating black holes associated with very small plunging regions (which is the case of the sources presented in our study) the impact is negligible for present and near future X-ray data.…”

Returning radiation in strong gravity around black holes: reverberation from the accretion disc

“…The passage of photons in the curved space-time around the black hole was followed using General Relativistic ray-tracing simulations. Rays were traced by integrating the null geodesic equations in the Kerr space-time using the CUDAKERR GPU-based ray-tracing code (Wilkins & Fabian 2012Wilkins et al 2016;Wilkins, Reynolds & Fabian 2020).…”

“…The extension of the decaying tail of the response function, however, is encoded in the higher frequency Fourier components (Wilkins et al 2020). The delayed, scattered photons from the inner disc lead to a steepening of the decay of the time lag at high frequencies.…”

Towards Precision Measurements of Accreting Black Holes Using X-Ray Reflection Spectroscopy

“…The plunging region can thus generate a reflection spectrum. However, the gas in the plunging region is highly ionized (Reynolds & Begelman 1997, Wilkins et al 2020a) and therefore its spectrum looks like a power-law component: even if we employ a relativistic reflection model that ignores the reflection from the plunging region, the impact on the measurements of the model parameters is often modest (Cardenas-Avendano et al 2020).…”