Continuum emission from within the plunging region of black hole discs (2405.09175v1)

Abstract: The thermal continuum emission observed from accreting black holes across X-ray bands has the potential to be leveraged as a powerful probe of the mass and spin of the central black hole. The vast majority of existing ``continuum fitting'' models neglect emission sourced at and within the innermost stable circular orbit (ISCO) of the black hole. Numerical simulations, however, find non-zero emission sourced from these regions. In this work we extend existing techniques by including the emission sourced from within the plunging region, utilising new analytical models which reproduce the properties of numerical accretion simulations. We show that in general the neglected intra-ISCO emission produces a hot-and-small quasi-blackbody component, but can also produce a weak power-law tail for more extreme parameter regions. A similar hot-and-small blackbody component has been added in by hand in an ad-hoc manner to previous analyses of X-ray binary spectra. We show that the X-ray spectrum of MAXI J1820+070 in a soft-state outburst is extremely well described by a full Kerr black hole disc, while conventional models which neglect intra-ISCO emission are unable to reproduce the data. We believe this represents the first robust detection of intra-ISCO emission in the literature, and allows additional constraints to be placed on the MAXI J1820+070 black hole spin which must be low $a_\bullet < 0.5$ to allow a detectable intra-ISCO region. Emission from within the ISCO is the dominant emission component in the MAXI J1820+070 spectrum between $6$ and $10$ keV, highlighting the necessity of including this region. Our continuum fitting model is made publicly available.

• The paper extends conventional accretion disc models by incorporating notable intra-ISCO emissions driven by magnetic stresses.
• It validates the analytic approach against GRMHD simulations and X-ray observations, capturing a distinct hot quasi-blackbody and a weak power-law tail.
• The model refines black hole property estimates by inferring low spin (a < 0.5) and enhancing accuracy in mass and accretion rate determinations.

Continuum Emission from Within the ISCO in Black Hole Accretion Discs

The research conducted by Mummery et al. addresses a critical gap in existing models of black hole accretion discs, specifically concerning the emission characteristics from within the plunging region, located inside the innermost stable circular orbit (ISCO). Historically, continuum fitting models have often excluded this region from their calculations, operating under the assumption of a negligible ISCO stress boundary. However, numerical simulations have persistently reported non-zero emission arising from this region, fueled by magnetic interactions that extend into the plunging area, suggesting the inadequacy of traditional models.

The authors extend standard accretion disc models by refining analytical methods that incorporate accurate representations of this inner-region emission, drawing from previously established simulation data and theoretical developments. Their model successfully reproduces the thermodynamic properties noted in GRMHD simulations, thus validating the analytical approach against complex computational results.

Key Findings

1. Emission Characteristics: The paper identifies that emission from within the ISCO introduces a significant, hot-and-small quasi-blackbody component to the overall spectrum. In scenarios with extreme parameters, a weak power-law tail can also emerge from this region, analogous to features occasionally added manually in previous spectral analyses of X-ray binaries.
2. Constraints and Observations: The X-ray observations of the binary system MAXI J1820+070, particularly during a soft-state outburst, affirm the necessity of incorporating intra-ISCO emissions in spectral modelling. Conventional disc models fail to account for observed spectral data without additional components, which this paper attributes to intra-ISCO processes.
3. Spin and Accretion Rate Implications: The analysis infers a low spin for the MAXI J1820+070 black hole, with spin parameter $a_\bullet < 0.5$, highlighting the potential for intra-ISCO emission to refine constraints on black hole properties. This opens a pathway for more robust investigations into black hole characteristics when observed emissions align with predictions from extended disc models.

Theoretical and Practical Implications

This paper demonstrates that overlooking emissions from the plunging region can lead to substantial inaccuracies in inferred black hole attributes like mass and spin. As these emissions are intrinsically linked to the magnetic stress phenomena within accretion discs, their inclusion not only advances the observational toolkit for black hole paper but also bridges fundamental theoretical gaps in our understanding of accretion disc physics and the effects of magnetic dynamics.

The researchers propose that these refined models could provide observational insights into magnetic turbulence processes within disc environments, offering a tangible connection between observed spectral features and underlying physical mechanisms. This has significant implications not just for black hole spectroscopy, but for broader applications in astrophysics where similar accretion models could be applied.

Future Directions

The research invites further exploration into the detailed physics governing photon starvation and radiative transfer within these regions, potentially integrating more sophisticated radiative transfer models to capture emission nuances. Additional focus on simulating varied accretion regimes and spin states could refine these constraints and expand the practical effectiveness of continuum fitting as a diagnostic tool for black hole systems. As these models continue to evolve, their role in providing a unified framework for the interpretation of black hole spectra will undoubtedly be a vital asset in both high-energy astrophysics and the paper of relativistic physics.

By introducing intra-ISCO effects into continuum fitting models, this paper lays crucial groundwork for re-evaluating black hole observations with a more comprehensive understanding of disc dynamics and emission processes. This development presents a significant incremental step toward aligning spectral analysis techniques with the intricate realities of astrophysical black hole environments.