Radiatively efficient accreting black holes in the hard state: the case study of H1743-322

Abstract: In recent years, much effort has been devoted to unraveling the connection between the accretion flow and the jets in accreting compact objects. In the present work, we report new constraints on these issues, through the long term study of the radio and X-ray behaviour of the black hole candidate H1743-322. This source is known to be one of the outliers' of the universal radio/X-ray correlation, i.e. a group of accreting stellar-mass black holes displaying fainter radio emission for a given X-ray luminosity than expected from the correlation. Our study shows that the radio and X-ray emission of H1743-322 are strongly correlated at high luminosity in the hard spectral state. However, this correlation is unusually steep for a black hole X-ray binary: b ~ 1.4 (with L_{Radio} \propto L_{X}^{b}). Below a critical luminosity, the correlation becomes shallower until it rejoins the standard correlation with b ~ 0.6. Based on these results, we first show that the steep correlation can be explained if the inner accretion flow is radiatively efficient during the hard state, in contrast to what is usually assumed for black hole X-ray binaries in this spectral state. The transition between the steep and the standard correlation would therefore reflect a change from a radiatively efficient to a radiatively inefficient accretion flow. Finally, we investigate the possibility that the discrepancy betweenoutliers' and `standard' black holes arises from the outflow properties rather than from the accretion flow.

Radiatively Efficient Accreting Black Holes in the Hard State: Insights from H1743−322

The study of black hole X-ray binaries (BHXBs) continues to offer important insights into accretion processes and jet formation in astrophysical systems. The paper by Coriat et al. focuses on the BHXB H1743−322, aiming to unravel the connection between accretion flows and jets in these systems. H1743−322 is somewhat anomalous among stellar-mass black holes because it is an "outlier" in the universal radio/X-ray correlation, showing fainter radio emission at given X-ray luminosities.

Key Findings

The paper presents long-term observations of the radio and X-ray emissions from H1743−322. Key findings from this comprehensive study include:

• Radio/X-ray Correlation: In the hard spectral state and at high luminosities, a strong correlation exists between the radio and X-ray emissions of H1743−322, showing a steep slope with $L_{\rm Radio} \propto L_{X}^{1.4}$. This deviates from the more common $L_{\rm Radio} \propto L_{X}^{0.6-0.7}$ observed in typical BHXBs.
• Transition at Low Luminosities: Below a critical X-ray luminosity, the correlation becomes shallower and aligns with the standard $b \approx 0.6$ correlation, suggesting a shift in the accretion flow or outflow properties.
• Radiatively Efficient Flow: The steep correlation observed implies that the inner accretion flow remains radiatively efficient during the hard state, which contrasts with typical assumptions that such flows are inefficient.

Implications and Theoretical Considerations

The implications of these findings suggest that H1743−322 and possibly other outliers possess distinct accretion processes:

1. Radiatively Efficient to Inefficient Transition: The transition in correlation behavior might indicate a change from a radiatively efficient to an inefficient accretion flow as the luminosity decreases, possibly moving from a Luminous Hot Accretion Flow (LHAF) regime to an Advection Dominated Accretion Flow (ADAF).
2. Accretion Flow Models: The study's results challenge models that assume a homogeneous nature of the accretion flow in BHXBs, suggesting instead a complex interplay where multiple components contribute differentially based on the accretion state.
3. Outflow Properties: The suggestion to reframe outliers as "X-ray loud" implies that outflow properties, such as magnetic field strength and jet composition, could differ substantially, impacting their radiative efficiency and the observed emissions.

Speculations for Future Developments

This study lays foundational questions for future research in BHXBs:

• Determining Fundamental Parameters: Identification of parameters that determine whether a system is a standard BHXB or an outlier could focus on the companion star, orbital characteristics, or magnetic field influences.
• Refined Theoretical Models: Models incorporating variable jet power dependencies on accretion rate, incorporating dynamics of disc coronae, and more comprehensive jet-disc coupling mechanisms are necessary.
• Further Observational Campaigns: Systematic observations of other outlier sources, integrating multi-wavelength datasets, could refine the understanding of the correlation anomalies and transition points.

In conclusion, the investigation of H1743−322 provides critical insights into the complex physics governing accretion and jet emission in BHXBs. By challenging existing models and proposing a radiatively efficient framework for certain states, this paper underscores the need for reevaluation of accretion paradigms and highlights exciting avenues for future research in astrophysical black hole studies.