The "universal" radio/X-ray flux correlation : the case study of the black hole GX 339-4

Abstract: The existing radio and X-ray flux correlation for Galactic black holes in the hard and quiescent states relies on a sample which is mostly dominated by two sources (GX 339-4 and V404 Cyg) observed in a single outburst. In this paper, we report on a series of radio and X-ray observations of the recurrent black hole GX 339-4 with the Australia Telescope Compact Array, the Rossi X-ray Timing Explorer and the Swift satellites. With our new long term campaign, we now have a total of 88 quasi-simultaneous radio and X-ray observations of GX 339-4 during its hard state, covering a total of seven outbursts over a 15--year period. Our new measurements represent the largest sample for a stellar mass black hole, without any bias from distance uncertainties, over the largest flux variations and down to a level that could be close to quiescence, making GX 339-4 the reference source for comparison with other accreting sources (black holes, neutrons stars, white dwarfs and active galactic nuclei). Our results demonstrate a very strong and stable coupling between radio and X-ray emission, despite several outbursts of different nature and separated by a period of quiescence. The radio and X-ray luminosity correlation of the form L_X ~L_Rad^0.62 +/-0.01 confirms the non-linear coupling between the jet and the inner accretion flow powers and better defines the standard correlation track in the radio-X-ray diagram for stellar mass black holes. We further note epochs of deviations from the fit that significantly exceed the measurement uncertainties, especially during the formation and destruction of the compact jets ...[abridged]. We incorporated our new data in a more global study of black hole candidates strongly supporting a scale invariance in the jet-accretion coupling of accreting black holes, and confirms the existence of two populations of sources in the radio/X-ray diagram.

Long-Term Radio/X-ray Flux Correlation in GX 339-4: A Comprehensive Analysis

The paper investigates the long-term radio/X-ray flux correlation in the black hole GX 339-4, a key topic in the study of accreting black holes (BHs). Using 88 quasi-simultaneous radio and X-ray observations collected over 15 years, this study provides an extensive dataset that significantly enlarges the sample size used in previous analyses. These observations cover seven outbursts over this period, allowing for a detailed examination of the correlation dynamics across different hard states.

Methodology and Observations

The researchers utilized data from major observational facilities, including the Australia Telescope Compact Array, the Rossi X-ray Timing Explorer, and the Swift satellites. This comprehensive campaign amassed a total of 88 radio/X-ray data points for GX 339-4, establishing it as the largest dataset for a stellar-mass black hole.

Main Findings

1. Stable Correlation Over Time: The relationship between radio and X-ray luminosities, traditionally characterized by a non-linear correlation, is confirmed to be robust across multiple outbursts. The paper reports a correlation of the form ( L_X \propto L_{\text{Rad}}^{0.62 \pm 0.01} ), a finding consistent with prior indices yet improved in precision due to the expanded dataset.

2. Deviation Periods: Significant deviations from the fitted correlation were observed, particularly during the formation and destruction of self-absorbed compact jets. During these intervals, the radio luminosity often appeared brighter during the decay phase than in the rise for equivalent X-ray luminosity levels, implying potential variability in jet properties or accretion dynamics.

3. Connection to Near-Infrared Emissions: By linking radio/X-ray data with near-infrared/X-ray correlations, the study underscores a coupled emission characteristic spanning these frequency ranges. This finding emphasizes the potential for jet influence on emissions across a wide spectrum.

4. Populations of BH Sources: The global analysis positions GX 339-4 as a reference point for understanding accreting sources, suggesting a scale invariance in jet-accretion coupling across mass scales, encompassing BHs, neutron stars, and possibly active galactic nuclei.

5. Extended Analysis & Implications: When plotted on a global scale alongside other BH candidates, GX 339-4 and V404 Cyg follow the "standard" track on the radio/X-ray diagram, differentiating them from "outliers" like H 1743-322. This separation highlights two distinct population behaviors and supports theoretical considerations of radiatively inefficient states and potentially varying radio and X-ray emission mechanisms.

Implications and Future Directions

This comprehensive analysis establishes GX 339-4 as a foundational source for reference in BH studies. The persistence of the "universal" correlation suggests a fundamental relationship between jet activity and accretion dynamics, reinforced by the connection to near-infrared emissions. The observed deviations offer a pathway to further explore the interplay between jet formation, disc properties, and state transitions, potentially leading to refined models of BH accretion physics.

Continued monitoring across diverse frequency bands and the extension of these techniques to other BH systems are likely to deepen our understanding of the underlying mechanisms driving these correlations. Future efforts might focus on more sensitive observations in quiescence states, further testing the limits and universality of these findings. Overall, the results facilitate a robust platform for future comparative studies across different accreting systems, potentially informing scaling laws applicable to AGNs as suggested by the "fundamental plane" of BH activity.