Variability as a predictor for the hard-to-soft state transition in GX 339-4. ☄️
Published in the AAS American Astrophysical Journal — link
I investigated characteristics of X-ray binary black holes with an MIT postdoc at the Kara Black Hole Group. We discovered that variability (powered by spectral hue) is a predictor for state transitions in certain black hole systems.
Overview 🦾
During the outbursts of black hole X-ray binaries (BHXRBs), their accretion flows transition through several states. The source luminosity rises in the hard state, dominated by non-thermal emission, before transitioning to the blackbody-dominated soft state. As the luminosity decreases, the source transitions back into the hard state and fades to quiescence.
This picture does not always hold, as ≈ 40% of the outbursts never leave the hard state. Identifying the physics that govern state transitions remains one of the outstanding open questions in black hole astrophysics.
To address this question, we analyzed archival RXTE data of multiple outbursts of GX 339-4. We compared the properties of the X-ray variability and time-averaged energy spectrum and demonstrate that the variability (quantified by the power spectral hue) systematically evolves ≈ 10-40 days ahead of the canonical state transition (quantified by a change in spectral hardness); no such evolution is found in hard state only outbursts.
This indicates that the X-ray variability can be used to predict if and when the hard-to-soft state transition will occur. Finally, we found a similar behavior in ten outbursts of four additional BHXRBs with more sparse observational coverage.
Based on these findings, we deduced that state transitions in BHXRBs might be driven by a change in the turbulence in the outer regions of the disk, leading to a dramatic change in variability. This change is only seen in the spectrum days to weeks later, as the fluctuations propagate inwards towards the corona.
Our findings are illustrated in example X-ray binary black hole systems, shown below. These trends are replicated in every BHXRB we investigated.
Takeaways 👣
・For several months, we struggled to find any factor that could be influencing the hardness (state transition) of these black hole systems. We graphed countless parameters against each other, parsed years of retrospective spectral radiation data, and met with our lab's head every week to discuss any patterns observable from our graphs. However, we still couldn't reach any definite trends — but we still kept searching. Finally, my postdoc suggested graphing variability (powered by spectral hue) as one of our final resorts, which had a clear predictive pattern.
・This experience taught me to be patient and the importance of consistent, regular effort when working on research or development projects!
・I also gained strong data analysis skills, especially data preprocessing, statistical analysis (variations of chi-squared models), and data visualization (as shown above).
