| Name: | Biki Ram |
| Affiliation: | IIT Indore |
| Conference ID: | ASI2025_508 |
| Title: | Unveiling State Transitions and Inner Disk-Corona Dynamics in Black Hole X-ray Binaries Using Power-Color and QPO Analysis |
| Authors: | Biki Ram, Dr. Manoneeta Chakraborty
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| Authors Affiliation: | 1. Indian Institute of Technology Indore (IITI), India
2. Indian Institute of Technology Indore (IITI), India |
| Mode of Presentation: | Poster |
| Abstract Category: | High Energy Phenomena, Fundamental Physics and Astronomy |
| Abstract: | The power spectra of Black Hole (BH) Low-Mass X-ray Binaries (LMXBs) evolve systematically across states, influenced by the fluctuations in the corona and accretion disk. We used an innovative power-color technique based on the ratio of variability amplitude at distinct frequency ranges to estimate spectral states using hue, representing angular positions on the power-color wheel. This method provides a promising alternative to the standard Hardness-Intensity Diagram (HID) method, which is often ambiguous. In this study, we analyzed the power spectra of ten BH-LMXB sources using archival AstroSat data during outburst phases. By employing a root-mean-square (RMS) variability versus hue analysis, we identified distinct states.
Quasi-periodic oscillations (QPOs) in LMXBs provide valuable insights into the emission mechanisms of the inner accretion disk, constraining parameters in the strong field regime. We detected multiple such QPOs and associated harmonic in our sample and conducted a study of their energy dependence. We observed the evolution of QPO RMS spectra with both hue and QPO frequency, shedding light on the inner accretion disk dynamics and disk-corona geometry in various states. A notable finding was the detection of a sign reversal in the average QPO time lag between hard and soft photons near a QPO frequency of 2 Hz, concurrent with the hard to hard-intermediate state transition. Our analysis further revealed a change in the slope of the QPO RMS spectra around this same frequency and hue value.
This observed time lag reversal (hard to soft) which occurs during this state transition phase is consistent with a transition from an elongated jet-like corona (responsible for the hard lag) to a compact corona (responsible for soft lags through reverberation) model. The transition frequency may signify the critical radius for Lense-Thirring precession, above which the state transition occurs. This study offers insights into the state evolution of BH-LMXB.
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