Description
The “Fundamental Plane of Black Hole Activity (fundamental plane)” defines a correlation between X-ray and radio emission of Active Galactic Nuclei (AGN) and the mass of the accreting black hole (BH), serving as a primary indicator of disk-jet coupling in AGN across all accretion rates. However, the impact of intrinsic X-ray and radio variability on this relationship remains poorly understood. Testing the dependence of the fundamental plane on X-ray/radio variability is essential to understand the dynamic connection between AGN accretion disks and jets/outflows originating from the AGN core. Motivated by the empirical observations of a diverse class of AGN accreting at different rates, we have compiled a sample of AGN that have multi-epoch near-simultaneous observations in radio (5GHz) and X-ray (0.5-10 keV) bands. We present results from the X-ray/radio variability study of selected sources with different types of accretion and emission properties.
Additionally, we present a radio follow-up study of a Weak Line Quasar (WLQ) SDSSJ1539+3954 (Ayushi Chhipa et al 2026 ApJ 996 23), which exhibited an exceptional case of X-ray variability in 2019-2020. Its X-ray flux increased by over 20 times from 2013 to 2019 and subsequently dropped by at least a factor of 9 in 2020. We carried out a follow-up radio study in the 0.3-10 GHz range using the Giant Metrewave Radio Telescope (GMRT; 2020, 2022, 2024) and Very Large Array (VLA; 2022), and analyzed archival Very Large Array Sky Survey (VLASS) 3 GHz data (2017-2023) to investigate the source’s radio properties and potential connection with the X-ray behavior. Our observations reveal a compact radio source with a spectral index of -0.65 $\pm$ 0.15 in the frequency range 0.3-1.4 GHz and -1.09 $\pm$ 0.16 in 3-10 GHz. While the source was undetected in VLA-FIRST (1994) and VLASS epochs, the GMRT and VLA observations show no statistically significant variability over the monitored period. The absence of detectable changes in the radio flux, despite strong X-ray variability, suggests no direct connection between the X-ray variability and the radio emission, consistent with the thick-disk-plus-outflow (TDO) model for WLQs, accreting at super-Eddington rates. However, the sensitivity limit of the surveys prevents us from drawing definitive conclusions regarding no radio variability on longer timescales between the VLA-FIRST and GMRT epochs. Our analysis supports an AGN core wind/outflow-driven emission origin of the radio emission from the quasar.