Probing the disc–jet coupling in S4 0954+65, PKS 0903−57, and 4C +01.02 with γ-rays

Abstract

ABSTRACT We present a comprehensive variability study on three blazars, S4 0954+65, PKS 0903−57, and 4C +01.02, covering a mass range of log(M/M⊙) = 8–9, by using ∼15-yr-long γ-ray LCs from Fermi-LAT. The variability level is characterized by the fractional variability amplitude that is higher for γ-rays compared with optical/UV and X-rays emissions. A power spectral density (PSD) study and damped random walk (DRW) modelling are done to probe the characteristic time-scale. The PSD is fitted with a single power law (PL) and bending PL models and the corresponding success fraction was estimated. In the case of PKS 0903−57, We observed a break in the γ-ray PSD at 256 d, which is comparable to the viscous time-scale in the accretion disc, suggesting a possible disc–jet coupling. The non-thermal damping time-scale from the DRW modelling is compared with the thermal damping time-scale for AGNs including our three sources. Our sources lie on the best fit of the $\mathrm{\tau ^{\rm rest}_{\rm damping}}\!-\!M_{\rm BH}$ plot derived for AGN, suggesting a possible accretion disc–jet connection. If the jet’s variability is linked to the disc’s variability, we expect a log-normal flux distribution, often connected to the accretion disc’s multiplicative processes. Our study observed a double log-normal flux distribution, possibly linked to long- and short-term variability from the accretion disc and the jet. In summary, PSD and DRW modelling results for these three sources combined with blazars and AGNs studied in literature favour a disc–jet coupling scenario. However, more such studies are needed to refine this understanding.