Exploring the Origin of Broadband Emissions of Blazar PKS 2005−489

Abstract BL Lac object PKS 2005–489 is well-known for relatively stable γ -ray flux variation, whose broadband spectral energy distributions (SEDs) generally extend from radio to TeV energies. Three contemporaneous SEDs, obtained from the multi-frequency observations during 2008–2009, 2011–2012 and 2020, present a bimodal structure for the low-energy synchrotron hump, where the distributions of data points in X-ray and optical/UV bands exhibit a bump morphology, but both have significantly different flux levels. Additionally, the X- and γ -ray fluxes display a notable increase during high-flux state. These observational characteristics are difficult to reconcile with a one-zone homogeneous synchrotron self-Compton (SSC) model. In this study, we analyzed the Fermi-LAT data and obtained seven light curves spanning from 2008 to 2024, along with four sets of γ -ray energy distributions. Three SED modelings are performed based on the one-zone homogeneous SSC scenario, in which both the accretion disk (AD) and the host galaxy are invoked to interpret the optical/UV emissions. Our study of PKS 2005–489 demonstrates that: (1) the source has been in a low γ -ray active state during these observational periods; (2) our model can satisfactorily reproduce the broadband SEDs, particularly for the broader optical/UV excess; (3) the γ -ray emissions likely arose from the turbulent acceleration processes, with the emitting regions being strongly matter-dominated and the jet power is almost entirely in the form of electron energy; and (4) the Blandford–Znajek (BZ) process can extract sufficient energy from the central engine to sustain the jet activity.