A hidden jet emission component revealed by the anomalous X-ray low state of 1ES 0229+200

Abstract In August 2021, simultaneous Swift-XRT and NuSTAR observations caught the extreme high-synchrotron-peaked BL Lac object 1ES 0229+200 in an exceptionally faint X-ray state. The spectrum was concave, showing tentative hardening above ∼ 25 keV. We investigate three representative interpretations within a common decelerating-jet framework: a one-zone hybrid electron–proton syn-chrotron model (Model A), and external-Compton (EC) and pure synchrotron self-Compton (SSC) realizations of a two-zone leptonic model (Model B). All three reproduce the simultaneous X-ray spectrum. In Model A, electron synchrotron emission accounts for the soft X-rays, while proton synchrotron emission produces the hard component. In both realizations of Model B, a compact inner zone produces the hard component through EC or SSC emission, while an outer zone approximately describes the historical broadband emission. The three solutions therefore lead to the same observational inference: the low state reveals an additional radiative component that is normally hidden beneath the dominant soft-X-ray continuum. The present data do not determine whether this component arises from a second particle population in one zone or from a distinct dissipation region. Simultaneous hard-X-ray, MeV, GeV, and VHE observations will distinguish these possibilities.