Probing the High-energy Emission of the Very-high-energy-emitting Changing-look Blazar B2 1420+32

Abstract We present a multiwavelength temporal and spectral study of the changing-look blazar B2 1420+32 using Fermi Large Area Telescope, Swift X-Ray Telescope, and Swift UV Optical Telescope data from MJD 58818–60721. The source reached a peak 0.1–300 GeV photon flux of (4.62 ± 0.29) × 10 −6 ph cm −2 s −1 around MJD 60488, about 60 times the 4FGL-DR4 average, during which the photon index hardened to 2.19 ± 0.14; the flux–index evolution shows only weak evidence for global harder-when-brighter behaviour. The fractional variability is strongly energy dependent, largest in gamma rays, substantial in the optical/UV, and low in X-rays. Strong gamma ray–optical/UV correlations and a moderate gamma ray–X-ray correlation are observed. We identified five activity states for broadband spectral energy distribution (SED) modelling. Their X-ray spectra are described by a log-parabola, with significant negative curvature in four of the five states. The high-energy (X-ray and gamma-ray) components of their SED are modeled using synchrotron self-Compton (SSC), external Compton (EC), and SSC+EC scenarios. The SSC-only and EC-only models either require physically disfavoured parameters or fail to reproduce the very-high-energy (VHE) emission, whereas SSC+EC provides the most self-consistent description, with a seed-photon temperature of ∼10 3 K favouring an infrared torus origin. The brighter states require larger bulk Lorentz factors and higher jet powers, while the magnetic field is comparable across most states but drops sharply in the VHE state. The break energy also changes significantly between outbursts, indicating that the spectral shape evolves between activity episodes. These results suggest that Doppler boosting and jet energetics set the activity level, and the break energy governs the spectral shape between outbursts.