Anjum Peer, Zahir Shah, Sikandar Akbar, Bari Maqbool, Ranjeev Misra

We present a multi-wavelength temporal and spectral study of the changing-look blazar B2~1420+32 using \emph{Fermi}-LAT, \emph{Swift}-XRT, and \emph{Swift}-UVOT data from MJD~58818--60721. The source reached a peak 0.1--300~GeV photon flux of $(4.62 \pm 0.29) \times 10^{-6}\,\mathrm{ph\,cm^{-2}\,s^{-1}}$ around MJD~60488, about 60 times the 4FGL-DR4 average, during which the photon index hardened to $2.19 \pm 0.14$; the flux--index evolution shows only weak evidence for global harder-when-brighter behaviour. The fractional variability is strongly energy dependent, largest in $γ$-rays, substantial in the optical/UV, and low in X-rays. Strong $γ$-ray--optical/UV correlations and a moderate $γ$-ray--X-ray correlation indicate that the X-ray emission tracks the $γ$-ray variability less closely than the optical/UV emission. The X-ray spectra are best described by a log-parabola, and the negative curvature measured in four of the five states suggests that the X-ray band samples the transition between the high-energy tail of the synchrotron component and the onset of the inverse-Compton component. We identified five activity states and modelled the high-energy (X-ray and $γ$-ray) component of their broadband spectral energy distributions (SEDs) 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 VHE emission, whereas SSC+EC provides the most self-consistent description, with a seed-photon temperature of $\sim 10^{3}$~K favouring an infrared torus origin. The brighter states require larger bulk Lorentz factors and higher jet powers, while the magnetic field varies only modestly, indicating that the flux evolution is governed by a combination of Doppler boosting and jet energetics.