Signature of Stochastic Acceleration and Cooling Processes in an Outburst Phase of the TeV Blazar ON 231

We present a detailed spectral and temporal study of the intermediate-type blazar ON 231 during the TeV outburst phase in 2008 June with observations performed by Swift and XMM-Newton. The X-ray flux of the source, which was significantly dominated by the soft photons (below $3-4$ keV), varies between 27$\%$ and 38$\%$ on day timescales, while mild variations were observed in the optical/UV emissions. We found a maximum soft lag of $\sim 1$ hr between the UV and soft X-ray bands, which can be understood if the magnetic field of the emitting region is $\sim 5.6~ \delta^{-1/3}$ G. The $0.6-10$ keV spectra can be well represented by a broken power-law model, which indicates the presence of both synchrotron and inverse Compton components in the studied X-ray regime. The synchrotron part of the SEDs constructed with simultaneous optical/UV and X-ray data follows a log-parabolic shape. A time-resolved spectral analysis shows that the break energy varies significantly between 2.4 and 7.3 keV with the changing flux state of the source, and the similar variations of the spectral slopes of the two components support the SSC scenario. The synchrotron tail, following a log-parabolic function, shows that the peak frequency ($\nu_{p}$) varies by two orders of magnitude ($\sim 10^{14}-10^{16}$ Hz) during the event. A significantly positive $E_{p}-\beta$ relation is observed from both SED and time-resolved spectral analyses. The most feasible scenario for the observed trend during the flaring event could be associated with a magnetic-field-driven stochastic process evolving toward an equilibrium energy level.