Suzaku observation of TeV blazar the 1ES 1218+304: Clues on particle acceleration in an extreme TeV blazar

We observed the TeV blazar 1ES 1218+304 with the X-ray astronomy satellite Suzaku in May 2006. At the beginning of the two-day continuous observation, we detected a large flare in which the 5-10 keV flux changed by a factor of ∼2 on a timescale of 5×10⁴ s. During the flare, the increase in the hard X-ray flux clearly lagged behind that observed in the soft X-rays, with the maximum lag of 2.3 ×10⁴ s observed between the 0.3-1 keV and 5-10 keV bands. Furthermore we discovered that the temporal profile of the flare clearly changes with energy, being more symmetric at higher energies. From the spectral fitting of multi-wavelength data assuming a one-zone, homogeneous synchrotron self-Compton model, we obtain a magnetic field strength B - 0.047 G, an emission region size R = 3.0 × 10¹⁶ cm for an appropriate beaming with a Doppler factor of δ = 20. This value of B is in good agreement with an independent estimate through the model fit to the observed time lag ascribing the energy-dependent variability to differential acceleration timescale of relativistic electrons provided that the gyro-factor ξ is 10⁵.