Suzaku observation of the tev blazar íes 1218+304: clues on particle acceleration in an extreme tev blazar

We observed the TeV blazar IES 1218+304 with the X-ray astronomy satellite Suzaku in 2006 May. At the beginning of the 2 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 multiwavelength data assuming a one-zone, homogeneous synchrotron self-Compton model, we obtain a magnetic field strength B ~ 0.047 G and 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 the differential acceleration timescale of relativistic electrons provided that the gyrofactor ξ is 10⁵.