2010ApJ...724.1509U

We report results from the observations of the well-studied TeV blazar Mrk 421 with the Swift and the Suzaku satellites in 2008 December. During the observation, Mrk 421 was found in a relatively low activity state, with the corresponding 2-10 keV flux of 3x10^–10 erg/s/cm2. For the purpose of robustly constraining the UV-to-X-ray emission continuum we selected only the data corresponding to truly simultaneous time intervals between Swift and Suzaku, allowing us to obtain a good-quality, broadband spectrum despite a modest length (0.6 ks) exposure. We analyzed the spectrum with the parametric forward-fitting SYNCHROTRON model implemented in XSPEC assuming two different representations of the underlying electron energy distribution, both well motivated by the current particle acceleration models: a power-law distribution above the minimum energy γ_min with an exponential cutoff at the maximum energy γ_max, and a modified ultra-relativistic Maxwellian with an equilibrium energy γ_eq. We found that the latter implies unlikely physical conditions within the blazar zone of Mrk 421. On the other hand, the exponentially moderated power-law electron distribution gives two possible sets of the model parameters: (1) flat spectrum dN'_e/dγ ∝ γ^–1.91 with low minimum electron energy γ_min< 10³, and (2) steep spectrum ∝γ^–2.77 with high minimum electron energy γ_min≃ 2x10⁴. We discuss different interpretations of both possibilities in the context of a diffusive acceleration of electrons at relativistic, sub- or superluminal shocks. We also comment on exactly how the γ-ray data can be used to discriminate between the different proposed scenarios.