Radiation Mechanisms and Physical Properties of GeV-TeV BL Lac Objects

Abstract: BL Lac objects are the best candidates to study the jet properties since their spectral energy distributions (SEDs) are less contaminated by the emission from the accretion disk and external Compton processes. We compile the broadband SEDs observed with Fermi/LAT and other instruments from literature for 24 TeV BL Lac objects. Two SEDs, which are identified as a low or high state according to its flux density at 1 TeV, are available for each of ten objects. These SEDs can be explained well with the synchrotron+synchrotron-self-Compton model. We constrain the magnetic filed strength B and the Doppler factor delta of the radiation region by incorporating the chi^{2}-minimization technique and the gamma-ray transparency condition. Twenty-four out of the 34 SEDs can constrain B and delta in 1 sigma significance level and others only present the limits for both B and delta. The typical values of $B$ and delta are 0.1 ~ 0.6 G and 10 ~ 35, respectively. The derived values of gamma_b are significantly different among sources and even among the low and high states of a given source. Prominent flux variations with a clear spectral shift are observed and a tentative correlation between the ratio of the flux density at 1 TeV and the ratio of gamma_b in the low and high states is presented, likely indicating that the relativistic shocks for the electron acceleration may be responsible for the flux variations and the spectral shift. A weak anti-correlation between the jet power and the mass of the central black hole is observed, i.e., P_{jet} propto M^{-1}_{BH}, which disfavors the scenario of a pure accretion-driven jet. Implications for the blazar sequence and the intergalactic magnetic field from our results are also briefly discussed.