RADIATION MECHANISMS AND PHYSICAL PROPERTIES OF GeV–TeV BL Lac OBJECTS

Abstract

Broadband spectral energy distributions (SEDs) simultaneously or quasisimultaneously observed with Fermi/LAT and the other instruments are complied from literature for 24 TeV BL Lac objects. Two SEDs are available for each of 11 objects, and the state of the sources is identified as a low or high state according to its flux density at 1 TeV. The well-sampled, clean SEDs without contaminations of the accretion disk and external Compton process of these sources are the best candidates for investigating the radiation mechanisms and the physical properties of the jets. Assuming that the electron spectrum is a broken power-law with a break at γ b and using the peak frequencies and their corresponding lumminosities (ν s , ν c , L s , and L c ) of the SEDs, we fit the SEDs with the single-zone synchrotron + synchrotron-self-Compton (SSC) model and determine the physical parameters of the jets, including the Doppler boosting factor (δ), the magnetic field strength (B), the size of radiating region (R), the bolometric luminosity (L bol ), and the jet total power (P jet ). The model well represents the SEDs, and the observed relation between ν s and ν c also favors the model. In this scenario, we find that γ b is significantly different among sources and even among the low and high states of a given source, but B is distributed narrowly within the range of 0.1−0.6 G, indicating that the shocks in the jets are significantly different among sources and the magnetic field may be independent of the shocks. δ ranges from 14 to 30, and R = (2.6 ∼ 100) × 10 15 cm, suggesting that flux variations with a minimum timescale from an hour to one day may be observed for these sources. Prominent flux variations with a clear spectral shift are observed and the ratio of the flux density at 1 TeV is correlated with the ratio of the γ b in the low and high states, indicating that the relativistic shocks in the jets may be responsible for the flux variations and the spectral shift. δ of the high state is systematically larger than that of the low state, but the ratios of δ and the flux density in the high and low states are not correlated. The ratio of L c /L s is anti-correlated with ν s in the co-moving frame for the sources in both the high and low states, but the slopes of the anti-correlations are significantly different. This anti-correlation is possibly due to the Klein-Nishina effect, but not the cooling effect of the photon fields outside the jet, as proposed for explaining the blazar sequence. No excess in the GeV band due to the interaction between the TeV photons and the extragalactic background light is observed, implying that the strength of the intergalactic magnetic field would be much larger than 10 −16 G. The observed L bol is not correlated with P jet . An anti-correlation between P jet and the mass of the central black hole is observed, i.e., P jet ∝ M −1 BH , disfavoring the scenario of a pure accretion-driven jet. We suggest that the spin energy extraction may be significant for powering jets in these sources, implying t...

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