Blazar anti-sequence of spectral variation within individual blazars: Cases for MRK 501 and 3C 279

The jet properties of Mrk 501 and 3C 279 are derived by fitting broadband spectral energy distributions (SEDs) with lepton models. The derived γ_b (the break Lorenz factor of the electron distribution) is 10⁴-10⁶ for Mrk 501 and 200 ∼ 600 for 3C 279. The magnetic field strength (B) of Mrk 501 is usually one order of magnitude lower than that of 3C 279, but their Doppler factors (δ) are comparable. A spectral variation feature where the peak luminosity is correlated with the peak frequency, which is opposite from the blazar sequence, is observed in the two sources. We find that (1) the peak luminosities of the two bumps in the SEDs for Mrk 501 depend on γ_b in both the observer and co-moving frames, but they are not correlated with B and δ and (2) the luminosity variation of 3C 279 is dominated by the external Compton (EC) peak and its peak luminosity is correlated with γ_b and δ, but anti-correlated with B. These results suggest that γ_b may govern the spectral variation of Mrk 501 and δ and B would be responsible for the spectral variation of 3C 279. The narrow distribution of γ_b and the correlation of γ_b and B in 3C 279 would be due to the cooling from the EC process and the strong magnetic field. Based on our brief discussion, we propose that this spectral variation feature may originate from the instability of the corona but not from the variation of the accretion rate as does the blazar sequence.