ASCAand Contemporaneous Ground‐based Observations of the BL Lacertae Objects 1749+096 and 2200+420 (BL Lac)

We present ASCA observations of the radio-selected BL Lacertae objects1749+096 (z=0.32) and 2200+420 (BL Lac, z=0.069) performed in 1995 Sept andNov, respectively. The ASCA spectra of both sources can be described as a firstapproximation by a power law with photon index Gamma ~ 2. This is flatter thanfor most X-ray-selected BL Lacs observed with ASCA, in agreement with thepredictions of current blazar unification models. While 1749+096 exhibitstentative evidence for spectral flattening at low energies, a concave continuumis detected for 2200+420: the steep low-energy component is consistent thehigh-energy tail of the synchrotron emission responsible for the longerwavelengths, while the harder tail at higher energies is the onset of theCompton component. The spectral energy distributions from radio to gamma-raysare consistent with synchrotron-self Compton emission from a single homogeneousregion shortward of the IR/optical wavelengths, with a second component in theradio domain related to a more extended emission region. For 2200+420,comparing the 1995 Nov state with the optical/GeV flare of 1997 July, we findthat models requiring inverse Compton scattering of external photons provide aviable mechanism for the production of the highest (GeV) energies during theflare. An increase of the external radiation density and of the power injectedin the jet can reproduce the flat gamma-ray continuum observed in 1997 July. Adirectly testable prediction of this model is that the line luminosity in2200+420 should vary shortly after (~1 month) a non-thermal synchrotron flare.Comment: 28 pages,6 figures, 5 tables; LaTeX document. accepted for publication in the Astrophysical Journa