BL Lac objects are extragalactic, highly variable, polarized sources with significant emission from radio to X-ray wavelengths. However, despite extensive research, their nature and origin remain unclear. We propose here that a significant fraction of the BL Lac objects are optically violently variable quasars (OVVs) whose continuum emission has been greatly amplified, relative to the line emission, by point-like gravitational lenses (either stars or black holes) in intervening galaxies. Several anomalous physical and statistical properties of BL Lacs can be understood on the basis of this model, which is immediately testable on the basis of absorption line studies and by direct imaging.
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Ostriker, J., Vietri, M. Are some BL Lac objects artefacts of gravitational lensing?. Nature 318, 446–448 (1985). https://doi.org/10.1038/318446a0
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