PKS 2155−304 is a BL Lac object, a galaxy hosting a type of blazar. It is a strong emitter from radio to high-energy frequencies. PKS 2155−304 is at redshift z = 0.116 (Falomo, Pesce & Treves 1993). It is one of the brightest and most studied BL Lacs, one of the first identified through X-ray observations.
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