PKS 2155-304 is a BL Lac object, a galaxy hosting a type of

blazar A blazar is an active galactic nucleus (AGN) with a relativistic jet (a jet composed of ionized matter traveling at nearly the speed of light) directed very nearly towards an observer. Relativistic beaming of electromagnetic radiation from the ...

. 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.

References

BL Lacertae objects Piscis Austrinus {{Stars of Piscis Austrinus