Markarian 421 is a relatively nearby BL Lacertae object (400 million light-years) and is one of the brightest gamma-ray sources in the sky.
Astronomers now classify these objects as blazars, a term coined in 1978 by Columbia University astronomer Edward Spiegel to encompass two classes of objects: Optically Violently Variable quasars and BL Lacertae objects.
Beyond our galaxy, at a distance of more than a billion light-years, the BL Lacertae object
PKS 2155-304 radiates profusely across the spectrum from the visible to hard X-rays.
Indeed, it is classified as a BL Lacertae object
- a starlike point that is probably a hot jet from near a black hole that we see almost end on.
Although there are several classes of AGNs, only three will be addressed here: Seyfert galaxies, BL Lacertae objects
(BLL), and quasars--see the website above for a summary of their characteristics.
Gravitational lensing may explain, for example, the puzzling, quasar-like properties of so-called BL Lacertae objects
sometimes found embedded in nearby galaxies.
I crossed-checked this with Wolfgang Steinicke's Catalogue of Bright Quasars and BL Lacertae Objects
and investigated problematic entries with the SIMBAD Astronomical Database and the NASA/IPAC Extragalactic Database.