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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.
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.
The passage of stars across the line of sight to the quasar could account for the rapid brightness changes typically associated with BL Lacertae objects.
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.