Markarian 421 (Mrk 421, Mkn 421) is a blazar located in the constellation Ursa Major.

It is one of the closest blazars to Earth, making it one of the brightest quasars in the night sky.

It is suspected to have a supermassive black hole (SMBH) at its center due to its active nature.

It was first determined to be a very high energy gamma ray emitter in 1992 by M. Punch at the Whipple Observatory,[5] and an extremely rapid outburst in very-high-energy gamma rays (15-minute rise-time) was measured in 1996 by J. Gaidos at Whipple Observatory.

[6] Markarian 421 also had an outburst in 2001 and is monitored by the Whole Earth Blazar Telescope project.