[4] It is a blazar or BL Lac object, which is an active galactic nucleus with a jet that is shooting towards the Earth.

[5] Mrk 501 is an extremely variable source of gamma rays, undergoing violent outbursts.

[5] During an outburst in 1997, it was the brightest object in the sky in the very-high-energy gamma ray region of the spectrum, at energies above 1011 eV (100 GeV).

[7] It was first determined to be a very high energy gamma ray emitter in 1996 by John Quinn at the Whipple Observatory.

[10] This delay has led to various theories, including that space is bigger at small dimensions with a foamy quantum texture.

From the core an extremely high-speed blast of plasma emerges in a narrow cone shape as a one-sided jet.

This implies that the jet is relativistic with Γ about 15 (that is, the plasma is moving at 99.8% of the speed of light) and at an angle between 15° and 25° from the line of sight from the Earth.

[16] In March 2022, scientists led by Ioannis Liodakis studied Markarian 501 during an average state while discerning how blazars make such a bright light using Imaging X-ray Polarimetry Explorer (IXPE).

The researchers were "able to show that the particles in these jets are supercharged by shock fronts, resolving a longstanding 'unanswered question' about the dynamics of these brilliant objects.