However, if the energy is sufficiently great it can be seen indirectly with sensitive telescopes located on mountain tops. Markarian 421 is an elliptical galaxy some 400 million light- years from Earth. All models of these have a black hole at its center that's eating nearby material to get its energy, and then it spews material out into intergalactic space in the form of a jet. A jet of material is emanating from each of the two poles of Markarian 421. The axis of one of the jets is pointing in our direction.
Most astronomical objects give off thermal radiation.
This is extremely non-thermal. Theorists think that these very high-energy gamma rays are produced in the interaction of cosmic particles at even greater energy with ambient particles or photons in these jets. The cosmic particles may be electrons or protons and they must be accelerated by a process derived from the enormous gravity of a black hole. The energies they achieve are greater than those attained by human-built atom smashers, like at Fermilab near Chicago. If the primary particles are protons, their energies must be millions of times greater than the observed very high-energy gamma rays.