close to the line of sight, can be very faint and difficult to observe Doppler factor a mathematical expression which measures the strength (or weakness) of relativistic effects in AGN, including beaming, based on the jet speed and its angle to the line of sight with Earth flat spectrum a term for a non-thermal spectrum that emits a great deal of energy at the higher frequencies when compared to the lower frequencies intrinsic luminosity the luminosity from the jet in the rest frame of the jet jet (often termed 'relativistic jet') a high velocity (close to c) stream of plasma emanating from the polar direction of an AGN observed luminosity the luminosity from the jet in the rest frame of Earth spectral index a measure of how a non-thermal spectrum changes with frequency. The smaller α is, the more significant the energy at higher frequencies is. Typically α is in the range of 0 to 2. steep spectrum a term for a non-thermal spectrum that emits little energy at the higher frequencies when compared to the lower frequencies Physical quantities angle to the line-of-sight with Earth jet speed intrinsic luminosity (sometimes called emitted luminosity) observed Luminosity spectral index where Speed of light m/s Mathematical expressions relativistic beta Lorentz factor Doppler factor See also Astrophysical jet List of plasma physics articles Relativistic Doppler effect Relativistic particle Relativistic plasma Relativistic similarity parameter Relativistic wave equations References External links Detailed explanation of relativistic aberration Astrophysics