In recent decades the scope of observational astronomy has extended over the entire electromagnetic spectrum, and now ranges from radio waves to high-energy photons with teraelectronvolt (1 TeV = 10¹² eV) energies. The different bands reveal different aspects of the universe. In the x-ray domain, at photon energies between 0.1 and 500 keV, thermal radiation from high-temperature plasmas or nonthermal radiation produced by relativistic electrons interacting with magnetic fields (synchrotron radiation) or intense photon fields (the inverse Compton effect) is observed.