Active Galactic Nuclei and Pulsars

With the discovery of radio emission of extraterrestrial origin, it had opened a new and broader window in the electromagnetic spectrum to observe the sky. Two of the common sources of radio emissions are active galactic nuclei (AGN) and pulsars. AGN are very luminous at radio wavelength and are powered by the accretion disk surrounding supermassive black holes at the center. AGN are useful as an alternative standard ruler to determine cosmological parameters. Pulsars, on the other hand, is a type of neutron star that is highly magnetized and rotates at enormous speed. This cosmic lighthouse produces a very precise pulsation period that can be used as cosmic clock.