‘Radio-loud’ quasars – which are brighter at radio wavelengths than in the optical range – emit two narrow beams of radiation that emerge from opposite edges of the quasar. Blazars are a type of radio-loud quasar that periodically send violent pulses of radiowaves into space and were originally defined by the ‘synchrotron’ radiation that they emit when electrons are accelerated in a magnetic field. But astronomers proposed five years ago that this characteristic spectrum is only seen if the beams of radiowaves are viewed end-on. This would mean that many blazars had gone undetected. As the angle of observation increases away from the beam, the synchrotron profile weakens, and the spectrum changes further as the angle grows.

Ma and Wills compared two sets of spectra of 62 quasars – one gathered between 1998 and 2000, and the other ten years earlier. Some of the recent spectra exhibited emission lines over 20% stronger than those in the corresponding older spectra. These jumps in intensity occurred over the same timescale as the peaks in brightness seen in blazars. This suggests that many of the objects that astronomers thought were quasars are actually blazars. “Our observations offer support for the unified scheme for radio-loud quasars”, claim Ma and Wills. “The emission line variations provide the most direct evidence for the existence of violent blazar outbursts in every radio-loud quasar”.

Distant quasars are also among the objects catalogued by the ambitious Sloan Digital Sky Survey based at Pasadena in California. Project scientists have just released a huge instalment of data containing redshift information for 50 000 galaxies and 500 quasars. The survey aims to build the biggest digital map of the sky and the extensive archive – which has imaged both ‘local’ and remote bodies from the 2.5-metre Sloan telescope in New Mexico – will be available to the astronomical community for further study.