Radio-loud flares from microquasars and radio-loudness of quasars

The low-frequency power spectra of the X-ray and radio emission from four microquasars suggest that two distinct modes of energy output are at work: (i) the 'coupled' mode in which the X-ray and radio luminosities are closely coupled and vary only weakly, and (ii) the 'flaring' mode, which dramatically boosts the radio luminosity but makes no impact on the X-ray luminosity. The systems are in the flaring mode only a few per cent of the time. However, flares completely dominate the power spectrum of radio emission, with the consequence that sources in which the flaring mode occurs, such as GRS 1915+105 and Cyg X-3, have radio power spectra that lie more than an order of magnitude above the corresponding X-ray power spectra. Of the four microquasars; for which we have examined data, in only one, Cyg X-1, is the flaring mode seemingly inactive. While Cyg X-1 is a black hole candidate, one of the three flaring sources, Sco X-1, is a neutron star. Consequently, it is likely that both modes are driven by the accretion disc rather than black hole spin. Radio imaging strongly suggests that the flaring mode involves relativistic jets. A typical microquasar is in the flaring mode a few per cent of the time, which is similar to the fraction of quasars that are radio-loud. Thus there may be no essential difference between radio-loud and radio-quiet quasars; radio-loudness may simply be a function of the epoch at which the source is observed.