The low-frequency power spectra of the X-ray and radio emission from fourmicroquasars suggest that two distinct modes of energy output are at work: (i)the `coupled' mode in which the X-ray and radio luminosities are closelycoupled and vary only weakly, and (ii) the `flaring' mode, which dramaticallyboosts the radio luminosity but makes no impact on the X-ray luminosity. Thesystems are in the flaring mode only a few percent of the time. However, flarescompletely dominate the power spectrum of radio emission, with the consequencethat sources in which the flaring mode occurs, such as GRS 1915+105 and CygX-3, have radio power spectra that lie more than an order of magnitude abovethe corresponding X-ray power spectra. Of the four microquasars for which wehave examined data, in only one, Cyg X-1, is the flaring mode seeminglyinactive. While Cyg X-1 is a black-hole candidate, one of the three flaringsources, Sco X-1, is a neutron star. Consequently, it is likely that both modesare driven by the accretion disk rather than black-hole spin. Radio imagingstrongly suggests that the flaring mode involves relativistic jets. A typicalmicroquasar is in the flaring mode a few percent of the time, which is similarto the fraction of quasars that are radio loud. Thus there may be no essentialdifference between radio-loud and radio-quiet quasars; radio loudness maysimply be a function of the epoch at which the source is observed.
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