THE OPTICAL BULLETS OF SS-433 AS A RADIATIVE INSTABILITY PHENOMENON

The fact that the transient 'bullet' phenomenon and major variability seen in the optical spectrum of SS433 are essentially absent in X-ray data from the 'inner' jets seems to preclude their being 'true' bullet s - i.e. comprising intermittent large mass ejections. We propose inst ead that they are a radiative instability phenomenon occurring when mi nor variations in the jet filling factor (or other parameters such as jet mass loss rate or opening angle) take the jet through a critical t hreshold only below which can jet cooling to optical temperatures occu r. At other times the jet remains hot until cooled solely by expansion , by which time the optical emission measure is small. The effect is a nalysed for the case where the jets are heated by interaction with the stellar wind from the companion star. The parameter criticality of th is threshold could also help explain the rarity of detected SS433s.