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Re: [vsnet-chat 6125] U Gem flickering > Last night I observed U Gem "flickering". It changed magnitudes wildly > between 13.3 and 13.9 in a matter of seconds. All other stars in the field > were steady, it was a very fine night. This is the second time I recall > seeing this phenomenon. > > What is the most plausible explanation for such variation in this short time > scale? These phenomena are usually referred to as "flickering", which is almost always present in CVs. Once some phenomenon is identified as flickering, there is a prevalent tendency (at least in some community in the professional world) to simply ignore them as "silly flickering", if I correctly remember the wording what a some astronomer used. This is because flickering plays a simple role of "noise" when one wish to determine the periodicity. There is also a concept of "flickering and noise". However, the it has become more evident that flickering plays a very important role in identifying the physical process in the accretion disk. In X-ray binaries, particularly in black-hole binaries, the "flickering" (in X-rays) component is known to show a power law dependence of amplitudes versus frequency. This means that the flickering amplitude (in average) gets smaller in high frequency (i.e. shorter time scale). There is often a "break" in the power law (the slope of the amplitude(power) vs. frequency; all need to be in logarithmic scale), where an increase in power (amplitude) is frequently seen. This increase in power is referred to as quasi-periodic oscillations (QPOs), which are also seen in CVs. From these observations, there has been an idea of "self-organized criticality" (SOC), in which a superposition of "shots" in a self-organized critical state (this itself is an ubiquitous and interesting phenomenon, and search the web for a good representative reference; there was a good review article in Scientific American), which was originally introduced to explain time variations (see e.g. Mineshige et al. 1994, PASJ 46, 97; Takeuchi et al. 1995, PASJ 47, 617). The same explanation was extended to explain the CV flickering (Yonehara et al. 1997, ApJ 486, 388). In CVs, Bruch made a series of pioneering study (Bruch 1992, A&A 266, 237; 1996, A&A 312, 97; 2000, A&A 359, 988), which are confirmed to be phenomenologically in line with X-ray binaries (the history of CV flickering was actually old -- back to the 1940's -- but had not received sufficient attention under the influence of a well-known paradigm of stream impact-type variation since the 1970's until very recent years). The flickering phenomenon now receives more attention in view of the modern advance of magnetic instabilities in the accretion disk. (See a further historical summary of flickering in Kato et al. 2002, PASJ 54, 1033, which is also available in astro-ph or as a VSNET preprint; regarding optical variations of black-hole binaries, the phenomenon in V4641 Sgr last year is still fresh to our mind -- see Uemura et al. 2002, PASJ 54, L79 "Rapid Optical Fluctuations in the Black Hole Binary V4641 Sagittarii", which is also available in astro-ph or as a VSNET preprint and which clearly demonsrates the power-law dependence). The field of X-ray binary/CV flickering is still a novel field with respeect to its physical interpretation, and is now in progress! There might be no explanation or answer to your question at present, but your observation, possibly in relation to the outburst phase, may have recorded a new fundamental aspect of CV variability. Regards, Taichi Kato
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