MULTIWAVELENGTH MONITORING OF THE BL LACERTAE OBJECT PKS-2155-304 IN 1994 MAY .3. PROBING THE INNER JET THROUGH MULTIWAVELENGTH CORRELATIONS

In 1994 May, the BL Lac object PKS 2155-304 was observed continuously for similar to 10 days with the International Ultraviolet Explorer and the Extreme Ultraviolet Explorer and for 2 days with ASCA, as well as with ROSAT and with ground-based radio, infrared, and optical telesco pes. The light curves show a well-defined X-ray flare followed by a br oader, lower amplitude extreme-ultraviolet flare similar to 1 day late r and a broad, low-amplitude UV flare similar to 2 days later. X-ray f luxes obtained at three well-separated times the preceding week indica te at least one previous flare of comparable amplitude or perhaps ongo ing stochastic X-ray variations, and additional rapid variability was seen at the beginning of the IUE observation, when extremely sharp cha nges in UV flux occurred. The X-ray flux observed with ASCA flared by a factor of similar to 2 in about half a day and decayed roughly as fa st. In contrast, the subsequent UV flare had an amplitude of only simi lar to 35% and lasted longer than 2 days. Assuming that the X-ray, EUV , and UV events are associated, the lags, the decrease of amplitude wi th wavelength, and the broadening of the temporal profile with wavelen gth are all qualitatively as expected for synchrotron emission from an inhomogeneous, relativistic jet. Because of the high quality of the d ata, we can rule out that the observed flares were caused by either a Fermi-type shock acceleration event or a pair cascade in a homogeneous synchrotron-emitting region. A homogeneous region is still possible i f there was an instantaneous (t much less than hours) injection of hig h-energy electrons that emit first at X-ray energies. Alternatively, t he data are consistent with a compression wave or other disturbance cr ossing a region with stratified particle energy distributions. This ki nd of situation is expected to occur behind a shock front and/or in an inhomogeneous jet. The present light curves are in sharp contrast to the multiwavelength variability observed in 1991 November, when the am plitude was wavelength independent and the UV lagged the X-rays by les s than similar to 3 hr. This means that the origin of rapid multiwavel ength variability in this blazar is complex, involving at least two di fferent modes.