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Results of optical identification of the ASCA Lynx Deep Survey are presented. Six X-ray sources are detected in the 2-7 keV band using the Solid-State Imaging Spectrometer in a ~20' x 20' field of view with fluxes larger than ~4 x 10-14 ergs s-1 cm-2 in the band. Follow-up optical spectroscopic observations were made, and five out of six sources are identified with active galactic nuclei/quasi-stellar objects (AGNs/QSOs) at redshifts of 0.5-1.3. We also identify two more additional X-ray sources detected in a soft X-ray band with AGNs/QSOs. We find that three QSOs identified are located at z ~ 1.3. Two rich clusters and several groups of galaxies are also placed at the same redshift in the surveyed field, and projected separations between the QSOs and the clusters are 3-8 Mpc at the redshift.

PKS 2155-304, the brightest BL Lac object in the ultraviolet sky, was monitored with the International Ultraviolet Explorer satellite at ~1 hr time resolution for 10 nearly uninterrupted days in 1994 May. The campaign, which was coordinated with Extreme Ultraviolet Explorer, ROSAT, and ASCA monitoring, along with optical and radio observations from the ground, yielded the largest set of spectra and the richest short-timescale variability information ever gathered for a blazar at UV wavelengths. The source flared dramatically during the first day, with an increase by a factor of ~2.2 in an hour and a half. In subsequent days, the flux maintained a nearly constant level for ~5 days, then flared with ~35% amplitude for 2 days. The same variability was seen in both short- and long-wavelength IUE light curves, with zero formal lag ([?]2 hr), except during the rapid initial flare, when the variations were not resolved. Spectral index variations were small and not clearly correlated with flux. The flux variability observed in the present monitoring is so rapid that, for the first time, based on the UV emission alone, the traditional DL/Dt limit indicating relativistic beaming is exceeded. The most rapid variations, under the likely assumption of synchrotron radiation, lead to a lower limit of 1 G on the magnetic field strength in the UV-emitting region. These results are compared with earlier intensive monitoring of PKS 2155-304 with IUE in 1991 November, when the UV flux variations had completely different characteristics.

In 1994 May, the BL Lac object PKS 2155-304 was observed continuously for ~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 telescopes. The light curves show a well-defined X-ray flare followed by a broader, lower amplitude extreme-ultraviolet flare ~1 day later and a broad, low-amplitude UV flare ~2 days later. X-ray fluxes obtained at three well-separated times the preceding week indicate at least one previous flare of comparable amplitude or perhaps ongoing stochastic X-ray variations, and additional rapid variability was seen at the beginning of the IUE observation, when extremely sharp changes in UV flux occurred. The X-ray flux observed with ASCA flared by a factor of ~2 in about half a day and decayed roughly as fast. In contrast, the subsequent UV flare had an amplitude of only ~35% and lasted longer than 2 days. Assuming that the X-ray, EUV, and UV events are associated, the lags, the decrease of amplitude with wavelength, and the broadening of the temporal profile with wavelength are all qualitatively as expected for synchrotron emission from an inhomogeneous, relativistic jet. Because of the high quality of the data, 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 if there was an instantaneous (t [?] hours) injection of high-energy electrons that emit first at X-ray energies. Alternatively, the data are consistent with a compression wave or other disturbance crossing a region with stratified particle energy distributions. This kind 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 amplitude was wavelength independent and the UV lagged the X-rays by less than ~3 hr. This means that the origin of rapid multiwavelength variability in this blazar is complex, involving at least two different modes.