OPTICAL-IUE OBSERVATIONS OF THE GAMMA-RAY LOUD BL-LACERTAE OBJECT S5-0716- DATA AND INTERPRETATION(714 )

We monitored the BL Lac object S5 0716+714 in the optical band during the period November 1994 - April 1995, which includes the time of a ga mma-ray observation by the Energetic Gamma Ray Experiment Telescope (E GRET) on February 14-28, 1995. The light curves in the R and B bands s how fast fluctuations superimposed on longer timescale variations. The color index correlates with intensity during the rapid flares (the sp ectrum is flatter when the flux is higher), but it is rather insensiti ve to the long term trends. Over the 5 month observational period the light curve shows an overall brightening of about 1 mag followed by a fast decline. The EGRET pointing covers part of the very bright phase (V similar to 13.2) and the initial decline. An ultraviolet spectrum w as also obtained with the International Ultraviolet Explorer (IUE) (12 00-3000 Angstrom) during the EGRET observations. The variability of th e optical emission of S5 0716+714 by itself sets important constraints on the magnetic field strength and on the physical processes responsi ble for it. Interpreting the whole electromagnetic spectrum with synch rotron self Compton models leads to the prediction of a bright gamma-r ay state during the EGRET pointing. We discuss how the gamma-ray data could be used as a diagnostic of the proposed models.(1).