Broad-band optical observations of the extraordinarily bright optical afterglow of the intense gamma-ray burst GRB 991208 started ∼ 2.1 days after the event and continued until 4 Apr 2000. The flux decay constant of the optical afterglow in the R-band is −2.30 ± 0.07 up to ∼ 5 days, which Send offprint requests to: A. J. Castro-Tirado (ajct@laeff.esa.es)is very likely due to the jet effect, and after that it is followed by a much steeper decay with constant −3.2 ± 0.2, the fastest one ever seen in a GRB optical afterglow. A negative detection in several all-sky films taken simultaneously to the event, that otherwise would have reached naked eye brightness, implies either a previous additional break prior to ∼ 2 days after the 2 A. J. Castro-Tirado et al.:The extraordinarily bright optical afterglow of GRB 991208.

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Abstract. Optical polarization of seven selected BL Lac objects in UBV RI bands was studied with the Nordic Optical Telescope from December 10-14, 1999. Two of them, 3C 66A and PKS 0735+178, were monitored for 4 nights for a total integration time of 4.75 and 5.5 hours, respectively. Other objects (1 Jy 0138−097, H 0414+009, PKS 0823−223, OJ287 and BL Lac) were observed sparsely during the run. Apart from PKS 0823−223 (more polarized than observed in the past), the sources show levels of flux and polarization consistent with results at previous epochs. 3C 66A and PKS 0735+178 were intensively observed during December 11 and 12 and exhibited variability of polarization, both on internight and intranight time scales. Wavelength dependence of polarization has been investigated, as well as circular polarization. The results are discussed within the standard model for BL Lacs.

The SIMBIO-SYS (Spectrometer and Imaging for MPO BepiColombo Integrated Observatory SYStem) is a complex instrument suite part of the scientific payload of the Mercury Planetary Orbiter for the BepiColombo mission, the last of the cornerstone missions of the European Space Agency (ESA) Horizon + science program. The SIMBIO-SYS instrument will provide all the science imaging capability of the Bepi-Colombo MPO spacecraft. It consists of three channels: the STereo imaging Channel (STC), with a broad spectral band in the 400-950 nm range and medium spatial resolution (at best 58 m/px), that will provide Digital Terrain Model of the entire surface of the planet with an accuracy better than 80 m; the High Resolution Imaging Channel (HRIC), with broad spectral bands in the 400-900 nm range and high spatial resolution (at best 6 m/px), that will pro-The BepiColombo mission to Mercury Edited by Johannes Benkhoff, Go Murakami and Ayako Matsuoka B G. Cremonese

The polarized and total Ñux of the BL Lac object PKS 2155[304 were monitored intensively and simultaneously in the optical UBV RI bands with the Turin photopolarimeter at the CASLEO 2.15 m telescope during four campaigns in 1998 June, August, and November and 1999 August. The e †ective observation time amounted to D47 hr. PKS 2155[304 showed a linear polarization percentage (P) usually ranging between 3% and 7% and a polarization position angle (P.A.) mainly between 70¡ and 120¡. The highest temporal resolution of our observations, 15 minutes, is unprecedented for polarimetric monitoring of this source and has allowed us to detect amplitude variations of the linear polarization percentage from 6% to 7.5% in timescales of hours. In some nights the polarization percentage seems to increase toward shorter wavelengths ; however, the polarized spectrum does not vary signiÐcantly with time. The most remarkable variability event occurred on 1998 June 18, when the degree of linear polarization decreased by more than a factor 2 in 1 day in all bands, while the P.A. rotated by 90¡. This is consistent with the presence of two emission components, of di †erent polarization degree and position angle. Intranight variability of P and P.A. can be interpreted with small-amplitude physical or geometrical changes within the jet. Measurements of the circular polarization over time intervals of days set upper limits of 0.2%. Simultaneous photometry taken with the Turin Photopolarimeter and with a CCD camera at Astronomical Observatory did not show light variations correlated with those of the Co rdoba linearly polarized Ñux.