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Astron. Astrophys. 356, 1-10 (2000)

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4. Optical observations

4.1. Observations and data reduction

The optical data were obtained using the Dutch 0.9 m ESO telescope at La Silla, Chile, between May 17 and 27 1996. The telescope was equipped with a TEK CCD 512[FORMULA]512 pixels detector and Bessel BVR filters were used for the observations. The pixel size is 27 µm and the projected pixel size in the plane of the sky is 0.442 arcsec, providing a field of view of 3´.77 [FORMULA] 3´.77.

The original frames were flat fielded and bias corrected using MIDAS package and photometry was performed using the Robin procedure, developed at the Torino Observatory, Italy, by L. Lanteri. This procedure fits the PSF with a circular gaussian and evaluates the background level by fitting it with a 1st order polynomial surface. The magnitude of the object and the error are derived by comparison with reference stars in the same field of view. The typical photometric error is [FORMULA] mag in all bands.

4.2. Results

The light curves (Table 7 and Fig. 4) show an increase of luminosity of about 20% ([FORMULA]-0.25 mag), between the starting low level of May 17-18 and the maximum of May 24. The flux is then decreasing during the last two days. The behavior is very similar in all of the three filters.

[FIGURE] Fig. 4. Optical light curves of PKS 2155-304


Table 7. RVB light curves.

Assuming that the optical spectrum is described by a power law, we calculated the mean spectral indices using the simultaneous pairs data of the light curves. The results are [FORMULA] and [FORMULA] and indicate that the optical spectrum is steeper than the IR one.

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© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000