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Astron. Astrophys. 362, 105-112 (2000)

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3. Persistence of the variations

In Fig. 7 we overlaid the light curves measured in November 1991 (open circles) and in November 1996 (open squares). The only adjustments made are a shift of the time axis by -11 days for the November 1996 data and the conversion of the PSPC count rates to equivalent HRI count rates according to the conversion factor as described in Sect. 2.2. The errors of the PSPC data are generally smaller than the symbol sizes.

[FIGURE] Fig. 7. Overlay of the light-curves of the November 1991 observation (open circles) and the November 1996 observation (open squares). The time axis of the Nov. 96 data has been shifted by -11 days, the Nov 91 PSPC data have been converted to equivalent HRI count rates.

There is a striking correspondence between these two data sets, taken five years apart. Both times the source was obviously in a very similar intensity state, the shots occured with nearly exactly the same temporal separations, even the shapes of the shots (or the modulation) seem to have changed only slightly.

Although a chance coincidence of the occurrence of the shots cannot be ruled out, given the short data stretches, the persistence of the phenomenon over several years indicates that the intensity variations in PKS 2155-304 do not appear randomly but in a rather well defined pattern. Whether this is related to some underlying periodicity in the system cannot be judged from the short data stretches available.

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© European Southern Observatory (ESO) 2000

Online publication: October 30, 19100