## 3. ISO results## 3.1. The light curvesThe data and the corresponding light curves at 4.0 (SW5 filter),
14.3 (LW3), 60 (C1_60) and 90
When the purpose is to verify whether the flux is variable, the contribution of the pixel responsitivity to the absolute error can be neglected and a smaller uncertainty can be associated to the relative flux values of the light curves. However, this can be done only for the two light curves of the photometer (see Table 5), due to the way the photometric error was determined.
The relative errors on the flux are, in any case, quite large,
about 10 - 12% for the camera observations and from 20 to more than
50% for the photometer (see Appendix A). Within these uncertainties
the light curves show no evidence of variability. To quantify this
statement, we fitted the light curves with a constant term and the
reduced chi-square values were computed in order to test the goodness
of the fits. We first fitted the values of the best sampled period,
from 1996 May 13 to May 27. The results are
mJy at 4.0 However, the large uncertainty on the flux can hide smaller
variations. We calculated the mean relative error and obtained 3 sigma
limits for the lowest detectable variations of 32%, 36%, 76% and 132%
at 4.0, 14.3, 60 and 90 ## 3.2. The infrared spectrumThe infrared spectral shape of PKS 2155-304 was sampled, using 16
filters, from 2.8 to 170
In Fig. 3 it is also shown the result of a power law fit, that gives an energy spectral index of . The lower and upper limits were not considered in the fit; the reduced chi-square is , with 9 d.o.f., that gives a confidence level of 77.4%. From each simultaneous pairs of flux values of the SW5 and LW3
light curves, we obtained the spectral indices between 4.0 and 14.3
The mean value is , which is fully consistent with the index derived using 11 filters on a larger IR band. The fit with a constant term of the spectral indices
vs. time has a reduced chi-square of
0.26, with 13 d.o.f., which corresponds to a confidence level of less
than 1%. This indicates that the source showed no spectral variability
in the 4.0 - 14.3 © European Southern Observatory (ESO) 2000 Online publication: March 28, 2000 |