Ώρα 12:00 μ. Αστεροσκοπείο
Περίληψη:
he blazar emission - attributed to incoherent synchrotron processes
involving relativistic electrons - is expected to be highly polarised
especially in the optical bands. The specific conditions required for
the emergence of polarisation, the emission and radiative transfer
processes subsequently make the polarisation parameters and their
dynamics, unique probes of the microphysics of the emitting plasma as
well as the conditions and the configuration of the magnetic field at
the emission site. The recent discovery of rotations of the polarisation
angle seen associated with high energy flares observed in the GeV
energy bands, opened up a new field dealing with the nature of the
emission and variability mechanisms, the configuration of the magnetic
field and high energy emission site etc. The degree of difficulty
involved, has limited the polarisation studies - and especially
monitoring - to hand-picked cases rather than unbiased samples
forbidding systematic population studies. In order to lay the ground for
systematic and extensive polarimetric studies we have concieved,
designed and constructed a novel-design optical polarimeter that has
already been commissioned at the Skinakas telescope in Crete. The
program aims at monitoring the linear polarisation for a sample of
almost 100 gamma-ray loud sources that comprise an unbiased set and 15
gamma-ray quiet "comparison sample sources'' with a duty cycle of close
to 3 days for non active sources and for a fraction of a night for cases
in active state. The achieved precision in the polarisation degree
reaches a fraction of the percentage for sources of 18-th magnitude.
Here we present: (a) the scientific motivation, (b) the design of the
instrument, (c) the sample selection and (d) the results from the first
season of operation. We show that while the fractional polarisation for
both gamma-ray--loud and gamma-ray--quiet sources are well-described by
exponential distributions, the two classes have different optical
polarisation properties. This is the first time this statistical
difference is demonstrated in optical wavelengths.