The incidence of short time scale variability on different types of Blazars

ANDRUCHOW, I.; CELLONE, S. A.; ROMERO, G. E.

Florianópolis

Congreso; XIV Latin American Regional IAU Meeting; 2013

IAU

Blazars constitute the most extreme subclass of active galactic nuclei (AGN). It is generally accepted that their extreme behaviour is due to relativistic beaming of radiation from a jet pointing close to the observer´s direction. The jet thus dominates the electromagnetic emission of these objects along the whole spectrum, from radio to gamma-rays. In particular. the optical flux is synchrotron radiation from relativistic electrons, while high-energy (X through gamma, up to TeV energies) emission is thought to originate from inverse-Compton scattering of seed photons by the relativistic electrons (or, alternatively, hadrons) in the jet. Optical variability studies are thus useful to look for a link between the emission at different frequencies, especially when simultaneous or quasi-simultaneous observations are possible. In particular, optical polarization observations provide information on the properties of the magnetic field. A variable polarized flux component may be thus tracing either changes in the orientation of the jet, or in the ordering of the magnetic field. We present recent results from intensive optical photopolarimetric monitoring of a sample of blazars that have been detected at TeV energies. Since these are relatively nearby objects (z<0.1) -if not, TeV gamma-rays would be absorbed by the extragalactic background light (EBL)- their host galaxies are relatively bright and spatially resolved. We address particularly the depolarizing effect introduced by the host galaxies, as well as spurious changes in the polarimetric light-curves when seeing changes and hence the relative contribution of the AGN and the host galaxy within the photometric aperture changes consequently. On the other hand, since the sub-class of BL Lac objects (by definition) has optical spectra with very faint or even absent emission lines, reliable redshift determinations are still lacking for some objects. This prevents the determinations of their intrinsic properties (luminosity, variability time-scale, etc.). We also present results from a spectro-photometric study, using Gemini-GMOS, of a selected sample of BL Lacs, their host galaxies and galaxies in the field. Our aim is to provide firm limits to their redshifts as well as to characterize their immediate environment.