Microvariabilidad óptica de núcleos activos australes

ROMERO, G. E.; CELLONE, S. A.; COMBI, J. A.

Gonnet

Congreso; 42a. Reunión Anual AAA; 1998

AAA

We present results of a search for optical microvariability in aselected sample of 23 southern AGNs, which includes both radio-quiet andradio-loud objects. Microvariations were clearly detected in 60 % of theradio-loud sources, with amplitudes from 2.2 % up to 8 % within a singlenight. Night-to-night variability with amplitudes of ~20 % was alsoobserved in the BL Lac object 0537-441. On the contrary, no rapidvariability was detected at all in 8 radio-quiet quasars. We have usedmicrovariability data previously reported for northern objects alongwith our new results for southern AGNs in order to estimate duty cyclesof each class of objects from the largest possible sample. The dutycycles obtained for radio-loud, radio-quiet, and X-ray selected objectsare 68 %, 6.9 %, and 27.9 %, respectively. Most of the microvariabilityin radio-loud objects could be originating in interactions betweenrelativistic shocks and features in the inner jets, althoughcontributions from superluminal microlensing and accretion diskinstabilities can be present in some sources. It is possible that thelatter phenomenon is responsible for the microvariability observed innorthern radio-quiet quasars. We suggest that the difference in themicrovariability behaviour of radio and X-ray selected BL Lacs could bedue to the effect of stronger magnetic fields in the latter group ofobjects, fields that can prevent the formation of features like densityinhomogeneities and bends in the base of the jets by Kelvin-Helmholtzmacroscopic instabilities.