Issue |
Astron. Astrophys. Suppl. Ser.
Volume 135, Number 3, March II 1999
|
|
---|---|---|
Page(s) | 477 - 486 | |
DOI | https://doi.org/10.1051/aas:1999184 | |
Published online | 15 March 1999 |
Optical microvariability of southern AGNs*
1
Instituto Argentino de Radioastronomía, C.C.5, (1894) Villa Elisa, Bs. As., Argentina
2
Departamento de Física, UNLP, C.C. 67, 1900 La Plata, Argentina
3
Facultad de Ciencias Astronómicas y Geofísicas, UNLP, Paseo del Bosque, 1900 La Plata, Argentina
Send offprint request to: G.E. Romero, first address.
Received:
6
August
1998
Accepted:
9
November
1998
We present results of a search for optical microvariability in a selected
sample of 23 southern AGNs, which includes both radio-quiet and radio-loud
objects. Microvariations were clearly detected in 60% of the radio-loud
sources, with amplitudes from 2.2% up to 8% within a single night.
Night-to-night variability with amplitudes of % was also observed in
the BL Lac object 0537-441. On the contrary, no rapid variability was
detected at all in 8 radio-quiet quasars. We have used microvariability
data previously reported for northern objects
along with our new results for southern AGNs in order to estimate duty
cycles of each class of objects from the largest possible sample.
Most of the microvariability in radio-loud objects could be originating
in interactions between relativistic shocks and features in the inner jets,
although contributions from superluminal microlensing and accretion disk
instabilities can be present in some sources. It is possible that the latter
phenomenon is responsible for the microvariability observed in northern
radio-quiet quasars. We suggest that the difference in the microvariability
behaviour of radio and X-ray selected BL Lacs could be due to the effect
of stronger magnetic fields in the latter group of objects, fields that can
prevent the formation of features like density inhomogeneities and bends
in the base of the jets by Kelvin-Helmholtz macroscopic instabilities.
Key words: galaxies: active / BL Lacertae objects: general / quasars: general / galaxies: photometry
© European Southern Observatory (ESO), 1999