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Issue A&A Volume 390, Number 2, August I 2002 Page(s) 431 - 438 Section Extragalactic astronomy DOI 10.1051/0004-6361:20020743

A&A 390, 431-438 (2002)
DOI: 10.1051/0004-6361:20020743

Optical microvariability of EGRET blazars

G. E. Romero¹, S. A. Cellone², J. A. Combi¹ and I. Andruchow<sup>1

1</sup>  Instituto Argentino de Radioastronomía, C.C.5, (1894) Villa Elisa, Buenos Aires, Argentina
²  Facultad de Ciencias Astronómicas y Geofísicas, UNLP, Paseo del Bosque, 1900 La Plata, Buenos Aires, Argentina

(Received 15 January 2002 / Accepted 13 May 2002)

Abstract
We present results of a photometric CCD study of the incidence of microvariability in the optical emission of a sample of 20 blazars detected at gamma-ray energies by the EGRET instrument of the Compton Gamma-Ray Observatory. We have observed strong outbursts in some sources, but many others displayed no significant variability on timescales of hours. The typical minimum timescale is found to be of ~several hours, not tens of minutes as claimed by some authors. The duty cycle for optical intranight microvariations of gamma-ray blazars, as estimated from our observations, seems to be ~50%, lower than what is usually assumed. For night-to-night variations, instead, the duty cycle approaches that observed in radio-selected BL Lacs and flat-spectrum radio quasars (i.e. ~70%).

Key words: galaxies: active -- galaxies: photometry -- BL Lacertae objects: general -- gamma-rays: observations

Offprint request: S. A. Cellone, scellone@fcaglp.unlp.edu.ar

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