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Astron. Astrophys. 360, L47-L50 (2000)

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Letter to the Editor

Extreme intranight variability in the BL Lacertae object AO 0235+164

G.E. Romero **  1, S.A. Cellone **  2 and J.A. Combi ** *,  1

1 Instituto Argentino de Radioastronomía, C.C.5, (1894) Villa Elisa, Bs. As., Argentina
2 Facultad de Ciencias Astronómicas y Geofísicas, UNLP, Paseo del Bosque, 1900 La Plata, Argentina

Received 17 July 2000 / Accepted 27 July 2000


We present results of two-colour photometry with high time resolution of the violently variable BL Lac object AO 0235+164. We have found extreme intranight variability with amplitudes of [FORMULA]% over time scales of 24 hours. Changes of 0.5 magnitudes in both R and V bands were measured within a single night, and variations up to 1.2 magnitudes occurred from night to night. A complete outburst with an amplitude [FORMULA]% was observed during one of the nights, while the spectrum remained unchanged. This seems to support an origin based on a thin relativistic shock propagating in such a way that it changes the viewing angle, as recently suggested by Kraus et al. (1999) and Qian et al. (2000).

Key words: galaxies: BL Lacertae objects: individual: AO 0235+164 – galaxies: photometry – radiation mechanisms: non-thermal

* Member of CONICET
** Visiting Astronomer, Complejo Astronómico El Leoncito, operated under agreement between CONICET and the National Universities of La Plata, Córdoba, and San Juan.

Send offprint requests to: G.E. Romero

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© European Southern Observatory (ESO) 2000

Online publication: August 23, 2000