Bibcode
Villata, M.; Raiteri, C. M.; Kurtanidze, O. M.; Nikolashvili, M. G.; Ibrahimov, M. A.; Papadakis, I. E.; Tosti, G.; Hroch, F.; Takalo, L. O.; Sillanpää, A.; Hagen-Thorn, V. A.; Larionov, V. M.; Schwartz, R. D.; Basler, J.; Brown, L. F.; Balonek, T. J.; Benítez, E.; Ramírez, A.; Sadun, A. C.; Boltwood, P.; Carini, M. T.; Barnaby, D.; Coloma, J. M.; Ros, J. A.; Dai, B. Z.; Xie, G. Z.; Mattox, J. R.; Rodriguez, D.; Asfandiyarov, I. M.; Atkerson, A.; Beem, J. L.; Bloom, S. D.; Chanturiya, S. M.; Ciprini, S.; Crapanzano, S.; de Diego, J. A.; Efimova, N. V.; Gardiol, D.; Guerra, J. C.; Kahharov, B. B.; Kapanadze, B. Z.; Karttunen, H.; Kato, T.; Kimeridze, G. N.; Kudryavtseva, N. A.; Lainela, M.; Lanteri, L.; Larionova, E. G.; Maesano, M.; Marchili, N.; Massone, G.; Monroe, T.; Montagni, F.; Nesci, R.; Nilsson, K.; Noble, J. C.; Nucciarelli, G.; Ostorero, L.; Papamastorakis, J.; Pasanen, M.; Peters, C. S.; Pursimo, T.; Reig, P.; Ryle, W.; Sclavi, S.; Sigua, L. A.; Uemura, M.; Wills, W.
Referencia bibliográfica
Astronomy and Astrophysics, v.421, p.103-114 (2004)
Fecha de publicación:
7
2004
Revista
Número de citas
121
Número de citas referidas
113
Descripción
BL Lacertae has been the target of four observing campaigns by the Whole
Earth Blazar Telescope (WEBT) collaboration. In this paper we present
UBVRI light curves obtained by the WEBT from 1994 to 2002, including the
last, extended BL Lac 2001 campaign. A total of about 7500 optical
observations performed by 31 telescopes from Japan to Mexico have been
collected, to be added to the ˜15,600 observations of the BL Lac
Campaign 2000. All these data allow one to follow the source optical
emission behaviour with unprecedented detail. The analysis of the
colour indices reveals that the flux variability can be interpreted in
terms of two components: longer-term variations occurring on a few-day
time scale appear as mildly-chromatic events, while a strong
bluer-when-brighter chromatism characterizes very fast (intraday)
flares. By decoupling the two components, we quantify the degree of
chromatism inferring that longer-term flux changes imply moving along a
˜ 0.1 bluer-when-brighter slope in the B-R versus R plane; a
steeper slope of ˜ 0.4 would distinguish the shorter-term
variations. This means that, when considering the long-term trend, the
B-band flux level is related to the R-band one according to a power law
of index ˜ 1.1. Doppler factor variations on a ``convex" spectrum
could be the mechanism accounting for both the long-term variations and
their slight chromatism.