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Paper: Optical Variability of BL Lac: is it Redder-when-Fainter?
Volume: 350, Blazar Variability Workshop II: Entering the GLAST Era
Page: 71
Authors: Benitez, E.; Ramirez, A.
Abstract: Broad band BVRI (Johnson-Cousins) observations of BL Lac were done as part of the optical WEBT 2000 monitoring Campaign extension at the OAN-SPM. Using the 1.5m telescope we observed intraday variations in B,V,R filters in July 27th, 28th and 30th (UT). In particular, we detect variations in B ∼ 0.133 ± 0.029 mag, V ∼ 0.147 ± 0.035 mag, and R ∼ 0.114 ± 0.020 mag from 27th to 28th nights, and B ∼ 0.199 ± 0.021 mag, V ∼ 0.141 ± 0.030 mag, and R∼ 0.167 ± 0.074 mag from 28th to 30th nights. Intraday variations show that BL Lac was getting fainter each night. Optical microvariations were also detected in the three nights, with a maximum change of ΔR = 0.147±0.020 mag in Δt = 2.5hrs. We also observed that the maximum spectral changes obtained with the three filters were: ΔαBV ∼ 1.8, ΔαV R ∼ 1.12 and ΔαBR ∼ 0.57. From ΔαBR we find that the object is redder-when-fainter as has been found in previous studies. However, when using the V filter, we observed that the spectrum becomes flatter (ΔαBV ) from July 27th to 28th and also from ΔαV R from July 28th to 30th. Our results led us to conclude that the source is not bluer nor redder-when fainter.
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