Abstract:
We report the results of optical monitoring for a sample of 11 blazars including 10 BL Lacertae
objects (BL Lacs) and one flat spectrum radio quasar (FSRQ).We have measured the multiband
optical flux and colour variations in these blazars on intraday and short-term time-scales of
months and have limited data for two more blazars. These photometric observations were
made during 2009–2011, using six optical telescopes, four in Bulgaria, one in Greece and one
in India. On short-term time-scales we found significant flux variations in nine of the sources
and colour variations in three of them. Intraday variability was detected on six nights for two
sources out of the 18 nights and four sources for which we collected such data. These new
optical observations of these blazars plus data from our previous published papers (for three
more blazars) were used to analyse their spectral flux distributions in the optical frequency
range. Our full sample for this purpose includes six high-synchrotron-frequency-peaked BL
Lacs (HSPs), three intermediate-synchrotron-frequency-peaked BL Lacs (ISPs) and six lowsynchrotron-
frequency-peaked BL Lacs (LSPs; including both BL Lacs and FSRQs).We also
investigated the spectral slope variability and found that the average spectral slopes of LSPs
show a good accordance with the synchrotron self-Compton loss dominated model. Our
analysis supports previous studies that found that the spectra of the HSPs and FSRQs have
significant additional emission components. The spectra of all these HSPs and LSPs get flatter
when they become brighter, while for FSRQs the opposite appears to hold. This supports the
hypothesis that there is a significant thermal contribution to the optical spectrum for FSRQs.