Six-year Optical Monitoring of BL Lacertae Object 1ES 0806+52.4
  We present the results of the first systematic long-term multi-color optical
monitoring of the BL Lacertae object 1ES 0806+52.4. The monitoring was
performed in multiple passbands with a 60/90 cm Schmidt telescope from December
2005 to February 2011. The overall brightness of this object decreased from
2005 December to 2008 December, and regained after that. A sharp outburst
probably occurred around the end of our monitoring program. Overlapped on the
long-term trend are some short-term small-amplitude oscillations. No
intra-night variability was found in the object, which is in accord with the
historical observations before 2005. By investigating the color behavior, we
found strong bluer-when-brighter chromatism for the long-term variability of
1ES 0806+52.4. The total amplitudes at the c, i and o bands are 1.18, 1.12, and
1.02 mags, respectively. The amplitudes tend to increase toward shorter
wavelength, which may be the major cause of bluer-when-brighter. Such
bluer-when-brighter is also found in other blazars like S5 0716+714, OJ 287,
etc. The hard X-ray data collected from the Swift/BAT archive was correlated
with our optical data. No positive result was found, the reason of which may be
that the hard X-ray flux is a combination of the synchrotron and inverse
Compton emission but with different timescales and cadences under the leptonic
Synchrotron-Self-Compton (SSC) model.
