BVRI light curves of the BL Lac object Mrk 501

The observations were run with the 1.02m and 2.4m optical telescopes at Yunnan Observatories from 2005 April to 2012 May. The light curve at the R band on 2010 May 15 passes both variability tests (the F-test and the ANOVA test).

A flare within the light curve on 2010 May 15 has a magnitude change of {Delta}m=0.03+/-0.005_stat_+/-0.007_sys_mag, a darkening timescale of {tau}_d_=26.7min., and an amplitude of IDV Amp=2.9%+/-0.7% . A decline described by 11 consecutive flux measurements within the flare can be fitted linearly with a Pearson's correlation coefficient r=0.945 at the confidence level of >99.99% . Under the assumptions that the IDV is tightly connected to the mass of the black hole, and that the flare duration, being two times {tau}_d_, is representative of the minimum characteristic timescale, we can derive upper bounds to the mass of the black hole.