Abstract
We report results based on the monitoring of the BL Lac object Mrk 501 in the optical (B, V and R) passbands from March to May 2000. Observations spread over 12 nights were carried out using 1.2m Mount Abu Telescope, India and 61cm Telescope at Sobaeksan Astronomy Observatory, South Korea. The aim is to study the intra-day variability (IDV), short term variability and color variability in the low state of the source. We have detected flux variation of 0.05mag in the R-band in time scale of 15min in one night. In the B and V passbands, we have less data points and it is difficult to infer any IDVs. Short term flux variations are also observed in the V and R bands during the observing run. No significant variation in color (B–R) has been detected but (V–R) shows variation during the present observing run.Assuming the shortest observed time scale of variability (15min) to represent the disk instability or pulsation at a distance of five Schwarschild radii from the black hole (BH), mass of the central BH is estimated ∼1.20×108M⊙.
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