Multi-color Optical Variability of the TeV Blazar Mrk 501 in the Low-State

TL;DR

This study monitored the optical variability of the TeV blazar Mrk 501 in low state, detecting rapid flux changes and estimating the black hole mass based on observed variability timescales.

Contribution

First detailed optical variability analysis of Mrk 501 in low state across multiple passbands with rapid flux variation detection and black hole mass estimation.

Findings

Detected 0.05 mag flux variation in 15 min in R-band.

Observed short-term flux variations in V and R bands.

No significant color variation in B-R, but V-R varies.

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 1.2 meter Mount Abu Telescope, India and 61 cm 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 0.05 mag in the R-band in time scale of 15 min 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 (15 min) to represent the disk instability or pulsation at a distance of 5 Schwarschild radii from the black hole (BH), mass of the central BH is estimated $\sim$ 1.20 $\times$ 10$^{8} M_{\odot}$.