## Long-term optical flux and colour variability in quasars

Daniel Wysocki • • journal_club

# Paper

“Long-term optical flux and colour variability in quasars” by Sukanya et al.

http://arxiv.org/pdf/1508.05560.pdf

# Summary

Used quasar observations from MACHO (which was searching for dark matter using “massive compact halo objects”), as it was able to obtain light curves spanning 7.5 years.

F_var characterizes the flux variability of a source, and Figure 1 shows that the V-band exceeds that of the R-band in all but a few cases.

No lag between V- and R-band flux variations were detected using this dataset, which is likely due to how close the V- and R-bands are, relative to the spacing of observations.

Equation 5 states that the variation in color for a given quasar in the V- and R-bands follows a linear relation, with a slope S_{VR}). Depending on the fitting method used, the conclusions drawn may be slightly different. Using OLS (naive), one concludes that in all cases, quasars are bluer when brighter. Using more robust methods (MCMC, weighted OLS, and BCES) shows that there are in fact some cases where the opposite is true. Figure 2 shows histograms of the 4 methods’ results.