On the photometric error calibration for the `differential light curves' of point-like Active Galactic Nuclei

It is important to quantify the underestimation of rms photometric errors returned by the commonly used $\emph APPHOT$ algorithm in the $\emph IRAF$ software, in the context of differential photometry of point-like AGN, because of the crucial role it plays in evaluating their variability properties. Published values of the underestimation factor, $\eta$, using several different telescopes, lie in the range 1.3 - 1.75. The present study aims to revisit this question by employing an exceptionally large data set of 262 differential light curves (DLCs) derived from 262 pairs of non-varying stars monitored under our ARIES AGN monitoring program for characterizing the intra-night optical variability (INOV) of prominent AGN classes. The bulk of these data were taken with the 1-m Sampurnanad Telescope (ST). We find $\eta$ = 1.54$\pm$0.05 which is close to our recently reported value of $\eta$ = 1.5. Moreover, this consistency holds at least up to a brightness mismatch of 1.5 mag between the paired stars. From this we infer that a magnitude difference of at least up to 1.5 mag between a point-like AGN and comparison star(s) monitored simultaneously is within the same CCD chip acceptable, as it should not lead to spurious claims of INOV.