Abstract:
It is important to quantify the underestimation of rms pho-
tometric errors returned by the commonly used APPHOT algorithm in
the IRAF software, in the context of differential photometry of point-like
AGN, because of the crucial role it plays in evaluating their variabil-
ity properties. Published values of the underestimation factor, η, 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 Sampur-
nanad Telescope (ST). We find η = 1.54 ± 0.05 which is close to our
recently reported value of η = 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 simultane-
ously is within the same CCD chip acceptable, as it should not lead to
spurious claims of INOV.
