We propose a method for the flux calibration of reverberation mapping spectra based on accurate measurement of [ O iii ] \lambda 5007 emission by spectral fitting .
The method can achieve better accuracy than the traditional method of van Groningen & Wanders ( 50 ) , allowing reverberation mapping measurements for object with variability amplitudes as low as \sim 5 % .
As a demonstration , we reanalyze the data of the Seyfert 1 galaxy MCG–6-30-15 taken from the 2008 campaign of the Lick AGN Monitoring Project , which previously failed to obtain a time lag for this weakly variable object owing to a relatively large flux calibration uncertainty .
We detect a statistically significant rest-frame time lag of 6.38 _ { -2.69 } ^ { +3.07 } days between the H \beta and V -band light curves .
Combining this lag with FWHM ( H \beta ) = 1933 \pm 81 km~ { } s ^ { -1 } and a virial coefficient of f = 0.7 , we derive a virial black hole mass of 3.26 _ { -1.40 } ^ { +1.59 } \times 10 ^ { 6 } M _ { \odot } , which agrees well with previous estimates by other methods .