Improving the Flux Calibration in Reverberation Mapping by Spectral Fitting: Application to the Seyfert Galaxy MCG--6-30-15

TL;DR

This paper introduces an improved spectral fitting method for flux calibration in reverberation mapping, enabling more accurate measurements even for low-variability AGN, demonstrated through reanalysis of MCG--6-30-15 data.

Contribution

The authors develop a spectral fitting-based flux calibration technique that surpasses traditional methods, allowing for more precise reverberation mapping of weakly variable objects.

Findings

Successfully detected a time lag in MCG--6-30-15 after reanalysis.

Derived a black hole mass consistent with previous estimates.

Achieved better flux calibration accuracy for low-variability AGN.

Abstract

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 (1992), 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$\beta$) = $1933\pm81$ $\rm km~s^{-1}$ and a virial coefficient of $f$ = 0.7, we derive a virial black hole mass of $3.26_{-1.40}^{+1.59}\times10^6$ $M_{\odot}$, which agrees well with previous estimates by other methods.