The broad line region (BLR) size-luminosity relation has paramount importance
for estimating the mass of black holes in active galactic nuclei (AGNs).
Traditionally, the size of the Hβ BLR is often estimated from the optical
continuum luminosity at 5100\angstrom{} , while the size of the Hα BLR
and its correlation with the luminosity is much less constrained. As a part of
the Seoul National University AGN Monitoring Project (SAMP) which provides
six-year photometric and spectroscopic monitoring data, we present our
measurements of the Hα lags of 6 high-luminosity AGNs. Combined with the
measurements for 42 AGNs from the literature, we derive the size-luminosity
relations of Hα BLR against broad Hα and 5100\angstrom{}
continuum luminosities. We find the slope of the relations to be 0.61±0.04
and 0.59±0.04, respectively, which are consistent with the \hb{}
size-luminosity relation. Moreover, we find a linear relation between the
5100\angstrom{} continuum luminosity and the broad Hα luminosity across
7 orders of magnitude. Using these results, we propose a new virial mass
estimator based on the Hα broad emission line, finding that the previous
mass estimates based on the scaling relations in the literature are
overestimated by up to 0.7 dex at masses lower than 107~M⊙​.Comment: Accepted for publication in ApJ (Jun. 25th, 2023). 21 pages, 12
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