  Precise measurements of black hole masses are essential to understanding the
coevolution of these sources and their host galaxies. We develop a novel
approach for computing black hole virial masses using measurements of continuum
luminosities and emission line widths from partially overlapping, narrow-band
observations of quasars; we refer to this technique as single-epoch photometry.
This novel method relies on forward-modelling quasar observations for
estimating emission line widths, which enables unbiased measurements even for
lines coarsely resolved by narrow-band data. We assess the performance of this
technique using quasars from the Sloan Digital Sky Survey (SDSS) observed by
the miniJPAS survey, a proof-of-concept project of the Javalambre Physics of
the Accelerating Universe Astrophysical Survey (J-PAS) collaboration covering
$\simeq1\,\mathrm{deg}^2$ of the northern sky using the 56 J-PAS narrow-band
filters. We find remarkable agreement between black hole masses from
single-epoch SDSS spectra and single-epoch miniJPAS photometry, with no
systematic difference between these and a scatter ranging from 0.4 to 0.07 dex
for masses from $\log(M_\mathrm{BH})\simeq8$ to 9.75, respectively.
Reverberation mapping studies show that single-epoch masses present
approximately 0.4 dex precision, letting us conclude that our novel technique
delivers black hole masses with only mildly lower precision than single-epoch
spectroscopy. The J-PAS survey will soon start observing thousands of square
degrees without any source preselection other than the photometric depth in the
detection band, and thus single-epoch photometry has the potential to provide
details on the physical properties of quasar populations that do not satisfy
the preselection criteria of previous spectroscopic surveys.
