The JCMT Transient Survey: Identifying Submillimetre Continuum
  Variability over Several Year Timescales Using Archival JCMT Gould Belt
  Survey Observations
  Investigating variability at the earliest stages of low-mass star formation
is fundamental in understanding how a protostar assembles mass. While many
simulations of protostellar disks predict non-steady accretion onto protostars,
deeper investigation requires robust observational constraints on the frequency
and amplitude of variability events characterised across the observable SED. In
this study, we develop methods to robustly analyse repeated observations of an
area of the sky for submillimetre variability in order to determine constraints
on the magnitude and frequency of deeply embedded protostars. We compare
\mbox{850 $\mu$m} JCMT Transient Survey data with archival JCMT Gould Belt
Survey data to investigate variability over 2-4 year timescales. Out of 175
bright, independent emission sources identified in the overlapping fields, we
find 7 variable candidates, 5 of which we classify as \textit{Strong} and the
remaining 2 as \textit{Extended} to indicate the latter are associated with
larger-scale structure. For the \textit{Strong} variable candidates, we find an
average fractional peak brightness change per year of |4.0|\% yr$^{-1}$ with a
standard deviation of $2.7\%\mathrm{\:yr}^{-1}$. In total, 7\% of the
protostars associated with \mbox{850 $\mu$m} emission in our sample show signs
of variability. Four of the five \textit{Strong} sources are associated with a
known protostar. The remaining source is a good follow-up target for an object
that is anticipated to contain an enshrouded, deeply embedded protostar. In
addition, we estimate the \mbox{850 $\mu$m} periodicity of the submillimetre
variable source, EC 53, to be \mbox{567 $\pm$ 32 days} based on the archival
Gould Belt Survey data.
