The JCMT Transient Survey: Detection of sub-mm variability in a Class I
  protostar EC 53 in Serpens Main
  During the protostellar phase of stellar evolution, accretion onto the star
is expected to be variable, but this suspected variability has been difficult
to detect because protostars are deeply embedded. In this paper, we describe a
sub-mm luminosity burst of the Class I protostar EC 53 in Serpens Main, the
first variable found during our dedicated JCMT/SCUBA-2 monitoring program of
eight nearby star-forming regions. EC 53 remained quiescent for the first 6
months of our survey, from February to August 2016. The sub-mm emission began
to brighten in September 2016, reached a peak brightness of $1.5$ times the
faint state, and has been decaying slowly since February 2017. The change in
sub-mm brightness is interpreted as dust heating in the envelope, generated by
a luminosity increase of the protostar of a factor of $\ge 4$. The 850~$\mu$m
lightcurve resembles the historical $K$-band lightcurve, which varies by a
factor of $\sim 6$ with a 543 period and is interpreted as accretion
variability excited by interactions between the accretion disk and a close
binary system. The predictable detections of accretion variability observed at
both near-infrared and sub-mm wavelengths make the system a unique test-bed,
enabling us to capture the moment of the accretion burst and to study the
consequences of the outburst on the protostellar disk and envelope.
