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We present the four-year survey results of monthly submillimeter monitoringof eight nearby ($< 500 $pc) star-forming regions by the JCMT Transient Survey.We apply the Lomb-Scargle Periodogram technique to search for and characterizevariability on 295 submillimeter peaks brighter than 0.14 Jy beam$^{-1}$,including 22 disk sources (Class II), 83 protostars (Class 0/I), and 190starless sources. We uncover 18 secular variables, all of them protostars. Nosingle-epoch burst or drop events and no inherently stochastic sources areobserved. We classify the secular variables by their timescales into threegroups: Periodic, Curved, and Linear. For the Curved and Periodic cases, thedetectable fractional amplitude, with respect to mean peak brightness, is$\sim4$ % for sources brighter than $\sim$ 0.5 Jy beam$^{-1}$. Limiting oursample to only these bright sources, the observed variable fraction is 37 % (16out of 43). Considering source evolution, we find a similar fraction of brightvariables for both Class 0 and Class I. Using an empirically motivatedconversion from submillimeter variability to variation in mass accretion rate,six sources (7 % of our full sample) are predicted to have years-long accretionevents during which the excess mass accreted reaches more than 40 % above thetotal quiescently accreted mass: two previously known eruptive Class I sources,V1647 Ori and EC 53 (V371 Ser), and four Class 0 sources, HOPS 356, HOPS 373,HOPS 383, and West 40. Considering the full protostellar ensemble, theimportance of episodic accretion on few years timescale is negligible, only afew percent of the assembled mass. However, given that this accretion isdominated by events of order the observing time-window, it remains uncertain asto whether the importance of episodic events will continue to rise withdecades-long monitoring.

The JCMT Transient Survey: Four Year Summary of Monitoring the Submillimeter Variability of Protostars