A significant fraction of a star’s mass may be accreted during luminous outbursts. To constrain this fraction, space-based and sub-orbital observation are required for characterizing such outbursts during the primary accretion phase of protostars. Using photometry from Spitzer/IRAC spanning 2004 to 2017, we detect three outbursts from Class 0 protostars with higher than 2 mag changes at 3.6 or 4.5 μm. The number of detections implies that Class 0 protostars - i.e. protostars in their primary accretion phase - burst every 438 yr, with a 95% confidence interval of 161 to 1884 yr. Combining Spitzer and WISE/NEOWISE data spanning 2004–2019, we show that the bursts last for more than nine years with significant variability during each burst. To determine the changes in luminosity, and therefore the changes in the rate of mass accretion, we use 19–100 μm photometry from SOFIA, Spitzer, and Herschel to measure the amplitudes of the bursts. Based on the burst interval, a duration of 15 yr, and the observed amplitudes, we find that 3%–100% of the mass accretion is occurring during outbursts. We discuss how to make comparisons between Spitzer, Herschel, FORCAST and HAWC+, given their disparate bandpasses.
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