A Gigantic Mid-infrared Outburst in an Embedded Class I Young Stellar Object J064722.95+031644.6

We report the serendipitous discovery of a giant mid-infrared outburst from a previously unknown source near a star-forming region in the constellation Monoceros. The source gradually brightened by a factor of 5 from 2014-2016 before an abrupt rise by a factor of more than 100 in 2017. A total amplitude increase of >500 at 4.5 mu m has since faded by a factor of about 10. Prior to the outburst, it was only detected at wavelengths longer than 1.8 mu m in UKIDSS, Spitzer, and Herschel with a spectral energy distribution of a Class I young stellar object (YSO). It has not been detected in recent optical surveys, suggesting that it is deeply embedded. With a minimum distance of 3.5 kpc, the source has a bolometric luminosity of at least 9 L-circle dot in the quiescent state and 400 L-circle dot at the peak of the eruption. The maximum accretion rate is estimated to be at least a few 10(-5) M-circle dot yr(-1). It shares several common properties with another eruptive event, WISE J142238.82-611553.7: exceptionally large amplitude, featureless near-infrared spectrum with the exception of H-2 lines, intermediate eruption duration, an embedded Class I YSO, and a low radiative temperature (<600-700 K) in outburst. We interpret that the radiation from the inner accretion disk and young star is obscured and reprocessed by either an inflated outer disk or thick dusty outflow on scales >6.5 au during the outburst.