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We report on the Swift discovery of a second high-amplitude (factor 100)outburst of the Seyfert 1.9 galaxy IC 3599, and discuss implications foroutburst scenarios. Swift detected this active galactic nucleus (AGN) again inFebruary 2010 in X-rays at a level of (1.50\plm0.11)$\times 10^{36}$ W (0.2-2.0keV), which is nearly as luminous as the first outburst detected with ROSAT in1990. Optical data from the Catalina sky survey show that the optical emissionwas already bright two years before the Swift X-ray high-state. Our new Swiftobservations performed between 2013 and 2015 show that IC 3599 is currentlyagain in a very low X-ray flux state. This repeat optical and X-ray outburst,and the long optical duration, suggest that IC 3599 is likely not a tidaldisruption event (TDE). Instead, variants of AGN-related variability areexplored. The data are consistent with an accretion disk instability around ablack hole of mass on the order 10$^6$--10$^7$ M$_{\odot}$; a value estimatedusing several different methods.

IC 3599 did it again: A second outburst of the X-ray transient Seyfert 1.9 Galaxy