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

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 for outburst scenarios. Swift detected this active galactic nucleus (AGN) again in February 2010 in X-rays at a level of (1.50\plm0.11)$\times 10^{36}$ W (0.2-2.0 keV), which is nearly as luminous as the first outburst detected with ROSAT in 1990. Optical data from the Catalina sky survey show that the optical emission was already bright two years before the Swift X-ray high-state. Our new Swift observations performed between 2013 and 2015 show that IC 3599 is currently again 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 tidal disruption event (TDE). Instead, variants of AGN-related variability are explored. The data are consistent with an accretion disk instability around a black hole of mass on the order 10$^6$--10$^7$ M$_{\odot}$; a value estimated using several different methods.