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 2010 February in X-rays at a level of (1.50 ± 0.11) ×10 W36 (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 2 yr before the Swift X-ray high state. Our new Swift observations performed between 2013 and 2015 show that IC 3599 is currently in a very low X-ray flux state once again. This repeat optical and X-ray outburst, and its long optical duration, suggest that IC 3599 is likely not a tidal disruption event. Instead, we explore variants of AGN-related variability. The data are consistent with accretion disk instability around a black hole of mass on the order of 106 –107 M⊙; a value estimated using several different methods