The Man behind the Curtain: X-Rays Drive the UV through NIR Variability in the 2013 Active Galactic Nucleus Outburst in NGC 2617
Im, M.; Choi, C.; Yoon, Y.; Kochanek, C. S.; Howell, D. A.; Komossa, S.; Peterson, B. M.; Dietrich, M.; Pogge, R. W.; Stanek, K. Z.; Brimacombe, J.; Shappee, B. J.; Hsiao, E. Y.; Morrell, N.; Phillips, M. M.; Grupe, D.; Mathur, S.; Giustini, M.; Koss, M.; Contreras, C.; Valenti, S.; De Rosa, G.; Holoien, T. W. -S.; Mudd, D.; Zu, Y.; Adams, S.; Prieto, J. L.; Pojmanski, G.; Jencson, J.; Terndrup, D. M.; Szczygieł, D. M.; Campillay, A.; Leighly, K. M.; Parrent, J.; Basu, U.; Beacom, J. F.; Dubberley, M.; Elphick, M.; Foale, S.; Gonzalez, C.; Hawkins, E.; Mullins, D.; Nugent, J. M.; Rosing, W.; Ross, R.; Sand, D.; Walker, Z.
United States, Germany, South Korea, Poland, Australia, Chile, Spain
Abstract
After the All-Sky Automated Survey for SuperNovae discovered a significant brightening of the inner region of NGC 2617, we began a ~70 day photometric and spectroscopic monitoring campaign from the X-ray through near-infrared (NIR) wavelengths. We report that NGC 2617 went through a dramatic outburst, during which its X-ray flux increased by over an order of magnitude followed by an increase of its optical/ultraviolet (UV) continuum flux by almost an order of magnitude. NGC 2617, classified as a Seyfert 1.8 galaxy in 2003, is now a Seyfert 1 due to the appearance of broad optical emission lines and a continuum blue bump. Such "changing look active galactic nuclei (AGNs)" are rare and provide us with important insights about AGN physics. Based on the Hβ line width and the radius-luminosity relation, we estimate the mass of central black hole (BH) to be (4 ± 1) × 107 M ⊙. When we cross-correlate the light curves, we find that the disk emission lags the X-rays, with the lag becoming longer as we move from the UV (2-3 days) to the NIR (6-9 days). Also, the NIR is more heavily temporally smoothed than the UV. This can largely be explained by a simple model of a thermally emitting thin disk around a BH of the estimated mass that is illuminated by the observed, variable X-ray fluxes.