The Man Behind the Curtain: X-rays Drive the UV through NIR Variability
  in the 2013 AGN Outburst in NGC 2617
  After the All-Sky Automated Survey for SuperNovae (ASAS-SN) 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 (AGN)" are rare and provide us with important insights about AGN
physics. Based on the Hbeta line width and the radius-luminosity relation, we
estimate the mass of central black hole to be (4 +/- 1) x 10^7 M_sun. 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 black hole of the estimated mass that is illuminated by the
observed, variable X-ray fluxes.
