THE REMARKABLE POST-MAXIMUM BEHAVIOR OF THE CHANGING LOOK SEYFERT GALAXY NGC 2617

Abstract

Optical and near-infrared photometry, optical spectroscopy, and soft X-ray and UV monitoring of the changing-look active galactic nucleus NGC 2617 show that it continues to have the appearance of a type-1 Seyfert galaxy. An optical light curve for 20102016 indicates that the change of type probably occurred between 2010 October and 2012 February and was not related to the
brightening in 2013. In 2016, NGC 2617 brightened again to a level of activity close to that in 2013 April. We find variations in all pass bands and in boththe intensities and proles of the broad Balmer lines. A new displaced emission peak has appeared in Hβ. X-ray variations are well correlated with UV-optical variability and possibly lead by 2-3 d. The K band lags the J band by about
21.5 ± 2.5 d and lags the combined B + J filters by 25 d. J lags B by about 3 d. This could be because J-band variability arises from the outer part of the accretion disc, while K-band variability comes from thermal re-emission by dust. We propose that spectral-type changes are a result of increasing central luminosity causing sublimation of the innermost dust in the hollow bi-conical
outflow. We briefly discuss various other possible reasons that might explain the dramatic changes in NGC 2617.