Monitoring of the nuclear outburst in NGC 660 with the Arecibo 305m Telescope

We present updated results from the Arecibo monitoring of the nuclear outburst in NGC 660. Observations were made in continuum and in excited-OH emission and absorption at C-band (4.7 GHz). The full sequence covers pre-outburst observations in 2007, through the start of the outburst in 2008 and its peak in 2011, to observations taken in 2017 just before the Arecibo Telescope was damaged by Hurricane Maria, significantly reducing its sensitivity at the frequency band used. Based on the light-curve obtained, the outburst is unlikely to be either a tidal disruption event or a shift to a period of sustained AGN activity. Instead, it is most likely to be a single event caused by the accretion of a gas cloud over an extended period.