  Galaxy formation is significantly modulated by energy output from
supermassive black holes at the centers of galaxies which grow in highly
efficient luminous quasar phases. The timescale on which black holes transition
into and out of such phases is, however, unknown. We present the first
measurement of the shutdown timescale for an individual quasar using X-ray
observations of the nearby galaxy IC 2497, which hosted a luminous quasar no
more than 70,000 years ago that is still seen as a light echo in `Hanny's
Voorwerp', but whose present-day radiative output is lower by at least 2 and
more likely by over 4 orders of magnitude. This extremely rapid shutdown
provides new insights into the physics of accretion in supermassive black
holes, and may signal a transition of the accretion disk to a radiatively
inefficient state.
