Galaxy Zoo : 'Hanny's Voorwerp', a quasar light echo?
  We report the discovery of an unusual object near the spiral galaxy IC 2497,
discovered by visual inspection of the Sloan Digital Sky Survey (SDSS) as part
of the Galaxy Zoo project. The object, known as Hanny's Voorwerp, is bright in
the SDSS g band due to unusually strong OIII 4959-5007 emission lines. We
present the results of the first targeted observations of the object in the
optical, UV and X-ray, which show that the object contains highly ionized gas.
Although the line ratios are similar to extended emission-line regions near
luminous AGN, the source of this ionization is not apparent. The emission-line
properties, and lack of x-ray emission from IC 2497, suggest either a highly
obscured AGN with a novel geometry arranged to allow photoionization of the
object but not the galaxy's own circumnuclear gas, or, as we argue, the first
detection of a quasar light echo. In this case, either the luminosity of the
central source has decreased dramatically or else the obscuration in the system
has increased within 10^5 years. This object may thus represent the first
direct probe of quasar history on these timescales.
