  We report on the discovery of a bright Lyman alpha blob associated with the
z=3 quasar SDSSJ124020.91+145535.6 which is also coincident with strong damped
Lyman alpha absorption from a foreground galaxy (a so-called proximate damped
Lyman alpha system; PDLA). The one dimensional spectrum acquired by the Sloan
Digital Sky Survey (SDSS) shows a broad Lyman alpha emission line with a FWHM ~
500 km/s and a luminosity of L_{Lya} = 3.9e43 erg/s superposed on the trough of
the PDLA. Mechanisms for powering this large Lyman alpha luminosity are
discussed. We argue against emission from HII regions in the PDLA galaxy since
this requires an excessive star-formation rate ~ 500 Msun/yr and would
correspond to the largest Lyman alpha luminosity ever measured from a damped
Lyman alpha system or starburst galaxy. We use a Monte Carlo radiative transfer
simulation to investigate the possibility that the line emission is fluorescent
recombination radiation from the PDLA galaxy powered by the ionizing flux of
the quasar, but find that the predicted Lyman alpha flux is several orders of
magnitude lower than observed. We conclude that the Lyman alpha emission is not
associated with the PDLA galaxy at all, but instead is intrinsic to the
quasar's host and similar to the extended Lyman alpha 'fuzz' which is detected
around many AGN. PDLAs are natural coronagraphs that block their background
quasar at Lyman alpha, and we discuss how systems similar to
SDSSJ124020.91+145535.6 might be used to image the neutral hydrogen in the PDLA
galaxy in silhouette against the screen of extended Lyman alpha emission from
the background quasar.
