  The radio-quiet quasar SDSS J1240+1455 lies at a redshift of z=3.11, is
surrounded by a Ly-alpha blob (LAB), and is absorbed by a proximate damped
Ly-alpha system. In order to better define the morphology of the blob and
determine its emission mechanism, we gathered deep narrow-band images isolating
the Ly-alpha line of this object in linearly polarized light. We provide a deep
intensity image of the blob, showing a filamentary structure extending up to
16'' (or ~122 physical kpc) in diameter. No significant polarization signal
could be extracted from the data, but 95% probability upper limits were defined
through simulations. They vary between ~3% in the central 0.75'' disk (after
subtraction of the unpolarized quasar continuum) and ~10% in the 3.8-5.5''
annulus. The low polarization suggests that the Ly-alpha photons are emitted
mostly in situ, by recombination and de-excitation in a gas largely ionized by
the quasar ultraviolet light, rather than by a central source and scattered
subsequently by neutral hydrogen gas. This blob shows no detectable
polarization signal, contrary to LAB1, a brighter and more extended blob that
is not related to the nearby active galactic nucleus (AGN) in any obvious way,
and where a significant polarization signal of about 18% was detected.
