A large-scale, regular intergalactic magnetic field associated with
  Stephan's Quintet?
  Regular magnetic fields are frequently found within and in the outskirts of
galaxies, but their presence, properties, and origin has not yet been
established for galaxy groups. On the basis of broadband radio polarimetric
imaging with the Westerbork Synthesis Radio Telescope (WSRT), we made use of
Rotation Measure Synthesis to disentangle contributions from magnetic fields on
various scales for several polarised radio sources inside, behind, or in the
vicinity of the Stephan's Quintet (HCG92, SQ). We recognise the signature of a
large-scale, genuinely regular, magnetised screen, seemingly constrained to the
Quintet itself. Although we cannot exclude a contribution from the Milky Way,
our analysis favours a magnetic structure within the SQ system. If indeed
associated with the galaxy group in question, it would span a volume of at
least $60\,\times\,40\,\times\,20\,\mathrm{kpc}$ and have a strength at least
as high as that previously detected within large spiral galaxies. This field
would then surpass the extent of any other known galactic, regular magnetic
fields, have a considerable strength of a few microgauss, and would be the
first known example of such a structure in a galaxy system other than a galaxy
pair. Several other explanations are also presented and evaluated.
