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, we made use of rotation measure synthesis to disentangle contributions from magnetic fields on various scales for several polarized radio sources inside, behind, or in the vicinity of Stephan's Quintet (HCG 92, SQ). We recognize the signature of a large-scale, genuinely regular, magnetized screen, seemingly constrained to the Quintet itself. Although we cannot exclude a contribution from the Milky Way, our analysis favors 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 x 40 x 20 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.