  We report the discovery of a coherent magnetic spiral structure within the
nearby ringed Sab galaxy NGC 4736. High sensitivity radio polarimetric data
obtained with the VLA at 8.46GHz and 4.86GHz show a distinct ring of total
radio emission precisely corresponding to the bright inner pseudoring visible
in other wavelengths. However, unlike the total radio emission, the polarized
radio emission reveals a clear pattern of ordered magnetic field of spiral
shape, emerging from the galactic centre. The magnetic vectors do not follow
the tightly-wrapped spiral arms that characterize the inner pseudoring, but
instead cross the ring with a constant and large pitch angle of about 35deg.
The ordered field is thus not local adjusted to the pattern of star-formation
activity, unlike what is usually observed in grand-design spirals. The observed
asymmetric distribution of Faraday rotation suggests the possible action of a
large-scale MHD dynamo. The strong magnetic total and regular field within the
ring (up to 30microG and 13microG, respectively) indicates that a highly
efficient process of magnetic field amplification is under way, probably
related to secular evolutionary processes in the galaxy.
