Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Discovery of a strong spiral magnetic field crossing the inner
  pseudoring of NGC 4736   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.

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The categories of this paper are: q-alg
12
The categories of this paper are: nucl-th
9
The categories of this paper are: nlin
8
The categories of this paper are: hep-ex
22
The categories of this paper are: chao-dyn
19
The categories of this paper are: eess
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The categories of this paper are: comp-gas
15
The categories of this paper are: nucl-ex
25
The categories of this paper are: patt-sol
4
The categories of this paper are: cond-mat
5
The categories of this paper are: gr-qc
14
The categories of this paper are: math-ph
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The categories of this paper are: funct-an
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The categories of this paper are: stat
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The categories of this paper are: adap-org
36
The categories of this paper are: cmp-lg