Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Location of the Core in M81   We report on VLBI observations of M81*, the northwest-southeast oriented
nuclear core-jet source of the spiral galaxy M81, at five different frequencies
between 1.7 and 14.8 GHz. By phase referencing to supernova 1993J we can
accurately locate the emission region of M81* in the galaxy's reference frame.
Although the emission region's size decreases with increasing frequency while
the brightness peak moves to the southwest, the emission region seems sharply
bounded to the southwest at all frequencies. We argue that the core must be
located between the brightness peak at our highest frequency (14.8 GHz) and the
sharp bound to the southwest. This narrowly constrains the location of the
core, or the purported black hole in the center of the galaxy, to be within a
region of +/-0.2 mas or +/-800 AU (at a distance of ~4 Mpc). This range
includes the core position that we determined earlier by finding the most
stationary point in the brightness distribution of M81* at only a single
frequency. This independent constraint therefore strongly confirms our earlier
core position. Our observations also confirm that M81* is a core-jet source,
with a one-sided jet that extends to the northeast from the core, on average
curved somewhat to the east, with a radio spectrum that is flat or inverted
near the core and steep at the distant end. The brightness peak is
unambiguously identified with the variable jet rather than the core, which
indicates limitations in determining the proper motion of nearby galaxies and
in refining the extragalactic reference frame.

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The categories of this paper are: bayes-an
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The categories of this paper are: q-fin
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The categories of this paper are: funct-an
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The categories of this paper are: adap-org
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The categories of this paper are: hep-lat
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The categories of this paper are: mtrl-th
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The categories of this paper are: ao-sci
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The categories of this paper are: chem-ph
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The categories of this paper are: patt-sol
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The categories of this paper are: chao-dyn
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The categories of this paper are: nucl-th
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The categories of this paper are: plasm-ph
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The categories of this paper are: hep-ex
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The categories of this paper are: supr-con
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The categories of this paper are: hep-ph
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The categories of this paper are: cmp-lg