Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Discovery of a Nuclear X-Ray Source in NGC 7331: Evidence for a Massive
  Black Hole   Using ROSAT, we have made the first deep X-ray observations of NGC 7331,
discovering a nuclear X-ray source coincident with the previously determined
radio and optical nuclear centers. Positions, luminosities, and fluxes of X-ray
sources in the field are compared to previously identified radio and optical
sources. The nucleus of NGC 7331 has been analyzed to discern any new evidence
supporting the presence of a massive black hole (MBH), and comparisons are made
with other Low-Ionization Nuclear Emission-line Region (LINER) galaxies. The
flux ratio of core radio to soft X-ray emission in NGC 7331 is lower than in
other LINERs included in our sample.

15
The categories of this paper are: nucl-ex
18
The categories of this paper are: econ
16
The categories of this paper are: stat
12
The categories of this paper are: nucl-th
2
The categories of this paper are: math
28
The categories of this paper are: alg-geom
3
The categories of this paper are: physics
30
The categories of this paper are: q-alg
17
The categories of this paper are: q-fin
22
The categories of this paper are: chao-dyn
27
The categories of this paper are: comp-gas
31
The categories of this paper are: ao-sci
1
The categories of this paper are: cs
7
The categories of this paper are: hep-th
11
The categories of this paper are: hep-lat
4
The categories of this paper are: cond-mat