Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Discovery of an X-ray Nova in M31   We have obtained snapshot images of an X-ray nova in M31 from Chandra ACIS-I
and the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS). The
Chandra position of the X-ray nova was R.A.=00:44:06.68 +/- 1.74",
Dec.=+41:12:20.0 +/- 2.31". A follow-up HST observation 24 days later revealed
a source at R.A.=00:44:06.81, Dec.=+41:12:24.0 that was B=25.75 +/- 0.05. This
optical source faded to B=27.1 +/- 0.1 in 3 months. During this time period,
the X-ray flux decayed linearly from (3.6 +/- 0.2) X 10^{-4} to <(6.9 +/- 0.09)
X 10^{-5} ct cm^{-2} s^{-1}. The HST identification of an optical source in the
same region experiencing an obvious drop in brightness in concert with the
X-ray nova suggests that this optical source is the counterpart of the X-ray
nova. However, the precision of the X-ray position allows the possibility that
the optical source is a nearby variable star. We discuss the implications of
both possibilities.

23
The categories of this paper are: solv-int
3
The categories of this paper are: physics
8
The categories of this paper are: hep-ex
2
The categories of this paper are: math
32
The categories of this paper are: atom-ph
1
The categories of this paper are: cs
22
The categories of this paper are: chao-dyn
28
The categories of this paper are: alg-geom
20
The categories of this paper are: acc-phys
26
The categories of this paper are: bayes-an
35
The categories of this paper are: mtrl-th
34
The categories of this paper are: chem-ph
37
The categories of this paper are: supr-con
15
The categories of this paper are: nucl-ex
16
The categories of this paper are: stat
36
The categories of this paper are: cmp-lg