Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A possible close supermassive black-hole binary in a quasar with optical
  periodicity   Quasars have long been known to be variable sources at all wavelengths. Their
optical variability is stochastic, can be due to a variety of physical
mechanisms, and is well-described statistically in terms of a damped random
walk model. The recent availability of large collections of astronomical time
series of flux measurements (light curves) offers new data sets for a
systematic exploration of quasar variability. Here we report on the detection
of a strong, smooth periodic signal in the optical variability of the quasar PG
1302-102 with a mean observed period of 1,884 $\pm$ 88 days. It was identified
in a search for periodic variability in a data set of light curves for 247,000
known, spectroscopically confirmed quasars with a temporal baseline of $\sim9$
years. While the interpretation of this phenomenon is still uncertain, the most
plausible mechanisms involve a binary system of two supermassive black holes
with a subparsec separation. Such systems are an expected consequence of galaxy
mergers and can provide important constraints on models of galaxy formation and
evolution.

15
The categories of this paper are: nucl-ex
2
The categories of this paper are: math
31
The categories of this paper are: ao-sci
4
The categories of this paper are: cond-mat
7
The categories of this paper are: hep-th
32
The categories of this paper are: atom-ph
16
The categories of this paper are: stat
0
The categories of this paper are: hep-ph
3
The categories of this paper are: physics
12
The categories of this paper are: nucl-th
26
The categories of this paper are: bayes-an
30
The categories of this paper are: q-alg
24
The categories of this paper are: dg-ga
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The categories of this paper are: chao-dyn
25
The categories of this paper are: patt-sol
35
The categories of this paper are: mtrl-th