Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: First Search for an X-ray -- Optical Reverberation Signal in an
  Ultraluminous X-ray Source   Using simultaneous optical (VLT/FORS2) and X-ray (XMM-Newton) data of NGC
5408, we present the first ever attempt to search for a reverberation signal in
an ultraluminous X-ray source (NGC 5408 X-1). The idea is similar to AGN broad
line reverberation mapping where a lag measurement between the X-ray and the
optical flux combined with a Keplerian velocity estimate should enable us to
weigh the central compact object. We find that although NGC 5408 X-1's X-rays
are variable on a timescale of a few hundred seconds (RMS of 9.0$\pm$0.5%), the
optical emission does not show any statistically significant variations. We set
a 3$\sigma$ upper limit on the RMS optical variability of 3.3%. The ratio of
the X-ray to the optical variability is an indicator of X-ray reprocessing
efficiency. In X-ray binaries, this ratio is roughly 5. Assuming a similar
ratio for NGC 5408 X-1, the expected RMS optical variability is $\approx$2%
which is still a factor of roughly two lower than what was possible with the
VLT observations in this study. We find marginal evidence (3$\sigma$) for
optical variability on a $\sim$ 24 hour timescale. Our results demonstrate that
such measurements can be made, but photometric conditions, low sky background
levels and longer simultaneous observations will be required to reach the
optical variability levels similar to X-ray binaries.

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The categories of this paper are: atom-ph
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The categories of this paper are: math-ph
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The categories of this paper are: nucl-th
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The categories of this paper are: comp-gas
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The categories of this paper are: nucl-ex
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The categories of this paper are: bayes-an
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The categories of this paper are: acc-phys
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The categories of this paper are: stat
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The categories of this paper are: alg-geom
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The categories of this paper are: econ
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The categories of this paper are: hep-lat
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The categories of this paper are: chao-dyn
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The categories of this paper are: cond-mat
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The categories of this paper are: q-fin
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The categories of this paper are: nlin
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The categories of this paper are: adap-org