Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Enhanced optical activity 12 days before X-ray activity, and a 4 day
  X-ray delay during outburst rise, in a low-mass X-ray binary   X-ray transients, such as accreting neutron stars, periodically undergo
outbursts, thought to be caused by a thermal-viscous instability in the
accretion disk. Usually outbursts of accreting neutron stars are identified
when the accretion disk has undergone an instability, and the persistent X-ray
flux has risen to a threshold detectable by all sky monitors on X-ray space
observatories. Here we present the earliest known combined optical, UV, and
X-ray monitoring observations of the outburst onset of an accreting neutron
star low mass X-ray binary system. We observed a significant, continuing
increase in the optical i'-band magnitude starting on July 25, 12 days before
the first X-ray detection with Swift/XRT and NICER (August 6), during the onset
of the 2019 outburst of SAX J1808.4-3658. We also observed a 4 day optical to
X-ray rise delay, and a 2 day UV to X-ray delay, at the onset of the outburst.
We present the multiwavelength observations that were obtained, discussing the
theory of outbursts in X-ray transients, including the disk instability model,
and the implications of the delay. This work is an important confirmation of
the delay in optical to X-ray emission during the onset of outbursts in low
mass X-ray binaries, which has only previously been measured with less
sensitive all sky monitors. We find observational evidence that the outburst is
triggered by ionisation of hydrogen in the disk.

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The categories of this paper are: bayes-an
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The categories of this paper are: eess
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The categories of this paper are: physics
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The categories of this paper are: stat
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The categories of this paper are: chao-dyn
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The categories of this paper are: acc-phys
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The categories of this paper are: nucl-ex
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The categories of this paper are: atom-ph
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The categories of this paper are: cmp-lg
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The categories of this paper are: plasm-ph
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The categories of this paper are: gr-qc
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The categories of this paper are: math-ph
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The categories of this paper are: solv-int
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The categories of this paper are: funct-an
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The categories of this paper are: supr-con
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The categories of this paper are: q-fin