Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Chandra Observations of the QSO Pair Q2345+007: Binary Quasar or Massive
  Dark Lens?   The components of the wide (7.3") separation quasar pair Q2345+007A,B
(z=2.15) have the most strikingly similar optical spectra seen to date (Steidel
& Sargent 1991) yet no detected lensing mass, making this system the best
candidate known for a massive (1e14 Msun) dark matter lens system. Here we
present results from a 65ksec Chandra observation designed to investigate
whether it is a binary quasar or a gravitational lens. We find no X-ray
evidence for a lensing cluster to a (0.5-2keV) flux limit of 2e-15 cgs, which
is consistent with lensing only for a reduced baryon fraction. Using the
Chandra X-ray observations of the quasars themselves, together with new and
published optical measurements, we use the observed emission properties of the
quasars for further tests between the lens and binary hypotheses. Assuming
similar line-of-sight absorption to the images, we find that their X-ray
continuum slopes are inconsistent (Gamma_A=2.30 and Gamma_B=0.83) as are their
X-ray to optical flux ratios. The probability that B suffers absorption
sufficient to account for these spectral differences is negligible. We present
new optical evidence that the flux ratio of the pair is variable, so the
time-delay in a lens scenario could cause some of the discrepancies. However,
adequately large variations in overall spectral energy distribution are rare in
individual QSOs. All new evidence here weighs strongly toward the binary
interpretation. Q2345+007 thus may represent the highest redshift example known
of interaction-triggered but as-yet unmerged luminous AGN.

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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: eess
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The categories of this paper are: chao-dyn
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The categories of this paper are: hep-th
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The categories of this paper are: comp-gas
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The categories of this paper are: hep-lat
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The categories of this paper are: chem-ph
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The categories of this paper are: hep-ph
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The categories of this paper are: plasm-ph
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The categories of this paper are: cs
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The categories of this paper are: funct-an
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The categories of this paper are: nlin
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The categories of this paper are: nucl-th
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The categories of this paper are: q-bio
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The categories of this paper are: econ