Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Microlensing & Scintillation of Gravitationally Lensed Compact Radio
  Sources: Evidence for MACHOs?   We present the first unambiguous case of external variability of an
extra-galactic radio source, the CLASS gravitational lens B1600+434, consisting
of two images lensed by an edge-on disk galaxy. The VLA 8.5-GHz difference
light curve of the lens images shows external variability at the 14.6-sigma
confidence level. Although the current single-frequency VLA observations cannot
conclusively exclude scintillation, several lines of evidence show that it does
not dominate the short-term variability. This is supported by an ongoing
three-frequency WSRT campaign. If scintillation is the dominant source of this
external variability, it would require very different properties of the
Galactic ionized ISM towards the two lens images. Microlensing of a
superluminal jet component can explain both the rms and time-scale of
variability, but requires the halo of the lens galaxy to be filled with
>=0.5-Msun MACHOs. The current data appears to support microlensing as the
dominant source of the observed external variability.

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The categories of this paper are: q-fin
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The categories of this paper are: mtrl-th
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The categories of this paper are: cmp-lg
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The categories of this paper are: supr-con
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The categories of this paper are: chem-ph
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The categories of this paper are: acc-phys
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The categories of this paper are: math
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The categories of this paper are: chao-dyn
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The categories of this paper are: stat
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The categories of this paper are: cond-mat
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The categories of this paper are: patt-sol
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The categories of this paper are: math-ph
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The categories of this paper are: dg-ga
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The categories of this paper are: gr-qc
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The categories of this paper are: bayes-an
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The categories of this paper are: funct-an