Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Generalised model-independent characterisation of strong gravitational
  lenses III: perturbed axisymmetric lenses   In galaxy-galaxy strong gravitational lensing, Einstein rings are generated
when the lensing galaxy has an axisymmetric lensing potential and the source
galaxy is aligned with its symmetry centre along the line of sight. Using a
Taylor expansion around the Einstein radius and eliminating the unknown source,
I derive a set of analytic equations that determine differences of the
deflection angle of the perturber weighted by the convergence of the
axisymmetric lens and ratios of the convergences at the positions of the arcs
from the measurable thickness of the arcs. In the same manner, asymmetries in
the brightness distributions along an arc determine differences in the
deflection angle of the perturber if the source has a symmetric brightness
profile and is oriented parallel to or orthogonal to the caustic. These
equations are the only model-independent information retrievable from
observations to leading order in the Taylor expansion. General constraints on
the derivatives of the perturbing lens are derived such that the perturbation
does not change the number of critical curves. To infer physical properties
such as the mass of the perturber or its position, models need to be inserted.
The same conclusions about the scale of detectable masses and model-dependent
degeneracies as in other approaches are then found and supported by analysing
B1938 as an example. Yet, the model-independent equations show that there is a
fundamental degeneracy between the main lens and the perturber that can only be
broken if their relative position is known. This explains the degeneracies
between lens models already found in simulations from a more general viewpoint.
Depending on the properties of the pertuber, this degeneracy can be broken by
characterising the surrounding of the lens or by measuring the time delay
between quasar images embedded in the perturbed Einstein ring of the host
galaxy.

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The categories of this paper are: atom-ph
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The categories of this paper are: stat
21
The categories of this paper are: adap-org
3
The categories of this paper are: physics
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The categories of this paper are: nucl-ex
4
The categories of this paper are: cond-mat
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The categories of this paper are: cmp-lg
2
The categories of this paper are: math
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The categories of this paper are: bayes-an
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The categories of this paper are: dg-ga
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The categories of this paper are: hep-lat
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The categories of this paper are: q-bio
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The categories of this paper are: acc-phys
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The categories of this paper are: hep-th
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The categories of this paper are: patt-sol
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The categories of this paper are: eess