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Abstract: Linear spectropolarimetry is a ``photon-hungry- observing technique,requiring a specific sequence of observations to determine the Stokes Q and Uparameters. For dual-beam spectropolarimeters, the parameters can be ideallydetermined using observations at N=2 retarder plate positions. The additionalpolarization signal introduced by instrumental effects requires the redundancyof N=4 observations to correct for these effects. We wish to determine if the``instrumental signature corrections-, eQ and eU, are identical forobservations with dual-beam spectropolarimeters. For instances whenobservations were acquired at N=3 retarder plate angles, we wish to determineif the complete measurement of one Stokes parameter and the associatedinstrumental signature correction can be used to determine the other Stokesparameter. We constructed analytical and Monte Carlo models of a generaldual-beam spectropolarimeter to study the factors affecting the assumptioneQ=eU. We compared these models with VLT FORS1 spectropolarimetry. We find thateQ-eU=0, with the variance around zero \DeltaeQ-eU being directly relatedto the signal-to-noise ratio of the observations. Observations of a polarizedstandard star, over the period 2002-2007, show that the assumption of eQ-eU=0is generally true, although the absolute values of eQ and eU vary betweenobservational epochs. Incomplete spectropolarimetry datasets can be analyzedunder the assumption that eQ=eU. This property of the analysis ofspectropolarimetry, with dual beam spectropolarimeters, can be used to assessthe quality of the reduction for complete data.



Autor: J.R. Maund 1 1 University of Texas, Austin

Fuente: https://arxiv.org/







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