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Instrumental effects in IXPE observations of extended sources

Presentation #109.03 in the session IXPE.

Published onJul 01, 2023
Instrumental effects in IXPE observations of extended sources

The Imaging X-ray Polarimetry Explorer (IXPE) is a NASA Small Explorer mission in partnership with the Italian space agency (Agenzia Spaziale Italiana, ASI) capable of space-resolved spectro-polarimetry, which through the first year of its science program has studied for the first time the polarization properties of tens of sources in the soft x-ray band (2-8 keV). IXPE exploits three identical gas pixel detectors, placed at the focal plane of three WOLTER Type I optics, to image the tracks left by the photo-electrons generated by X-ray absorption. Track images are analyzed one at a time to assign them our best estimate of the impact point and the photo-electron emission direction. The polarization information is then recovered on a statistical basis starting from the azimuthal distribution of the reconstructed directions.Since the estimates of the photon absorption position and photo-electron emission direction are correlated, errors in the reconstruction of the former can create a bias in the latter. The coupling of this effect with gradients in the intensity of the X-ray flux can cause a polarization leakage that needs to be taken into account to recover the true polarization properties of the source. In this poster we describe the physical origin of this effect, introduce the relevant scaling with the PSF of the optics, the position resolution of the focal plane detectors and the morphology of the celestial source. We also present some real-life examples where the effect is important and report the status of the analysis tools that we are developing and that we plan to release to the Community to quantify the effect and correct it.

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