THE IMAGING PROPERTIES OF THE GAS PIXEL DETECTOR AS A FOCAL PLANE POLARIMETER

ABSTRACTX-rays are particularly suited to probe the physics of extreme objects. However, despite the enormousimprovements of X-ray Astronomyin imaging, spectroscopyand timing, polarimetryremains largelyunexplored. We propose the photoelectric polarimeterGas Pixel Detector (GPD) as an instrumentcandidate to fill the gap of more than thirty years of lack of measurements. The GPD, in the focusof a telescope, will increase the sensitivity of orders of magnitude. Moreover, since it can measurethe energy, the position, the arrival time and the polarization angle of every single photon, allows toperform polarimetryof subsets of data singled out from the spectrum, the light curve or the imageof source. The GPD has an intrinsic very fine imaging capability and in this work we report on thecalibrationcampaign carriedout in 2012at the PANTER X-raytest facility of the Max-Planck-Institutfu¨r extraterrestrische Physik of Garching (Germany) in which, for the first time, we coupled it to aJET-X optics module with a focal length of 3.5 m and an angular resolutionof 18 arcsec at 4.5 keV.This configuration was proposed in 2012 aboard the X-ray Imaging PolarimetryExplorer (XIPE) inresponse to the ESA call for a small mission. We derived the imaging and polarimetric performancefor extended sources like Pulsar Wind Nebulaeand Supernova Remnantsas case studies for the XIPEconfiguration, discussing also possible improvements by coupling the detector with advanced optics,having finer angular resolutionand larger effective area, to study with more details extended objects.Keywords: X-ray polarimetry, X-ray telescope, angular resolution