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Programs of X-ray polarimetry in Italy arise from the convergence of a long experience of X-ray astronomy missions with an outstanding tradition in development of radiation detectors. The gas pixel detector in the focus of X-ray optics can perform angular-resolved polarimetry with a breakthrough improvement in sensitivity, even with a moderate collecting surface. POLARIX makes a large use of already existing items and, in particular, of the three telescopes from the JET-X program. It can extend the X-ray polarimetry from one positive detection only, to tens of sources, including a few brighter extragalactics: an ambitious pathfinder on a very limited budget. Phase A study of POLARIX, and of four other missions, was performed in 2008 and ASI should select two missions to fly. Another pathfinder is under study: two short telescopes, designed with modern tight packing techniques, mounted as piggy-back on the Chinese mission HXMT.
The national context
X-ray polarimetry has been a line of research at IASF for many years. A scattering polarimeter was proposed for the XMM mission, but was not selected. An IASF team joined the collaboration headed by R. Novick for the Stellar X-ray Polarimeter, aboard the Spectrum X-Gamma mission, contributing the detectors as the sensors for both scattering and diffraction stages. SXRP was completed but the whole mission was not. Considering the large area of the telescope, SXRP was close to the best achievable with conventional techniques.
The advent of a new generation of X-ray polarimeters based on the photoelectric effect poses the problem of their calibration. We devised and built a calibration facility aimed at the study of the performances of photoelectric X-ray polarimeters such as the Gas Pixel Detector (GPD). The calibration facility exploits the 45° Bragg diffraction from crystals of both X-ray lines characteristic of X-ray tubes and from continuum. A set of linear and rotary stages allows the GPD to be calibrated on its whole surface. We successfully tested the GPD filled with a mixture of He-DME 30-70 at one atmosphere. We measured the modulation factor at 2.69 keV and 4.51 keV. We also studied the homogeneity of the modulation factor, of the angular phase and of the position reconstruction capability on the surface of the GPD.
Since early 2000 we have been developing true 2-D X-ray polarimeters based on the photoelectric effect. The Gas Pixel Detector (GPD), an evolution based on the use of an ASICCMOS readout chip, has been calibrated at energies greater then 5 keV with a Thomson based X-ray polarizer. However, the sensitivity of the GPD at the focus of conventional X-ray optics peaks below 5 keV. In this regime the photoelectric effect severely competes with Thomson scattering. For this reason we developed a facility aimed at the production of polarized X-rays in the few keV band.
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