Compton polarimeters are typically designed to be sensitive only to the azimuthal angle of the scattered photon, ignoring the scattering angle. Such a 2-dimensional reconstruction of the event is pursued for both simplicity and because the polarization of the incident photon influences only the azimuthal response of the instrument. While this is true for on-axis sources, when the source starts to be off-axis of several degrees the azimuthal response of the instrument is effectively a convolution of the azimuthal and polar scattering angles: measuring the latter would provide a better sensitivity and smaller systematic effects. In this contribution, we will present a design which allows to estimate the scattering angle in a Compton polarimeter through the read-out of the light signal at the two ends of scintillator bars. Such a design is being tested with a representative set-up and first results on the performance are presented.
A new generation of 3D gas detectors is available and applied today, for example, to track particles in accelerator vertex detectors. They apply the concept of Time Projection Chambers to 2D devices. When coupled with a gas system, Timepix3, or the most recent Timepix4, allows for 3D track reconstruction. Such capability promises to improve by far the sensitivity of the present 2D Gas Pixel Detector (GPD) on board the Imaging X-ray Polarimeter Explorer (IXPE), also solving the issue of its large dead time thanks to its parallel read-out. In this framework, we are developing a new simulation software based on the Geant4 tool to be used to develop these devices as a new generation of X-ray polarimeters for astrophysics. Moreover, a new read-out software capable of obtaining a 3D track reconstruction, taking advantage of the Timepix3 time capabilities and using the IXPE GPD legacy, has been developed, which helps improve the polarimetric sensitivity in view of future X-ray missions having on board this kind of detectors.
IXPE has been a highly successful mission, opening a new window in X-ray astronomy. IXPE observations have highlighted the importance of polarimetry along with spectroscopy in determining the geometry and physics behind many high-energy emissions from black hole X-ray binaries (BHXRBs), Pulsar Wind Nebulae (PWN), Active Galactic Nuclei (AGN) etc. However, IXPE is just the first step towards future wide band (0.1 to 100 keV) X-ray polarimetry. The future of this field demands larger effective areas, better energy resolution, and broader energy bands. IXPE is barely capable to address key scientific cases such as reflection features in X-ray binaries, molecular clouds around the Galactic Center, radio-quiet AGNs, non-thermal emission regions in supernova remnants etc. To take advantage of the recent advances in X-ray optics, gaseous detectors with different thickness, pressures and gas mixtures would be required. Using next-generation ASICs, like Timepix3, it is possible to have parallel fast readout, providing simultaneous time and charge information for each pixel, enabling 3D imaging of photoelectron tracks. In this article, we explore such a possibility using GridPix detectors.
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