On the Operation of X-Ray Polarimeters With a Large Field of View

Abstract

The measurement of the linear polarization is one of the hot topics of High Energy Astrophysics. Gas detectors based on photoelectric effect have paved the way for the design of sensitive instruments and mission proposals based on them have been presented in the last few years in the energy range from about 2 keV to a few tens of keV. As well, a number of polarimeters based on Compton scattering are approved or discussed for launch on-board balloons or space satellites at higher energies. These instruments are typically dedicated to pointed observations with narrow field of view telescopes or collimators, but there are also projects aimed at the polarimetry of bright transient sources, like Soft Gamma Repeaters or the prompt emission of Gamma Ray Bursts. Given the erratic appearance of such events in the sky, these polarimeters have a large field of view to catch a reasonable number of them and, as a result, photons may impinge on the detector off-axis. This changes dramatically the response of the instrument to polarization, regardless if photoabsorption or Compton scattering is involved. Instead of the simple cosine square dependency expected for polarized photons which are incident on-axis, the response is never purely cosinusoidal and a systematic modulation appears also for unpolarized radiation. We investigate the origin of these differences and present an analytic treatment which proves that actually such systematic effects are the natural consequence of how current instruments operate. Our analysis provides the expected response of photoelectric or Compton polarimeters to photons impinging with any inclination and state of polarization.

Science prospects for SPHiNX – A small satellite GRB polarimetry mission

“…This is achieved using computer simulations validated using both polarised and unpolarised beams in the laboratory [41]. A particular challenge for large field-of-view polarimeters, such as SPHiNX, is that the polarimetric response depends on the incidence angle and energy spectrum of the GRB [42]. SPHiNX is designed to allow these parameters to be reconstructed.…”

Imaging Polarimeter for a Sub-MeV Gamma-Ray All-sky Survey Using an Electron-tracking Compton Camera

“…Therefore, the degree of polarization of the incident photon is obtained from µ/µ 100 , where µ 100 is the modulation factor for 100% linearly polarized incident photons and is often used as an instrumental analyzing power for polarization. When the incident photon has an incident angle of δ relative to the optical axis of the detector (in the case of off-axis incidence), N (φ) is affected by a complicated dependence not only on the polarization but also on the incident direction and energy, and therefore it no longer follows a cos(2φ) curve (Lei et al 1997;Muleri 2014). To obtain a distribution that follows the form of Equation (3) for all incident angles, we need to move to the photon coordinate system, X p Y p Z p , which includes the Z p -axis along the incident direction and the X p -Y p plane perpendicular to the Z p -axis, as shown in Figure 1(b).…”

A high-energy Compton polarimeter for the POET SMEX mission

“…This would permit POET to determine its own GRB locations without requiring a dedicated instrument (such as TRALE for PETS). It would also provide added redundancy for the polarization measurements, since the polarization response of each HEP module would be slightly different due to the different incidence angles 11 . We will replicate the GRB polarization sensitivity calculation performed for TRAP (Fig.…”