XIPE: the X-ray Imaging Polarimetry Explorer

Abstract: X-ray polarimetry, sometimes alone, and sometimes coupled to spectral and temporal variability measurements and to imaging, allows a wealth of physical phenomena in astrophysics to be studied. X-ray polarimetry investigates the acceleration process, for example, including those typical of magnetic reconnection in solar flares, but also emission in the strong magnetic fields of neutron stars and white dwarfs. It detects scattering in asymmetric structures such as accretion disks and columns, and in the so-called molecular torus and ionization cones. In addition, it allows fundamental physics in regimes of gravity and of magnetic field intensity not accessible to experiments on the Earth to be probed. Finally, models that describe fundamental interactions (e.g. quantum gravity and the extension of the Standard Model) can be tested. We describe in this paper the X-ray Imaging Polarimetry Explorer (XIPE), proposed in June 2012 to the first ESA call for a small mission with a launch in 2017 but not selected. XIPE is composed of two out of the three existing JET-X telescopes with two Gas Pixel Detectors (GPD) filled with a He-DME mixture at their focus and two additional GPDs filled with pressurized Ar-DME facing the sun. The Minimum Detectable Polarization is 14 % at 1 mCrab in 10E5 s (2-10 keV) and 0.6 % for an X10 class flare. The Half Energy Width, measured at PANTER X-ray test facility (MPE, Germany) with JET-X optics is 24 arcsec. XIPE takes advantage of a low-earth equatorial orbit with Malindi as down-link station and of a Mission Operation Center (MOC) at INPE (Brazil).

• The paper presents XIPE’s innovative integration of X-ray imaging and polarimetry, achieving a sensitivity with a Minimum Detectable Polarization as low as 0.6% for solar flares.
• It details the use of proven technologies like JET-X telescopes and Gas Pixel Detectors to investigate particle acceleration in supernova remnants and AGN jets.
• The mission design aims to test quantum electrodynamics and General Relativity effects in strong magnetic and gravitational fields, despite not being selected by ESA.

Overview of XIPE: The X-ray Imaging Polarimetry Explorer

The paper discusses the X-ray Imaging Polarimetry Explorer (XIPE), a mission proposed to the European Space Agency (ESA) for conducting advanced X-ray polarimetry of celestial sources and solar flares. X-ray polarimetry is a crucial yet underutilized observational method that can shed light on various astrophysical phenomena, including particle acceleration and strong magnetic fields, which cannot be adequately explored through imaging, spectrometry, or timing alone.

Design and Objectives

XIPE was designed to leverage the synergy between X-ray spectroscopic and polarimetric measurements alongside imaging capabilities, using existing infrastructure where possible. The mission's design found basis in previously developed technologies and missions, such as the POLARIX and GEMS. Key components include the JET-X telescopes and Gas Pixel Detectors (GPDs) that facilitate high-resolution imaging and effective polarimetric observation.

The payload comprises two kinds of X-ray detectors: the Efficient X-ray Photoelectric Polarimeters (EXP) and the Medium Energy Solar Polarimeter (MESP). These instruments are capable of conducting precise measurements of X-ray polarization, with forecasts of a Minimum Detectable Polarization (MDP) reaching 0.6% for an X10 class solar flare. This rigorous sensitivity allows investigators to probe phenomena close to acceleration sites, assess magnetic field structures in neutron stars and white dwarfs, and study scattering in asymmetric structures such as accretion disks.

Astrophysical and Fundamental Physics Applications

The scientific scope of XIPE covers several high-impact astrophysical questions and fundamental physics tests. Particular areas of focus include:

1. Acceleration phenomena: By analyzing supernova remnants and jets from active galactic nuclei (AGNs), XIPE aims to gather insights into cosmic ray acceleration processes and the synchrotron emissions occurring in these high-energy environments.
2. Emission in strong magnetic fields: The mission will detect polarized emissions due to intense magnetic fields in systems like accreting white dwarfs and neutron stars, shedding light on the geometry and energy-dependent polarization effects predicted by quantum electrodynamics (QED).
3. Scattering in aspherical situations: Investigations into scattering environments around X-ray binaries and active galactic nuclei, utilizing the data to elucidate geometric configurations within these systems.
4. Fundamental physics tests: XIPE provides the capability to examine QED effects, such as vacuum birefringence, in extreme magnetic fields, and explore General Relativity effects in strong gravitational fields of black holes. It can also search for quantum gravity signatures and axion-like particles, probing physics beyond the Standard Model.

Practical Implementation and Challenges

The mission design economizes using existing hardware, optimizing budget constraints while ensuring technical feasibility. However, despite its promise and preparedness, the XIPE mission proposal was not selected for ESA's small mission slot in 2017. The decision underlines the challenges posed by competitive selection processes in space research funding, where logistical and financial constraints are as decisive as scientific merit.

Conclusion and Future Perspectives

XIPE represents a well-conceived approach to addressing the demands for high-quality X-ray polarimetric data in contemporary astrophysics. The anticipated breakthroughs in understanding cosmic acceleration, magnetic field interactions, and fundamental physical processes underscore the mission's potential to significantly advance the field of X-ray astronomy. The progress made in the design and readiness of such a mission signals a promising direction, with the expectation that its primary objectives and infrastructure may be integrated into future spacecraft efforts, ensuring the continued pursuit of X-ray polarimetry in the quest for a deeper understanding of the universe.