EXPO: a quantum leap in fast, wide-band x-ray polarimetry for astrophysics

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Abstract
The Enhanced X-ray Polarimetry Observatory (EXPO) is a mission concept proposed to ESA as an M8 candidate, with a prospective launch around 2041. Building on the scientific success of the Imaging X-ray Polarimetry Explorer (IXPE), EXPO is designed not only to overcome its two main limitations, the narrow 2–8 keV energy band and the very slow repointing time, but also to unlock new scientific capabilities. A wide energy band and fast repointing are essential for addressing key questions in high-energy astrophysics, including the origin of the hard X-ray emission in magnetars and black-hole binaries, particle acceleration in supernova remnants and pulsar-wind nebulae, radiative transfer in highly magnetized plasmas, X-ray reflection in accretion flows and active galactic nuclei, and the prompt and afterglow emission of gamma-ray bursts and of magnetar flares. The payload combines broadband imaging X-ray polarimetry with rapid time-domain capability. Five focusing X-ray telescopes employ gas photoelectric polarimeters based on the Timepix ASIC family with InGrid amplification, enabling true three-dimensional track reconstruction and operation over the 2–35 keV band through optimized low- and medium-energy detector configurations. The mirror modules are based on proven electroformed nickel technology with Au/C coatings and an XMM-like focal length of 7.5 m. The polarimeters are complemented by three dedicated detection systems: (i) a coded-mask Wide Field Instrument (WFI), derived from SVOM /ECLAIRs, providing continuous monitoring of a ∼2 sr field of view and autonomous transient localization; (ii) a Spectral Imaging Camera (SIC), based on stacked CMOS and CdTe detectors, providing simultaneous broadband imaging spectroscopy for accurate spectro-polarimetric decomposition; and (iii) an Instrument Control Unit (ICU), responsible for payload management, onboard WFI image reconstruction, transient identification, and autonomous spacecraft repointing. Together, these capabilities will extend X-ray polarimetry into the hard X-ray domain and open a new observational window on fast transients, time-domain astrophysics, and multi-messenger astronomy. This paper presents the concept of the mission EXPO, its payload architecture, and the scientific opportunities provided by its unique combination of broadband polarimetry and rapid response.

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