The NASA's Small Explorer (SMEX) mission "Imaging X-ray Polarimetry Explorer" (IXPE) is a pointing telescope exploiting the polarization state of the X-ray photons emitted by violent and dense astronomical objects (extreme gravitational, electric, magnetic fields), such as neutron stars and pulsar wind nebulae, as well as stellar and supermassive black holes. IXPE is an exclusive and bilateral collaboration between NASA and the Italian Space Agency (ASI), selected on January 3, 2017 and launched on December 9, 2021, at 6.00 UT, from the Kennedy Space Center in Cape Canaveral, Florida, USA, on board of a Falcon 9 (space X private company) rocket. The three identical X-ray telescopes and cameras onboard of the spacecraft provide imaging, energy, and timing data in the 2-8 keV band, with polarization-sensitive detectors (gas pixel detector GDP), and grazing-incidence optics for each one. The IXPE science team has a very long history in X-ray polarimetry (theoretical studies and first measurements in early '70s).

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Technical and science objectives: • improving polarization sensitivity by two orders of magnitude over the X-ray polarimeter onboard OSO-8; • providing simultaneous spectral, spatial, temporal X-ray measurements; • determining the geometry and emission mechanism of sources like pulsars (PSR) and pulsar wind nebulae (PWN), magnetars, accreting X-ray binaries, white dwarfs (WD), neutron stars (NS) and stellar-sized accreting black holes (BHs), microquasars, supernova remnants (SNR), the Milky Way Galactic center, radio-quiet active galactic nuclei (AGN), accreting supermassive black holes (SMBHs), blazars and radiogalaxies; • finding, in particular, the magnetic field configuration in magnetars and determining its magnitude; finding the mechanism for X rays production and geometry in pulsars (isolated and accreting), and determining how particles are accelerated in pulsar wind nebulae; • explore opportunities for coordinated observations with other, flying, X-ray satellites (Chandra, XMM-Newton, NuSTAR, SRG/eROSITA, Swift, NICER, MAXI, INTEGRAL, AstroSat, etc.), for broadband multifrequency studies and for multimessenger astro-particle physics with gravitational waves and cosmic neutrinos; • potential researches for fundamental physics effects in a such extreme astrophysical environments and systems, like the quantum gravity, birefringence, and new hypothetical exotic particles like the axion-like particles (ALPs).

The IXPE space observatory, an exclusive international NASA-ASI collaboration, took off punctually and successfully on December 9, 2021, at 6.00 UT, from historic Launch Complex 39 pad A (LC 39A) of the Kennedy Space Center in Cape Canaveral, Florida, USA, on board of a Falcon-9 rocket of the Space-X, private, company. After the launch acquisition of signal was confirmed at the Malindi ASI station, with the release of IXPE from the Falcon-9t 2nd stage and unfolding of the solar panels. The satellite is now placed on a circular equatorial orbit at an altitude of about 600 km with an inclination of only 0.2 degrees. After a week since the launch the 3.7-meter boom of IXPE deployed nominally and the fore and aft star-tracker heads acquired a solution, then spacecraft and instrument commissioning begin. About a month after launch, IXPE will be ready to begin its two-year primary science mission.

The IXPE space observatory is composed of three X-ray telescopes, with three detector units and GDP gas pixel detectors, funded by ASI and developed, built and calibrated by INFN (Turin and Pisa) and INAF-IAPS (Rome), exploit a technology developed over the last 15 years. It will provide simultaneous measures of polarization, time-variability, spectral and imaging properties. Polarized X-ray light carries unique details about where the energetic X-ray radiation comes from and what it passes through.
NASA and ASI, the USA and Italy, have a long tradition of bilateral cooperation on successful space missions, with IXPE representing another virtuous example, with this scientific instrumentation delivered on time, despite the challenge of the pandemic. Hundreds of engineers and scientists from more than 12 countries worked together to make IXPE a reality. The mission is led by principal investigator is Dr. Martin C. Weisskopf at NASA's Marshall Space Flight Center. Ball Aerospace is the main industry partner.