What is IXPE
IXPE, short for Imaging X-ray Polarimetry Explorer, is a NASA mission that launched a new telescope into orbit on December 9, 2021, aboard a Falcon-9 rocket from the American company SpaceX. IXPE belongs to NASA’s class of missions called SMall EXplorer (SMEX), designed to test a single new technological concept in orbit for the first time, while leveraging the vast expertise from other missions. The technological innovation brought by IXPE is fundamentally one: measuring the polarization of X-rays from certain types of highly energetic cosmic objects such as supermassive black holes at the centers of galaxies (billions of times the mass of the Sun) or tiny, extremely dense neutron stars (the mass of the Sun in just 20 km in diameter). The X-ray polarization that IXPE will be able to detect and measure will be crucial in determining which theoretical models are most valid for explaining the physics of these extreme objects. There are various theories in the field, but to date, this type of fundamental data is still missing to highlight one in particular.
The role of the National Institute for Astrophysics
Italy participates in IXPE as NASA’s sole partner through a partnership coordinated and funded by the Italian Space Agency (ASI), involving the National Institute for Astrophysics (INAF), the National Institute for Nuclear Physics (INFN), the University of Rome 3, and the private company OHB. Italy provided the core of the scientific mission, namely the detectors capable of recognizing the polarization of X-rays from celestial objects that the satellite will observe during its expected 3-year nominal lifespan. In total, the Italian team comprises nearly a hundred people, with thirty of them – including the scientific leader Paolo Soffitta – being INAF personnel, and in particular, four represent the INAF team from Cagliari: Alessio Trois, Matteo Bachetti, Maura Pilia, and Andrea Possenti.
Alessio Trois, a technological researcher from Cagliari, trained at the space astrophysics institutes in Milan and Rome before moving to Cagliari to work on another entirely Italian scientific mission, the AGILE high-energy astrophysics satellite. His contribution to the IXPE mission involves developing the scientific instrument (the set of detectors for studying X-ray polarization and the computer that manages them) as the person responsible for assembly, integration, verification, and testing, as well as overseeing the operations involving the scientific instrumentation provided by Italy from launch onwards.
“Delivering the IXPE instrument to our American colleagues in such a short time, just 3 years,” explains Trois, “was a success made possible only by the extraordinary organizational synergy of all the institutes involved. As INAF Cagliari, we made a substantial contribution in defining the onboard computer’s logic, integrating and testing the instrument, as well as its physical integration on the satellite module that will carry it into orbit and the qualification campaign of the entire observatory, composed of three identical and independent telescopes. We also support the in-flight operations of the instrument, which will be managed directly by the American industry.”
“Preparing a space mission,” adds Trois, “requires obsessive attention to detail. The costs for launching space telescopes are extremely high. Moreover, with the sole exception of the Hubble telescope, no mission is designed to be visited and repaired in orbit. Therefore, every transistor, every weld, every rivet must have a purpose and function perfectly. Everything must be validated, tested, and verified before launch to ensure that the satellite can survive the extreme conditions it will face from launch to its operational phase in orbit around the Earth, when it will be exposed to gamma rays and extreme thermal fluctuations.”
The instruments must therefore be tested and calibrated with meticulous care, which requires months of work (about 12 for IXPE) in hyper-protected environments, the so-called “clean rooms.” The analysis of all these delicate preparations is carried out with dedicated software that is specifically written and must also be thoroughly tested.
And writing the software has been (and will be) crucial not only for validating the technical part of the instrument assembly but also for assessing which observations can maximize the mission’s impact. To do this, INAF researcher Maura Pilia – a native of Villasalto and trained in Cagliari, Bologna, Milan, and the Netherlands – has worked extensively on programming and simulating scenarios and mathematical models to apply to some of IXPE’s preferred celestial objects: millisecond pulsars. These neutron stars rotate (and consequently “pulse”) at incredible speeds: up to more than 700 rotations per second, which is why they are considered the ultimate cosmic clocks. The polarized X-ray signals that IXPE will be able to observe will complement observations made in wavelengths other than X-rays: from weaker radio waves to even more energetic gamma rays, wavelengths well known to Maura Pilia thanks to years of observations with various types of telescopes. “These simulations,” Pilia specifies, “are not only essential for understanding which sources are optimal for observations with IXPE and thus establishing observational priorities, but they also allow us to be prepared for the actual observations, having already set up the tools for data analysis and models for comparison. We expect many discoveries and want to be able to communicate them fully and as soon as possible!”
No ground-based telescope can capture X-rays from space, as our atmosphere fortunately protects us from these radiations. Therefore, the only way to observe them is to send satellites into orbit. In 2012, NASA launched the NuSTAR mission, in the same class as IXPE, which has faced many challenges and technological improvements over the years that IXPE can now benefit from thanks to Matteo Bachetti’s role. The researcher from Selargius has experience in France and the United States and participated in the pre-launch calibration and commissioning of NuSTAR, becoming a reference point for temporal analyses with the satellite, even winning a medal awarded by NASA and an honor from his hometown. Today, he brings this experience to IXPE: “For example, to process each X-ray captured by NuSTAR and IXPE,” Bachetti explains, “the detector shuts down for a very brief interval, a phenomenon called dead time. This creates problems in searching for periodic signals in data from very bright sources. Over time, we have developed strategies to circumvent these problems in NuSTAR that will be useful for IXPE.”
Among the first supporters of the involvement of the Sardinian research center in the project is astrophysicist Andrea Possenti, the lead researcher and coordinator of INAF-OAC’s scientific activities related to the mission: “After the excitement of the launch,” Possenti states, “scientific discoveries will soon follow, for example in the field of magnetar stars, which are the most magnetized objects in the Universe. Observing polarized X-rays can reveal the geometry of these magnetic fields, a million billion times more intense than that of our Earth.”
The participation of the Cagliari Astronomical Observatory in the IXPE project was not accidental and represents a significant milestone because it is the first time that INAF-OAC has directly contributed to the preparation of an international space mission. This achievement was made possible by the visionary Sardinia Radio Telescope (SRT) project, desired and realized by the late INAF president, Nichi D’Amico. The Sardinian radio telescope has played a dual role. On one hand, it has facilitated the return of numerous Italian researchers and technologists working outside Sardinia while attracting many researchers from around the world (South Africa, China, Pakistan, Australia, the United States, to name a few) and with them their technological and scientific knowledge in various fields. Additionally, the construction of SRT has strengthened the collaboration between the Italian Space Agency and INAF, which share the same instrument on Sardinian soil, constantly improving it to their mutual advantage.
Concludes INAF-OAC Director, Emilio Molinari: “It is a very exciting and rewarding moment for the Cagliari Astronomical Observatory, which has confirmed itself as a driving force of excellence for the entire region. It is capable of offering opportunities for young researchers and consequently developing highly valuable expertise on the island, capable of playing leading roles in world-class scientific endeavors.”
IXPE, for its part, represents a dual significance for INAF Cagliari: a great achievement but also a springboard for further important missions, such as the major Sino-European space collaboration eXTP (enhanced X-ray Timing and Polarimetry mission), which will likely bring numerous Chinese researchers to Cagliari. However, this is another story; now it’s time for Sardinia to celebrate as a protagonist for the long-awaited launch of IXPE.