Neutron stars—the ultra-dense corpses of dead stars—are surrounded by atmospheres of superheated plasma, and a new computer code called MAUS will simulate how those atmospheres behave in magnetic fields a trillion times stronger than any on Earth. Existing codes are outdated and too slow to handle the full range of physical effects now visible with modern X-ray telescopes. Recent observations from NASA’s IXPE mission have revealed quantum-electrodynamic phenomena—such as vacuum polarisation and magnetic condensation—that older models cannot reproduce. Without a new tool, astronomers cannot properly interpret the data already collected by XMM-Newton, NuSTAR, and Chandra, nor prepare for next-generation observatories like Athena. MAUS will systematically model neutron-star atmospheres across a wide range of parameters, re-analysing all existing spectral, timing, and polarimetric data. It will also generate synthetic training data for AI and machine-learning applications, helping to design future X-ray missions. The code will be released publicly with a user-friendly interface, creating a lasting resource for the entire astrophysical community. This is fundamental science. It will not change a single power grid or medical scanner tomorrow. But understanding how matter and radiation behave under extreme magnetic fields tests the limits of quantum electrodynamics—and historically, such tests have reshaped physics in unexpected ways.
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We aim at carrying the first ever systematic numerical investigation of neutron stars (NSs) atmospheres embedded in quantising magnetic fields (B = 4 x1013 G), by developing MAUS (Magnetised Atmosphere Universal Simulator) a fast and robust numerical algorithm using state-of-the art knowledge of opacities and QED effects. The proposal is needed: the few existing codes dealing with this regime, including one by our team, rely on different microphysical inputs and are often built on outdated architectures which make them unsuitable for a systematic, fast modelling of NSs atmospheres exploring wide range of parameters. Despite these limitations, available atmospheric models have been used for the last two decades to interpret NSs spectral and timing data. However, the advent of X-ray polarimetry and the recent IXPE magnetar observations, have shown that previously unfathomable physical effects are now accessible to observations, including QED vacuum polarisation, particle back bombardment and magnetic condensation. These effects are also expected to modify the spectral properties (in particular regarding detectability of line emission, which will be accessible with future X-ray observatories such as Athena). Combining polarimetry with spectral and timing information can therefore reveal exquisite details on the physics of the atmospheric plasma around these compact objects, pushing our knowledge of quantising electron dynamics to a higher gear. The need for a novel atmospheric code is urgent: MAUS will allow us for the systematic investigation and re-analysis of all existing spectral/timing (XMM-Newton, Nustar, Chandra, etc..) and polarimetric (IXPE, and future XpoSat) data of currently observed thernally emitting NSs, available from different observatories. This has the potential to revolutionise our physical understanding of NS atmospheres. It will make it possible to perform systematic simulations and also generate simulated training data for future AI/ML applications, for the spectral, timing and polarimetric signal. This will inform the scientific development of future X-ray observatories (Athena, GoSoX, Colibri, eXTP, and potentially many others). We are in a unique position to carry out this challenging project, thanks to our long standing expertise in the field of radiative transfer in strong fields. The team of proposers is deeply involved in the design and scientific development of several of the above-mentioned missions, which will ensure maximum dissemination of the results within the relevant teams. We plan to make MAUS public in a user-friendly interface, and therefore to build a long standing legacy for the entire X-ray community.
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