Periodic Reporting for period 1 - DIANA (Disk Instabilities in Highly Accreting Neutron Stars)
Période du rapport: 2025-04-01 au 2027-03-31
Accretion disks are found throughout the Universe and power a large fraction of high-energy astrophysical sources, from neutron stars in our Galaxy to supermassive black holes at the centres of galaxies. These disks not only generate intense radiation, but also launch powerful outflows that strongly influence both their host systems and their wider environments. A striking example of this interaction is the production of highly collimated jets, in which matter is accelerated to a significant fraction of the speed of light.
Despite decades of study, many aspects of these systems remain poorly understood. We still do not know how the structure of the accretion flow changes as the luminosity evolves, nor do we understand the physical mechanisms that launch and power relativistic jets. These uncertainties limit our ability to quantify the fundamental role of accretion in shaping the evolution of the Universe.
In recent years, two observational techniques have emerged as true game changers for the study of compact objects: X-ray polarimetry and fast optical–infrared photometry. X-ray polarimetry provides powerful constraints on the geometry of the accretion flow, independently of the energy spectrum. This capability only became possible with the launch of the NASA–ASI Imaging X-ray Polarimetry Explorer (IXPE), whose detectors were developed at INAF-IAPS.
Fast optical–infrared photometry has, in parallel, revealed rapid non-thermal emission linked to the acceleration of electrons in the accretion flow and in the jet. By correlating sub-second variability at low energies with that observed in the X-rays, this technique offers an unprecedented way to probe the size, structure, and location of the emitting regions. In the last recent years, the development of new instruments of this kind has increased significant our understanding of these systems. In particular, the launch of the James Webb Space Telsescope (JWST) opened the possibility to study the mid infrared variability.
Accreting X-ray binaries in our Galaxy are particularly well suited to these photon-starved techniques thanks to their brightness and proximity, making them ideal laboratories for time-resolved and polarimetric studies.
The DIANA project is designed to take full advantage of these recent advances. Its goal is to combine X-ray polarimetry, fast optical–infrared observations, and multiwavelength data to clarify how accretion disks evolve, how jets are launched, and how energy is transported through these extreme systems.
Despite decades of study, many aspects of these systems remain poorly understood. We still do not know how the structure of the accretion flow changes as the luminosity evolves, nor do we understand the physical mechanisms that launch and power relativistic jets. These uncertainties limit our ability to quantify the fundamental role of accretion in shaping the evolution of the Universe.
In recent years, two observational techniques have emerged as true game changers for the study of compact objects: X-ray polarimetry and fast optical–infrared photometry. X-ray polarimetry provides powerful constraints on the geometry of the accretion flow, independently of the energy spectrum. This capability only became possible with the launch of the NASA–ASI Imaging X-ray Polarimetry Explorer (IXPE), whose detectors were developed at INAF-IAPS.
Fast optical–infrared photometry has, in parallel, revealed rapid non-thermal emission linked to the acceleration of electrons in the accretion flow and in the jet. By correlating sub-second variability at low energies with that observed in the X-rays, this technique offers an unprecedented way to probe the size, structure, and location of the emitting regions. In the last recent years, the development of new instruments of this kind has increased significant our understanding of these systems. In particular, the launch of the James Webb Space Telsescope (JWST) opened the possibility to study the mid infrared variability.
Accreting X-ray binaries in our Galaxy are particularly well suited to these photon-starved techniques thanks to their brightness and proximity, making them ideal laboratories for time-resolved and polarimetric studies.
The DIANA project is designed to take full advantage of these recent advances. Its goal is to combine X-ray polarimetry, fast optical–infrared observations, and multiwavelength data to clarify how accretion disks evolve, how jets are launched, and how energy is transported through these extreme systems.
During the DIANA project, I dedicated both time for training and for research. During my stay at INAF-IAPS I obtained an intensive training on X-ray polarimetry. I participated to the school on X-ray polarization at GSSI, during which I learned the basics of the technique and mastered the data analysis for IXPE. By participating to the regular group meetings (one every other week) I gained insight on the instrumental backgroud of the satellite; I learned the necessary features to assess the performance of an X-ray polarimeter. This opportunity also allowed me to gain insight on the various calibration/management activities behind a satellite.
I also investigated theoretical aspects on accretion discs. I participated to a workshop focused magneto-hydrodynamic in Warsaw, and worked on improving current models for highly accreting sources, where irradiation of the disc becomes a key component. In particular, I added the irradiation contribtuion to the radiation pressure instability code \lq\lq GLADIS" in order to quantify it's effect on the cyclic fast variability observed at high accretion rates. While this result will be soon made object of publication, my work shows that indeed irradiation significantly changed the shape on the variability. As a further development I plan to use this model for neutron stars, in which irradiation is even stronger than in BHs.
Regarding the research activity, thanks to my participation in the IXPE team, I participated to the observational campaigns of new transients that lead to key detections. My experience on X-ray timing revealed crucial in order to assess the properties of the accretion flow. Along with this, thanks to my observig time at telescopes such as the VLT, XMM-Newton and Nustar, I also managed to obtain multi-wavelength observations of active black hole transients, leading to strong constraints on their properties.
Last but not least, on the theoretical side I implemented the irradiation process in the radiation-pressure instability code GLADIS, and started working on including a boundary layer, to mimic the effect of a neutron star.
I also investigated theoretical aspects on accretion discs. I participated to a workshop focused magneto-hydrodynamic in Warsaw, and worked on improving current models for highly accreting sources, where irradiation of the disc becomes a key component. In particular, I added the irradiation contribtuion to the radiation pressure instability code \lq\lq GLADIS" in order to quantify it's effect on the cyclic fast variability observed at high accretion rates. While this result will be soon made object of publication, my work shows that indeed irradiation significantly changed the shape on the variability. As a further development I plan to use this model for neutron stars, in which irradiation is even stronger than in BHs.
Regarding the research activity, thanks to my participation in the IXPE team, I participated to the observational campaigns of new transients that lead to key detections. My experience on X-ray timing revealed crucial in order to assess the properties of the accretion flow. Along with this, thanks to my observig time at telescopes such as the VLT, XMM-Newton and Nustar, I also managed to obtain multi-wavelength observations of active black hole transients, leading to strong constraints on their properties.
Last but not least, on the theoretical side I implemented the irradiation process in the radiation-pressure instability code GLADIS, and started working on including a boundary layer, to mimic the effect of a neutron star.
The DIANA project delivered advances that push beyond the current state of the art in three major areas: multi-wavelength constraints on accretion flows, mid-infrared views of compact objects, and theoretical modelling of radiation-pressure–dominated disks in neutron stars.
A first key achievement was the combination of fast infrared variability with time-dependent X-ray polarimetry of accreting compact objects—an approach that had never been attempted before. Although Sco X-1 exhibited very low X-ray polarization, the fast IR analysis revealed strong sub-second variability, and the lack of correlated polarization changes places the tightest constraints so far on short-timescale geometric fluctuations in a neutron-star accretion flow. This result challenges the expectation—based on initial expectations—that radiation-pressure–dominated inner disks should produce detectable polarization swings. DIANA therefore provides evidence that the geometry of bright neutron-star systems is more stable than previously assumed.
Beyond neutron stars, DIANA contributed to ground-breaking IXPE campaigns on two black hole transients (MAXI J1744-294 and GRS 1739–278). These observations yielded some of the clearest constraints to date on the disc inclination and black hole spin derived from X-ray polarization. The measurements reveal a coherent picture of the inner flow geometry in these systems and strengthen the case that polarimetry can break degeneracies present in spectral modelling alone. Together with the broader IXPE collaboration, these results are helping reshape the theoretical framework used to describe the corona–jet configuration in black hole X-ray binaries.
The project also produced results that significantly extend the multi-wavelength view of compact objects, enabled by the first mid-infrared observations of X-ray binaries with JWST. DIANA contributed to the discovery of a powerful outflow in quiescence from A0620–00, the first of its kind ever detected in a stellar-mass black hole system, and to the identification of synchrotron emission in the soft state of GX 339–4, a result that overturns the textbook expectation that jets are fully quenched in disc-dominated phases. These findings demonstrate that the mid-IR band hosts rich, previously inaccessible diagnostics of jet and outflow physics.
On the theoretical side, DIANA delivered a major step forward by developing the first version of GLADIS-NS, a radiation-pressure–instability code tailored for accreting neutron stars. The project successfully incorporated X-ray irradiation of the disk, a key physical ingredient that had never been included in instability models for neutron stars. Preliminary simulations show that irradiation can strongly reshape the predicted variability patterns, opening a new pathway to connect theoretical predictions with observations from IXPE, JWST, and high-speed infrared instruments. The code will be made openly available, ensuring long-term impact and community uptake.
Finally, through workshops, invited talks, and close collaboration with IXPE and JWST teams, DIANA helped stimulate dialogue between observers and theorists working on accretion physics. The project has therefore not only produced new scientific results but also strengthened the methodological and collaborative foundations needed for the next generation of multi-wavelength and polarimetric studies of compact objects.
A first key achievement was the combination of fast infrared variability with time-dependent X-ray polarimetry of accreting compact objects—an approach that had never been attempted before. Although Sco X-1 exhibited very low X-ray polarization, the fast IR analysis revealed strong sub-second variability, and the lack of correlated polarization changes places the tightest constraints so far on short-timescale geometric fluctuations in a neutron-star accretion flow. This result challenges the expectation—based on initial expectations—that radiation-pressure–dominated inner disks should produce detectable polarization swings. DIANA therefore provides evidence that the geometry of bright neutron-star systems is more stable than previously assumed.
Beyond neutron stars, DIANA contributed to ground-breaking IXPE campaigns on two black hole transients (MAXI J1744-294 and GRS 1739–278). These observations yielded some of the clearest constraints to date on the disc inclination and black hole spin derived from X-ray polarization. The measurements reveal a coherent picture of the inner flow geometry in these systems and strengthen the case that polarimetry can break degeneracies present in spectral modelling alone. Together with the broader IXPE collaboration, these results are helping reshape the theoretical framework used to describe the corona–jet configuration in black hole X-ray binaries.
The project also produced results that significantly extend the multi-wavelength view of compact objects, enabled by the first mid-infrared observations of X-ray binaries with JWST. DIANA contributed to the discovery of a powerful outflow in quiescence from A0620–00, the first of its kind ever detected in a stellar-mass black hole system, and to the identification of synchrotron emission in the soft state of GX 339–4, a result that overturns the textbook expectation that jets are fully quenched in disc-dominated phases. These findings demonstrate that the mid-IR band hosts rich, previously inaccessible diagnostics of jet and outflow physics.
On the theoretical side, DIANA delivered a major step forward by developing the first version of GLADIS-NS, a radiation-pressure–instability code tailored for accreting neutron stars. The project successfully incorporated X-ray irradiation of the disk, a key physical ingredient that had never been included in instability models for neutron stars. Preliminary simulations show that irradiation can strongly reshape the predicted variability patterns, opening a new pathway to connect theoretical predictions with observations from IXPE, JWST, and high-speed infrared instruments. The code will be made openly available, ensuring long-term impact and community uptake.
Finally, through workshops, invited talks, and close collaboration with IXPE and JWST teams, DIANA helped stimulate dialogue between observers and theorists working on accretion physics. The project has therefore not only produced new scientific results but also strengthened the methodological and collaborative foundations needed for the next generation of multi-wavelength and polarimetric studies of compact objects.