Reflection in warped accretion disks around black holes

Project Information

REWARD

Grant agreement ID: 101107057

DOI

10.3030/101107057

Project terminated on 24 June 2025

EC signature date 21 April 2023

Start date 1 September 2023

End date 31 August 2025

Funded under

• Marie Skłodowska-Curie Actions (MSCA)

Total cost

No data

EU contribution

€ 172 750,08

Coordinated by

UNIVERSITA DEGLI STUDI DI MILANO
Italy

Periodic Reporting for period 1 - REWARD (Reflection in warped accretion disks around black holes)

Reporting period: 2023-09-01 to 2025-08-31

Summary of the context and overall objectives of the project

Black holes are among the most fascinating and extreme objects in the universe. They are formed when stars collapse at the end of their lives or lie at the centres of galaxies, where they can reach masses billions of times larger than the Sun. Despite being invisible, black holes profoundly shape their surroundings. On a small scale, they help us understand how stars live and die. On a large scale, they regulate the growth of galaxies and influence the structure of the universe itself.

Understanding black holes is therefore central to answering some of the biggest questions in modern astrophysics: How do galaxies evolve? How do stars end their lives? Do the laws of physics, as described by Einstein’s theory of relativity, hold true under the most extreme conditions in the cosmos? To make progress, scientists need to measure black holes more accurately. In particular, two key properties — their mass and their rate of rotation (spin) — are essential to connect theory with observations.

The main obstacle lies in the complex environment around black holes. Matter falling into them forms a disk of gas and, in many cases, powerful jets. But the exact shape and structure of these regions are still debated, and without this knowledge, current methods of measuring black hole properties remain uncertain.

A major breakthrough has recently become possible thanks to new space missions. For the first time, scientists can now measure the polarisation of X-ray radiations emitted by these objects — a property of light that is highly sensitive to geometry. In combination with radio observations of jets and with data from other telescopes, this opens up an unprecedented opportunity to reveal the structure of matter around black holes and to refine fundamental measurements.

The REWARD project was created to seize this opportunity. Its overall objective is to combine these new X-ray polarisation data with multiwavelength observations in order to clarify the geometry of the regions closest to black holes. At the same time, the project aims to build a broader picture of how black hole systems behave, avoiding the risk of drawing conclusions from only a few well-studied cases.

In this way, REWARD is not only advancing our knowledge of some of the most extreme objects in the universe, but also strengthening Europe’s role in a highly competitive area of space science. By aligning with recent international investments in new observatories, the project contributes to maximising the scientific return of these missions and prepares the ground for the next generation of astrophysical discoveries.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

During the REWARD project, my activities can be grouped into two main categories: exploiting X-ray polarisation observations to improve the current concept to model accreting black holes, and strengthening scientific collaborations and community interaction.

The first category focused on exploiting the potential of X-ray polarimetry to probe the geometry of accreting black holes. At the beginning of the fellowship, I joined the IXPE collaboration and gained the expertise required to carry out polarimetric analyses. This enabled me to contribute to several landmark studies reporting the first detections of X-ray polarisation in transient black hole binaries. These results opened a new observational window on the inner accretion flow and its connection to jet launching. Because black hole binaries are transient sources, it is essential to capture them during outbursts, when their polarisation properties evolve across accretion states. Within REWARD, I contributed to several observational campaigns, including GX 339-4, for which I successfully obtained IXPE observing time, and Swift J1727.8-1613 one of the brightest black hole binaries ever detected, which was observed across all accretion states of its outburst. Finally, I consolidated my long-term role in this area by joining the working group on strong gravity for the upcoming eXTP mission, ensuring continued involvement in next-generation X-ray polarisation studies.

The second category of work centred on expanding collaborations and strengthening ties within the high-energy astrophysics community. Within the BlackSTAR network, I contributed to investigations of the spectral and timing properties of black hole binaries, clarifying the interplay between different emission components. I was also part of a new Italian-led collaboration that obtained IXPE telescope time on GX 339-4, and I actively participated in the collaboration for the first analysis of black hole binaries using JWST data. These results mark a significant step in probing infrared emission and variability with next-generation instrumentation. The project also enabled me to carry out several research visits, including work at Universitat Autònoma de Barcelona (Spain) on the polarisation analysis of GX 339-4, at the Brera Observatory in Merate (Italy) on optical polarisation studies, and at the Institut de Planétologie et d’Astrophysique in Grenoble (France) on the spectral-polarimetric analysis of Swift J1727.8-1613.

Supervision and mentoring were also an integral part of the fellowship. At the University of Milan, I supervised undergraduate theses on X-ray and optical variability of accreting black holes, both of which produced novel results that will contribute to future publications. In addition, I mentored students through the Google Summer of Code programme, guiding them in the development of open-source astrophysical software. These activities not only advanced specific research objectives but also contributed to training the next generation of scientists and reinforced my own expertise in collaborative and open-source practices.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The REWARD project delivered results that significantly advance our understanding of accreting black holes, particularly through X-ray polarisation studies. A central achievement was the detection of polarisation in the black hole binary GX 339-4. The alignment of the polarisation vector perpendicular to the jet axis indicates an elongated corona orthogonal to the jet, offering a new perspective on the geometry of the inner accretion flow and its link to jet launching. This was the first low-mass X-ray binary where such an alignment was detected, and the result, published in the Astrophysical Journal Letters, challenges existing interpretations based on X-ray reverberation lags. Reconciling these findings with earlier models will require a change of paradigm in the theoretical framework used to describe black hole binaries.

Another important advance came from the extensive IXPE campaign on Swift J1727.8-1613 one of the brightest black hole binaries ever observed. This was the first system monitored across all accretion states of an outburst, yet it showed remarkably stable polarisation properties even as its spectral-timing characteristics changed significantly. The result suggests that the innermost accretion flow geometry may remain fixed throughout outburst evolution, posing a major challenge for current models. I contributed to four peer-reviewed articles reporting these findings. Observations of other systems, including Cygnus X-1 and IGR J17091-3624, further confirmed the difficulties in explaining polarisation degrees within existing theoretical frameworks, sparking a broader debate in the community on how to develop models capable of consistently accounting for all observables.

On the modelling side, a major achievement was the public release and improvement of the relativistic reverberation code RELTRANS. The model was extensively tested against other ray-tracing codes, refined in its treatment of relativistic effects, and made significantly more user-friendly through a dedicated GitHub repository. This open-source release ensures accessibility for the wider community and encourages further development. In parallel, I contributed to the creation of new tools such as StingrayExplorer and FlyingRay, designed to facilitate the analysis and visualisation of large volumes of X-ray binary data. FlyingRay, in particular, aims to provide a public database of spectral-timing features, supporting the construction of a general picture of black hole binary phenomenology beyond the study of individual sources.

Finally, REWARD also fostered dialogue between theoretical and observational communities through the organisation of an international workshop on accreting black holes at the University of Milan. With around 30 experts in attendance, the meeting highlighted the need for increasingly sophisticated models that combine multi-wavelength data with polarimetric results. The event reinforced my role as a bridge-builder between different research approaches and positioned me to contribute actively to shaping the next phase of black hole astrophysics.

X-ray and Optical polarisation aligned with the jet ejecta direction (background image) in GX 339-4

Schematic representation of emissions from accreting black holes