Tomaso Belloni passed away suddenly on August 26th. He was a leading scientist of the Italian astrophysics community and was well known internationally for his work on compact objects in the X-ray and gamma-ray bands. He had graduated at University of Milan, and continued his activity first at the Max Planck Institute of Garching (MPE) and then at the University of Amsterdam. He came back to Italy in 1999, and was Research Director at the Brera Astronomical Observatory.

Sara Motta & Luigi Stella

Figure 1: Tomaso Belloni. Credits: Melania Del Santo.

Mourning sometimes calls for silence, but Tomaso loved words, written, read, spoken,discussed, recited. He loved to laugh and to make people laugh. Having given so many talks - none of which was trivial - he resented highfalutin and rhetorical speeches. With this short note we intend to outline his style, that of a person of rare intelligence, occasionally rough, but certainly unconventional, full of passion and explosive enthusiasm, introverted and open at the same time.

Figure 2: Tomaso Belloni. Credits: Luigi Stella.

Tomaso used to discuss everything with everyone. He had strong and clear opinions, but also an intellectual honesty that few can boast: he rigorously applied his analytic approach to everything he encountered, and he was as insistent and sure of himself in arguing his theses as he was ready to admit his mistakes. And as soon as he discovered that he had supported the ”wrong” thesis, he changed his mind and did that with a smile, almost with joy.
He was recognized among the leading experts at the highest international level on his subjects. His huge scientific production has its core in the study and characterization of the variability in X-rays of stellar-mass black holes accreting star from a companion star. The very rapid quasi-periodic flux oscillations, the rich variety of emission modes of some of these systems, the variability classes identified in the GRS 1915+105 system and the characteristic variation to ”q” (or ”turtle-head” as Tomaso jokingly liked to call it) of the hardness of the energy spectrum as a function of intensity of transient black holes (subsequently identified also in other classes of accreting compact objects) are among the best known results inextricably linked to his work and his name. An anonymous reviewer of a project of his once described him as one of the "best minds" in the field.

Figure 3: "Surfing in LA", shot by Tomaso in Los Angeles. From Flickr (https://flic.kr/p/MAGkNm).

Some Fourier spectral modelling techniques he developed are now part of the basic knowledge of researchers and students working on fast timing studies. Nevertheless, Tomaso’s scientific activity extended very far: he studied pulsars of the most diverse classes, accreting neutron stars or isolated, magnetars, white dwarfs in binary systems, as well as the enigmatic ”ultraluminous” sources of X-rays in nearby galaxies. He laid the foundations to establish important analogies between the variability characteristics of black holes and neutron stars in X-ray binaries, and for the understanding of the conditions behind the emission of plasma winds and jets at very high speed from these systems.
Tomaso has been very involved in the activities of some of the most important X-ray
satellites, both under development (such as LOFT and eXTP), and in orbit. Among these successful missions, some of which made the history of X-ray astronomy, we recall ROSAT, for which Tomaso had worked at MPE since the early 1990s; RXTE, which he worked on for years while in Amsterdam at the Anton Pannekoek Institute for Astronomy and then later, when he moved definitively to Merate in a permanent position at the Brera Astronomical Observatory; INTEGRAL, for which Tomaso was a member of the Integral User Group and served on several occasions as chair of the Time Allocation Committee; AstroSAT, for which he wrote and maintained the official timing analysis software, GHATS. Tomaso collaborated with various Indian institutes, above all IUCAA, where the Astrosat operations center is located: he often visited IUCAA, establishing long standing collaborations with countless colleagues and students. Tomaso was also visiting professor at the University of Southampton, where he regularly spent time, and often gave his well-regarded lectures on timing, which many of us had the pleasure of attending at least once.

Figure 4: ”Looking ahead”, shot by Tomaso in Mumbai. From Flickr (https://flic.kr/p/21adXiY).

Tomaso was in fact an excellent speaker and popularizer, he loved the stage and knew how to keep it: in his talks he even managed to make the seemingly dry mathematics of Fourier techniques interesting. In a similar way he intrigued and fascinated the public of all ages talking about the strange features of black holes and dark matter in his outreach lectures as well as in the popular books which he authored. He took care of every detail in his presentations, which were as beautiful as they were clear and essential. He often taught the younger ones how to prepare their own presentations, recounting with amused horror about that single slide filled with over a hundred figures he had seen projected at a conference, or the jumble of colors and fonts from some presentation he had attended.
In recent years Tomaso assiduously devoted himself to institutional activities both within INAF and in other institutions. He was vice-president of the INAF Scientific Council, and during his term he worked hard for the institution with the enthusiasm and determination that distinguished him. He was a member of COSPAR and chair of the E panel ”Research in Astrophysics commission from Space”. He had a key role in the Scientific Assembly of Athens in 2022, and more recently he was fundamental - in his capacity as chair of the Scientific Organizing Committee - in laying out the initial proposal and organising the next Assembly, to be held in Florence in 2026. Tomaso was also an esteemed and very active member of the D Division (”High Energy Phenomena and Fundamental Physics”) of the International Astronomical Union.
Through decades of avid reading, ranging from novels to poetry, from art to all kinds of science, technologies and techniques, from the human sciences to statistics (but not the Bayesian one, which he did not like one bit!) Tomaso ammassed a boundless culture, maintained thanks to his prodigious memory and shared with spontaneity and irony. Novels, architecture, comics, economics, new discoveries, along with politics and football were among the topics that were examined every morning systematically and noisily argued in the corridors of the Observatory. Tomaso used to have work-related conversations with students and colleagues in his office sitting in front of a very bright window, unaware of the fact that they could not see his features against the light. When someone pointed that out to him he laughed out loud and changed his sitting place thereafter. Tomaso loved all forms of art, and was a passionate enthusiast. He had great sensitivity for music, which he knew about also in technical aspects. He was a refined connoisseur of classical authors, but he ranged over all genres: widely diverse literature excerpts resounded in his office, and pervaded the adjacent corridors. He was himself an artist, as testified by the massive body of photographs in which he captured the hidden beauty of unexpected places andsubjects with powerful shots, following light playing on lines, volumes and colours. In 2017, a series of his shots taken in the Sicilian city of Cefalù was exhibited in a town hall exhibit. We also like to remember him as semi-official photographer of many science meetings and workshops: many beautiful portraits were the result of his wandering among the participants during breaks, social dinners and evening entertainment with his camera in hand. Sometimes he carried also a tripod: it was for the self-timer, he too wanted to be in the group picture.

Your empty place at the table
talks narrates chats laughs out loud
...
(Vivian Lamarque)

Figure 10: Tomaso Belloni. Credits: Luigi Stella.