[2] After completing a postdoc at the University of Illinois at Urbana–Champaign studying black holes and neutron stars, he returned to SISSA as a research fellow, and ultimately an associate professor.
Together with collaborators in 2001, he has shown that r-modes in a neutron star will generate differential rotation, which could amplify the magnetic field and suppress the instability.
In 2003, he proposed that the QPOs that are measured in harmonic ratios in high-mass X-ray binaries can be explained simply in terms of trapped p-mode oscillations of an accretion torus around the black hole.
Starting from 2015, and as a member of the Executive Board of the collaboration Event Horizon Telescope, he has contributed to the international effort of producing the first image of a supermassive black hole at the center of the supergiant elliptical galaxy Messier 87 (M87).
Together with his group in Frankfurt, and using numerical simulations of plasma accreting onto black holes, he has contributed to the theoretical interpretation of the observational data.