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Our research on black holes

Imaging the event horizon of black holes

The defining feature of a black hole is the event horizon. Recent developments in mm-wave astronomy have made it possible to image the event horizon of the two largest black holes on the sky: Sgr A*, and the one in M87. Precise measurements of the black hole shadow confirm one of the most basic predictions of general relativity and allow one to test and constrain numerous alternative theories of gravity. Comparing the data with detailed numerical magneto- hydrodynamic simulations of accretion flows and jets, taking general relativistic light bending and radiation transport into account, also allows us to test models of black hole astrophysics in unprecedented detail.

Accretion disks and jets

Accretion disks and jets are two types of gas flows that have a fundamental impact on the evolution of single and binary stars, the evolution of galaxies (through potential feedback from supermassive black holes to their host galaxies), the acceleration of high-energy cosmic rays and the observational signatures of extreme events such as X-ray transients and gravitational-wave mergers.

Accretion, the process where material falls onto a gravitating object, is a fundamental astrophysical process occurring with young stars, compact objects and supermassive black holes alike. The fundamental physics of accretion is still poorly understood. Concerning accretion, the fundamental research questions addressed by our scientists are:

  • Is there universality to accretion physics at all length and time scales?
  • What is the dynamics of accretion disks? What are their flux variations and large-scale flows like?
  • How does gas accrete onto a black hole? Do intermediate-mass black holes exist and how are they formed?
  • Do gravitational wave mergers form a disk torus? What is the physics and can we detect it?

Jets are highly collimated outflows emanating from the vicinity of an accreting central source, invariably rooted in the inner region of an accretion disk:

  • What is the physics of jet formation and the process of collimation?
  • Are jets ubiquitous to all accreting sources, irrespective of the central object?
  • When are jets launched and quenched, in particular in compact objects and AGN?
  • Are jets and the shocks they create responsible for particle acceleration to ultra-high energies?
  • How are jets related to the accretion flow?

To answer these questions we combine physical theory, numerical modeling (3D magneto-hydrodynamic simulations, including radiative transfer), fast timing measurements (optical, X-rays), measurements of radio emission and mm-VLBI observations, gravitational wave detections, synoptic surveys and near-infrared imaging and spectroscopy.