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The physics of compact objects

Imaging the event horizon of black holes

The defining feature of a black hole is the event horizon. New developments in mm-wave astronomy within the next few years will make 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 would 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 testing 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 growth of supermassive black holes (including possible feedback 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 (through a disk) is a fundamental astrophysical process occurring in young stars, compact objects and supermassive black holes alike. The fundamental physics of accretion is still poorly understood. Concerning accretion, research questions addressed by the Department are:

  • Is there universality to accretion physics at all scales of length and time?
  • What is the dynamics of accretion disks: flux variations and large-scale flows
  • 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 use is made of theory, numerical modelling (3D magneto-hydrodynamic simulation, including radiation), fast timing (optical, X-rays), radio emission and submm-VLBI observations with BlackHoleCam, gravitational wave detections, synoptic surveys and near-infrared imaging and spectroscopy.

Goals for the next six years

A number of clear goals exist for this research line:

  • Image the event horizon and jet formation region in Sgr A* and M87, using submm-VLBI

  • Understand universality of accretion physics in compact objects and quasars e.g. through timing studies

  • Detect and understand the presence of an accretion disk in gravitational wave merger events.

  • Understand jet launching and particle acceleration and their links to accretion

  • Search for the existence of intermediate-mass black holes from X-ray/optical studies

  • An additional node to the BlackHoleCam project through the African Millimetre Telescope (AMT).

  • The deployment of the BlackGEM array, and its prototype the MeerLICHT telescope in conjunction with the MeerKAT radio array.

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