Accretion flows around black-holes, neutron stars or white dwarfs are studied since almost 60 years (Schmidt 1963). Although they are ubiquitous and somewhat similar over scales reaching billions in mass and size, their study has been limited because they remain unresolved point like sources in the optical/ultraviolet and X-rays, where they emit. Two main modes of accretion have been identified in Active Galactic Nuclei. In most sources the accretion rate is low and a high pressure, low density, low collision rate, optically thin, radiatively inefficient, two temperature plasma (Tion~1E12K, Te~1E10K) can form (Shapiro 1976; Narayan & Yi 1994,1995). This solution is stable only for low luminosities (<1% LEDD). The Event Horizon Telescope has recently resolved such flows in Sgr A* and M87, confirmed several aspects of the model and could detect particles accelerated close to the horizon of Sgr A* (Wielgus, 2022) a likely signature of the Blandford-Znajek (1977) process.

Image : Reconstruction of the photon flux surrounding the black hole M87.(Credit: EHT, 2019)

When the accretion rate is higher, momentum can be dissipated by viscosity and the flow proceeds via geometrically thin disk-shaped structures (Shakura & Sunyaev 1976). These accretion disks provide feedback to their environment by accelerating winds and launching jets in their central regions. The apparent size of accretion disks are of the order of 1-40μarcsec in nearby quasars, Seyfert galaxies and galactic cataclysmic variables and of 0.1-1μarcsec in of low mass X-ray binaries in our Galaxy.

Image : The two 6-meter class telescopes used by R.Hanbury-Brown and R.Twiss to determine the diameter of Sirius by using Intensity Interferometry. (Credit: ATNF Daily Astronomy)

Hanbury-Brown & Twiss (1954) invented intensity interferometry and measured the size of some bright stars by correlating the arrival times of photons detected by two optical telescopes. The physics has been explained as a quantum effect in the early 60s (Fano 1961) and has triggered the development of quantum optics (Glauber 1963). Its root is found in the quantum theory of statistical fluctuations in an ideal gas (Einstein 1925). The achievable signal-to-noise depends on the telescope size, the detector time resolution, and the number of spectral channels observed simultaneously. Extremely large telescopes (https://elt.eso.org) and 10ps resolution single photon detectors (Gramuglia et al 2021) bring the key improvements to reach in the optical angular resolutions better than these achieved in the radio by the Event Horizon Telescope and to obtain the first images of accretion disks around galactic and extragalactic compact objects, a breakthrough.

Achieving this is a two steps process. This proposal makes the first step i.e. demonstrating that intensity interferometry can work efficiently on large optical telescopes using 30ps time resolution and hundreds of independent wavelength channels, reaching a resolution of 15-100μarcsec for sources of magnitude 8-10, and enabling to resolve accretion disk around white dwarfs. This will be performed by building detectors and testing them on various telescope configurations. As a by-product we will obtain the most sensitive telescope array for sub-milliarcsec stellar intensity interferometry.

The second step, which could start in 2027, will be to design and build intensity spectro-interferometers for ESO’s 39m Extremely Large Telescope ELT (in construction at Cerro Armazones) and for the existing VLTs to allow imaging accretion discs in Seyfert galaxies and nearby quasars. Ultimately, copies of that instrument could be installed on the Giant Magellan Telescope (GMT), being built 600km south from the ELT, allowing together to study accretion disks around stellar mass black-holes and neutron stars or higher redshift quasars and reaching μarcsec resolution. Finally combining signals from the ELT and from the 655m2 thirty meter telescope, could allow to reach a resolution reaching 0.01μarcsec, enough to probe the central regions of accretion disks in low mass X-ray binaries.

Image : Size comparison between the ELT (in construction), the four 8-meter class ELT telescopes and the Colosseum. (Credit: ESO)

Intensity interferometry will allows to constrain parameters of accretion flows in a new way, e.g. making movies of accretion disks falling on white dwarfs (can be achieved with this proposal), determining accretion disk inclination, mapping the radial variations of disk temperature, or measuring the mass of super-massive black-holes. Our project will increase the spatial resolution reachable in the optical by thousands, opening new fields of investigation in astronomy in general. For instance, images could be obtained of the surface of Solar like stars and binary systems at a distance of 1kpc, of the surface of a core-collapse Supernova at 100kpc, or of the inner optical jets of blazars at 100Mpc