Seeing the unseeable

Abstract

The Event Horizon Telescope, an Earth-sized interferometer, aims to capture an image of a black hole's event horizon to test the theory of general relativity and probe accretion processes.Black holes are the most mysterious objects predicted by Einstein's theory of general relativity. Yet, despite compelling evidence for their existence, no black hole has been imaged with event-horizon-scale resolution. Supermassive black holes in particular convert the gravitational energy of infalling material into powerful outflows and luminous nuclei that redistribute matter and energy on galactic scales. Horizon-scale access to these central engines of galactic nuclei would enable the most detailed studies of black hole accretion and the formation of relativistic jets, while also providing new tests for the existence of black holes and the validity of general relativity at the boundary of a black hole.The Event Horizon Telescope (EHT) is an international project specifically designed to capture the first image of a black hole and to time-resolve the dynamics of the associated accretion flow on orbital timescales. To do this, our team links a network of millimetre and submillimetre wavelength observatories around the globe to form an Earth-sized very long baseline interferometry (VLBI) array with the required resolving power and sensitivity. Over the past decade, the EHT has built up the necessary infrastructure by deploying hydrogen maser atomic frequency standards and purpose-built ultrahigh data rate VLBI instrumentation to sites in Chile, Hawaii, Mexico, the South Pole, Arizona and Spain. For the first time this spring, the Atacama Large Millimeter/submillimeter Array (ALMA) facility in Chile joined the array with all of its antennas phased together, operating as a single VLBI station and boosting EHT sensitivity by a factor of ten. Inclusion of ALMA enables the EHT to detect sources within the atmospheric coherence time of several seconds. When observing at 1.3 mm wavelength, the longest baselines in less than 25 microarcseconds.This resolution is well matched to the two supermassive black holes with the largest apparent event horizons on the sky. Sagittarius A* (Sgr A*), the four million solar mass black hole at the centre of the Milky Way, has a Schwarzschild radius that subtends 10 microarcseconds, and Messier 87 (M87) hosts an approximately six billion solar mass black hole in Virgo A that has a similar angular size. However, extreme light bending near the event horizon forms a 'silhouette' feature bounded by the edge-brightened photon orbit, which is magnified to a size of ~50 microarcseconds. For these sources, observing this silhouette at its expected size would confirm that all the estimated mass is contained within the photon orbit -an extraordinary strengthening of the case for black holes and confirmation of general relativity. The EHT will also search for periodic signals in the VLBI data that correspond to orbits of inhomogeneities in the accretion flow, which are sensitively dependent on black hole spin.Th...

Modified gravity black hole lensing observables in weak and strong field of gravity

“…New baselines have been added to achieve an astonishing resolution of 15 µarcsec at 345 GHz [25]. For recent developments and reviews of experiments, see [25,[44][45][46][47][48][49][50][51][52][53].…”

Silhouettes of invisible black holes

“…Spherical orbits are defined by the common solutions of equations R(r) = dR(r)/dr = 0, where the radial effective potential R(r) is from (16), and the turning point in the polar direction are the zeros of effective polar potential Θ(θ) from (17). In the case of extreme Kerr black hole a = 1 the stable spherical orbits of particles corotating with a black hole at radius r = 1, which coincides in the Boyer-Lindquaist coordinates with the event horizon, form the one-parametric family [161]:…”

“…where the effective polar potential Θ(θ) is defined in (17). Parameter α is called the horizontal impact parameter and β, respectively, is called the vertical impact parameter.…”

“…Here θ min is a turning point in polar direction on the photon trajectory, defined by the condition Θ(θ) = 0, where Θ(θ) is an effective polar potential from Eq. (17). According to the Cunningham-Bardeen classification scheme of the multiple lensed images [166,167], the photons, providing the prime image of the emitting source do not intersect the black hole equatorial plane on their way from the source to a distant observer.…”

“…The most important task for the Event Horizon Telescope is recovering the image of the supermassive black hole candidate SgrA* at the center of Milky Way [8][9][10][11][12][13][14][15][16][17]. Our nearest and native supermassive black hole SgrA* with a mass M = (4.3 ± 0.3) × 10 6 M ⊙ [18][19][20][21][22] is evidently the object of intensive investigations .…”