Researchers from Astro Space Center have proved the possibility of observing supermassive black holes from the L2 libration point using a space-ground interferometer

Scientists from Astro Space Center have simulated the observations of supermassive black holes using the Millimetron space telescope in the space-to-Earth interferometer mode. As targets, they chose two supermassive black holes with the largest angular size. This is the core of the M87 galaxy and the black hole at the center of our Galaxy Sgr A*. The study showed that despite the extremely large distance to the L2 point from ground-based telescopes, such observations can be conducted  and the quality of the obtained images exceeds the combined capabilities of ground-based telescopes by 3-4 times. The work was published in Monthly Notices of the Royal Astronomical Society.

Simulated images of black hole shadow in the M87 galaxy. Two rightmost images (d) and (e) correspond to the results of the space-ground interferometer Millimetron-Event Horizon Telescope. Images (b) and (c) is a well-known result of the Event Horizon Telescope ground interferometer.


Radiation coming from the inner regions of the accretion disk of a supermassive black hole forms an image of the so-called "black hole shadow". The shape that the "shadow" takes, as well as the distribution of brightness across the disk, depend on the mass of the black hole and its angular velocity around its axis. On average, the shadow extends over distances of the order of five Schwarzschild radii. Thus, the observation of regions of space in the immediate vicinity of the event horizon of a supermassive black hole provides scientists with a unique opportunity to study the behavior of matter in extremely strong gravitational and magnetic fields. In addition, these observations provide information about the geometry of space-time in the immediate vicinity of the event horizon.

In 2019, the international collaboration Event Horizon Telescope (EHT) presented the first image of the shadow of a supermassive black hole located in the galaxy M87. The image was reconstructed from the data of very long baseline radio interferometric observations (VLBI) of eight telescopes at a wavelength of 1.3 mm. The maximum angular resolution achieved in these observations was 20 microarcseconds. This turned out to be enough to see the "shadow of the black hole". However, to study the individual details in the accretion disk, a higher angular resolution is required. The capabilities of the Event Horizon Telescope are limited by the size of our planet, as well as by the properties of the atmosphere, which does not allow reducing the wavelength at which observations are conducted. The way out of this situation is the development of ground-space VLBI networks.

The Millimetron space telescope, scheduled to be launched into space in 2029, will operate in two modes. In the first mode, the Millimetron will operate as a stand-alone ultra-sensitive space telescope with a mirror diameter of 10 meters. Its main tasks in this mode will be to study the inhomogeneity of the cosmic microwave background radiation and deviations in its spectrum, the study of the large-scale structure of the Universe, as well as the search for water and organic molecules in the interstellar medium.

The second mode assumes operation of Millimetron together with ground-based telescopes as an arm of the Space-to-Earth interferometer. The targets for it will be very compact astronomical objects, the study of which requires the highest possible angular resolution. This will include the vicinity of supermassive black holes.

However, a significant problem for interferometric observations of the Millimetron is its location. The space telescope will be located at libration point L2 of the Sun-Earth system, at an average distance of 1.5 million kilometers from the Earth. This means that the space-ground baseline (the distance between the telescopes) is about a hundred times greater than the ground ones. With this configuration of the interferometer, the reconstructed image of the black hole will degenerate into an almost one-dimensional line stretching in the direction of the longest baseline. Although this makes it possible to study one-dimensional brightness distributions in the accretion disk and the so-called "photon rings" with an angular resolution up to 100 nanoseconds of arc, but it does not make it possible to see a full-fledged image of the source.

Millimetron observatory trajectory projected on celestial sphere.


In the presented new work, the scientists from Astro Space Center have demonstrated that despite the large distance of the space telescope from the Earth, joint observation of the Millimetron and the Event Horizon Telescope could be done to obtain high-quality images. To do this, the researchers simulated Millimetron+EHT observations for two supermassive black holes at the center of our Galaxy and at the center of the M87 galaxy. Taking into account the sensitivity of each telescope, the influence of atmospheric irregularities on ground-based observatories, and the scattering of caused by interstellar plasma, the researchers determined the parts of the space telescope's orbit where the source image does not degenerate into a one-dimensional line. This happens when the Millimetron is approximately on the same straight line between the observed object and the Earth. During the year, this configuration is repeated once for each black hole. The quality of the reconstructed image turns out to be slightly worse than those for the space telescope located in the near-Earth orbit, but 3-4 times higher than for the ground only observations. The achieved angular resolution at 1.3 mm in this case was 5 microseconds of arc.

The new work proves for the first time the possibility to conduct space-ground observations using Millimetron observatory located in the L2 point orbit. The paper gives numerical estimates of the advantage of such a configuration over ground-based observations.


High-resolution imaging of a black hole shadow with Millimetron orbit around Lagrange point l2.

S F Likhachev, A G Rudnitskiy, M A Shchurov, A S Andrianov, A M Baryshev, S V Chernov, V I Kostenko

Monthly Notices of the Royal Astronomical Society, stac079, Published: 13 January 2022