Abstract
Black hole imaging challenges the third-generation space VLBI, the Very Long Baseline Interferometry, to operate on a 500[Formula: see text]GHz band. The coherent integration time needed here is 450[Formula: see text]s though the available space oscillators cannot offer more than 10[Formula: see text]s. Self-calibration methods might solve this issue in an interferometer formed by three antenna/satellite systems, but the need for the third satellite increases the mission costs. A frequency transfer is of special interest to alleviate both performance and cost issues. A concept of two-way optical frequency transfer is examined to investigate its suitability to enable space-to-space interferometry, in particular, to image the “shadows” of black holes from space. The concept, promising on paper, has been demonstrated by tests. The laboratory test set-up is presented and the verification of the temporal stability using standard analysis tool as TimePod has been passed. The resulting Allan Deviation is dominated by the 1/[Formula: see text] phase noise trend since the frequency transfer timescale of interest is shorter than 0.2[Formula: see text]s. This trend continues into longer integration times, as proven by the longest tests spanning over a few hours. The Allan Deviation between derived 103.2[Formula: see text]GHz oscillators is [Formula: see text]/[Formula: see text] within 10[Formula: see text][Formula: see text][Formula: see text]s that degrades twice towards the longest delay of 0.2[Formula: see text]s. The worst case satisfies the requirement with a margin of 11 times. The obtained coherence in the range of 0.997[Formula: see text]0.9998 is beneficial for space VLBI at 557[Formula: see text]GHz. The result is of special interest to future science missions for black hole imaging from space.
Highlights
Sagittarius A* is the handy object for the gravity theory choice, being a bright object of big angular size and well-known mass
The lowest-possible observation frequency which allows both mitigating the scattering by interstellar electrons in the Galactic plane and imaging planet-forming disk, is 557 GHz
This paper reports on the concept test results, including the high-level block diagram of the set-up and the measurement approach
Summary
Sagittarius A* is the handy object for the gravity theory choice, being a bright object of big angular size and well-known mass. The lowest-possible observation frequency which allows both mitigating the scattering by interstellar electrons in the Galactic plane (which would distort the image of the black hole in the center of our galaxy, Sagittarius A*) and imaging planet-forming disk, is 557 GHz. For the desired resolution of 5 as, the required distance between the radio-telescopes (i.e. the baseline of the interferometer) is more than twice the Earth's diameter. Atmospheric e®ects make observations from the ground at such submillimeter wavelength extremely challenging For these reasons, a space-based array for Very Long Baseline Interferometry (VLBI) at 557 GHz is required to image Sagittarius A* and water disks atne resolution. The design, follows an alternative concept of interferometry using telescope-satellites in Polar Medium Earth Orbits involving inter-satellite optical links for local oscillator connection between the satellites, data transfer and metrology.
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