Introduction to BHEX-The Black Hole Explorer (BHEX) is a mission that will discover and measure a black hole’s photon ring, capturing light that has orbited a black hole. BHEX will extend the Event Horizon Telescope (EHT) into space, producing the sharpest images in the history of astronomy, whilst operating a space-Earth hybrid VLBI observatory by sending the BHEX radio telescope into space to link down with the existing EHT telescope on Earth.
The mission aims to deepen our understanding of black holes by capturing and analyzing detailed images and data. Black holes, regions of spacetime with gravitational pulls so intense that nothing, not even light, can escape, are fundamental to the structure and evolution of the universe. Studying them can reveal insights into general relativity, mechanisms that form powerful relativistic jets in active galactic nuclei, and galaxy formation.
The BHEX mission seeks to overcome significant challenges, such as the need for extreme precision and sensitivity in observations due to the vast distances and faint signals involved. Advanced technologies like the Event Horizon Telescope (EHT) are employed, which combine data from multiple observatories worldwide to create a virtual telescope of Earth-sized proportions. This approach enabled the groundbreaking capture of the first image of a black hole's event horizon.
Understanding black holes also involves studying phenomena like photon rings—bright rings of light that encircle black holes, predicted by Einstein's theory of general relativity. These rings can provide further insights into the black hole's properties and the behavior of light in extreme gravitational fields. Despite the complexities, the mission's potential to uncover the secrets of black holes makes it a significant and exciting scientific endeavor.
The Science of Cryogenics in BHEX-Cryogenic systems are crucial for the BHEX mission, enabling the observation of black holes with unparalleled precision. The mission employs advanced 4 K spaceflight cryocoolers to maintain the required low temperatures for its sensitive detectors. These cryocoolers operate at multiple temperature stages, specifically cooling to 4.5 K and 20 K, which is essential for the optimal performance of the BHEX receiver systems.
The significance of cryogenics in space missions lies in its ability to reduce thermal noise, thereby enhancing the sensitivity of detectors used for astronomical observations. “BHEX will use superconducting detectors, so-called superconductor-insulator-superconductor mixers. They have to be operated at very low temperature, but in return they provide sensitivity that approaches the limits set by quantum mechanics”, according to Dan Marrone, BHEX Instrument Scientist and Professor of Astronomy at the University of Arizona.
BHEX utilizes a combination of cryogenic technologies, including Stirling and Joule-Thomson coolers, designed to meet stringent mass and power constraints. These technologies ensure effective heat load management and vibration control, crucial for maintaining the stability and functionality of the receiver system in the harsh space environment. These cryogenic systems are integral to achieving BHEX’s objectives of capturing high-resolution images of black holes and their photon rings, thus advancing our understanding of these enigmatic cosmic entities.
The Photon Ring-The photon ring around a black hole is a bright, thin ring created by photons that closely orbit the black hole before escaping. This ring is a unique feature resulting from extreme gravitational lensing near the event horizon, making it a key observational target for understanding black hole properties. Capturing the photon ring provides direct insights into the black hole's mass, spin, and inclination by measuring its shape and size. Black hole images are composed of a series of ever sharper photon rings, produced from light that has orbited the black hole increasingly more times before escaping. The properties of the black hole are imprinted on the size and shape of the photon ring, allowing us to directly study whether supermassive black holes are spinning.
Building the Mission-Led by Dr. Michael Johnson, BHEX PI, the BHEX team is a large collaboration consisting of scientists, researchers, engineers and other professionals from across multiple international organizations. From the cryogenic point of view, a Request for Information (RFI) is currently live and receiving responses for information on 4 K spaceflight cryocooling systems that could be used for the BHEX mission. Through the BHEX mission, the first demonstration of a closed-cycle 4 K spaceflight cryocooler on a US-led space mission will be performed. This is an incredibly exciting mission for the astrophysics community, and cryogenics plays a pivotal role in making it a reality. www.blackholeexplorer.org.
Image: High-resolution image of a black hole, showing the photon ring. Credit: George Wong (IAS)
