Design of the LEM-X chamber (left) and overall layout of the observatory (right)

The Lunar Electromagnetic Monitor in X-rays (LEM-X) is an All Sky Monitor for the X-ray band (2-50 keV) and employs pairs of compact and light-weight coded-aperture cameras. Each camera is equipped with: 

• a tungsten coded mask

• 4 large-area linear Silicon Drift Detectors (SDDs)

• 96 ultra low-noise front-end ASICs

• 8 ADC ASICs

• back-end electronics (BEE) and power-supply (PSU) boards

Such a configuration offers excellent spectral resolution (350 eV at 6 keV) over the 2–50 keV energy range. The SDDs are developed in Italy by a collaboration between INFN, INAF, ASI and FBK and produced in the foundries of the Bruno Kessler Foundation in Trento. The signal read-out and conditioning is performed by custom front-end and back-end ASICs developed by Polytechnic of Milan and University of Pavia. Each camera pair provides a field of view of 1 steradian at 25% effective area and achieves a Point-Source Location Accuracy (PSLA) of 1 arcminute, with an on-axis sensitivity better than 5 mCrab in 50 ks and 700 mCrab in 1 s.

Integrated LEM-X Detector Assembly 

The advantages of the LEM-X design are: high sensitivity, high level of maturity, compact and modular design, and high level of redundancy. The concept is to realize a "dome" composed of several identical modules that observe different directions, thus covering an overall simultaneous field of view of 2π. Each module is autonomous, compact (about 35 × 35 × 35 cm3), light (~10 kg), and with low power consumption (13 W). Moreover, thanks to its modular nature, LEM-X can be built gradually —even over a long period of time— and can eventually reach dimensions, weight, and complexity impossible to achieve on a free-flyer. 

The real breakthrough of LEM-X is represented by the simultaneous coverage of half of the sky in an energy band, that of “soft” X-rays, still not covered by wide-field experiments in orbit, with imaging and spectroscopic capabilities. The lunar site, as part of a wider infrastructure, can prove ideal for an all-sky monitoring experiment. The stability of the site allows for full exploitation of the instrument's capabilities, obtaining high-resolution images of transient events and detected sources. Adequately positioned (e.g., along the lunar equator), LEM-X will have simultaneous access to half of the celestial sphere and periodically to the entire sky. The site's accessibility will allow for maintenance and updates over time, making LEM-X a long-term observatory infrastructure, similar to a ground-based telescope. 

Simulation of 1 ks exposure on the galactic center (one camera pair) reconstructed with the Bloodmoon and Darksun packages. Full FoV reconstructed SNR image (left) and zoom of the ∼45° × 45° central FoV region with detected source names (right). 

The LEM-X project is developed in collaboration with ASI, INFN, Polytechnic of Milan, University of Pavia and Fondazione Bruno Kessler in the context of: 

• Earth-Moon-Mars (EMM) project (led by INAF in partnership with ASI and CNR) of the National Recovery and Resilience Plan (NRRP), Mission 4, Component 2, Investment 3.1: “Fund for the realisation of an integrated system of research and innovation infrastructures” - Action 3.1.1 funded by the European Union - NextGenerationEU

• Accordo Attuativo ASI-INAF “Supporto scientifico allo sviluppo dello strumento LEM-X in continuità con le attività EMM-PNRR”