LEM — THE FUTURE OF ASTRONOMY
The Line Emission Mapper (LEM) is an X-ray Probe for the 2030s. Its transformative observational capabilities will open a new discovery space, allowing comprehensive studies of the physics that drive the formation and growth of structures in the Universe: from planets and the Milky Way ecosystems to galactic halos and the cosmic web. The LEM concept is being prepared for the NASA 2023 Astrophysics Probes call for proposals.
THE LEM DESIGN
The spacecraft
The spacecraft is being developed in collaboration with the Lockheed Martin Corporation. It is a single telescope that consists of an X-ray mirror with a large collecting area and moderate angular resolution and a cryogenic microcalorimeter array. The design is optimized to study the soft X-ray band, where emission lines from the most abundant ion species of the circumgalactic and intergalactic mediums are detectable.
The X-ray mirror
The grazing-incidence X-ray mirror consists of many pairs of thin monocrystalline silicone shells coated with iridium or platinum to provide X-ray reflectivity. The mirror’s diameter is 1.5m, and the focal length is 4m. The geometry of the mirror is adjusted to maximize the effective area in the 0.2-2 keV band.
A revolutionary microcalorimeter array
LEM will use a large-area X-ray integral field unit, which will effectively map the soft X-ray line emission from objects that constitute galactic ecosystems. LEM’s 1–2 eV spectral resolution will make it possible to separate the faintest emission lines from the bright Milky Way foreground, providing groundbreaking measurements of the physics of these plasmas, from temperatures, densities, and chemical composition to gas dynamics
The LEM detector is a cryocooled array of Transition-Edge Sensor (TES) microcalorimeters consisting of 13,806 absorber pixels in a hexagonal arrangement. The inner 1062 absorber pixels will provide an energy resolution of 0.9 eV, while the resolution of the rest of the array will be 2 eV. A 1000-pixel prototype hybrid LEM array is shown along with part of the back side of the array with single pixels and hydras.
Laboratory measurements of the first LEM TES pixels demonstrate that the single pixel and the hydra (4 pixels co-added) can achieve their required 0.9 eV and 2 eV energy resolutions, respectively.
AN X-RAY INTEGRAL FIELD UNIT WITH A HUGE GRASP
The combination of a large collecting area and field of view make LEM an exquisite observatory to study faint diffuse structures. When compared with the requirements for future X-ray spectrometers, LEM stands out with its huge grasp and high spectral resolution.
Grasp is the product of the effective area and the field of view solid angle. Grasp is the critical quantity relevant for mapping faint extended objects with sizes comparable to or greater than the field of view, such as nearby galaxies, clusters, the IGM, or the Milky Way structures. LEM will have more than an order of magnitude larger grasp than Athena XIFU and more than three orders of magnitude larger grasp than XRISM Resolve.