Extending the high-resolution X-ray spectroscopy of Line Emission Mapper to UV/optically-bright sources

The Line Emission Mapper X-ray Probe-class mission concept is based on a microcalorimeter array tuned to energies in the range 0.1 to 2 keV. The study of cosmic ecosystems defines the directed portion of the Line Emission Mapper (LEM) mission, thus LEM has been optimized for observations of diffuse X-ray-emitting gas, largely with very low surface brightness. To broaden the range of targets that general observers can study with LEM, we have investigated the particular needs for UV/optical bright stars and solar-system objects. X-ray microcalorimeters are susceptible to degraded energy resolution that can result from thermal noise from residual UV, optical, and IR radiation. Using the present baseline design of the microcalorimeter thermal filters, we compute the UV-IR loading expected from bright stars over the effective temperature range 3500 to 39,000 K and from solar-system objects. The dominant leak of out-of-band energy is in the far-UV around 1500 Å, with a secondary peak of throughput around 4000 Å. For stars with magnitudes V<10 and for all solar-system planets as well as the Moon, the loading is significant, indicating that additional UV/optical blocking is essential if bright objects are to be observed. We have investigated the efficacy of several filter options for optical-blocking filters on the LEM filter wheel, demonstrating that new technology development is not necessary to open up many of these classes of objects to investigation with the high spectral resolution of LEM.