ALEXIS: An Ultrasoft X-Ray Monitor Experiment Using Miniature Satellite Technology

Abstract

Los Alamos and Sandia National Laboratories are, building an ultrasoft X-ray monitor experiment. This experiment, called ALEXIS (Array of Low-Energy X-Ray Imaging Sensors), consists of six compact normal-incidence telescopes. ALEXIS will operate in the 70 - 110 eV band. The ultrasoft X-ray/EUV band is nearly uncharted territory for astrophysics. ALEXIS, with its wide fields-of-view and well-defined wavelength bands, will complement the upcoming NASA Extreme Ultraviolet Explorer and ROSAT EUV Wide Field Camera, which are sensitive broadzband survey experiments. The program objectives of ALEXIS are to 1) demonstrate the feasibility of a wide field-of-view, normal incidence ultrasoft X-ray telescope system and 2) to determine ultrasoft X-ray backgrounds in the space environment. As a dividend, ALEXIS will pursue the following scientific objectives: 1) to map the diffuse background, with unprecedented angular resolution, in several emission line bands, 2) to perform a 3-color survey of point sources, 3) to search for transient phenomena in the ultrasoft X-ray band, and 4) to provide synoptic monitoring of variable ultrasoft X-ray sources such as cataclysmic variables and flare stars. The six ALEXIS telescopes are arranged in pairs to cover three 40° fields-of-view. During each spin of the satellite, ALEXIS will monitor more than half the sky. Each telescope consists of a layered synthetic microstructure (LSM) mirror, a curved microchannel plate detector, background-rejecting filters and magnets, and readout electronics. The mirrors will be tuned to 72 eV, 85 eV, and 95 or 107 eV bands, chosen to select and deselect interesting line features in the diffuse background. The geometric area of each ALEXIS telescope will be about 15 cm2. The telescopes employ spherical mirrors with the curved detector at prime focus and are limited by spherical aberration to a resolution of about 1°. Assuming nominal reflectivities, quantum efficiency, and filter transmission, the 5a survey sensitivity will be about 2 x 10-3 photons cm-2 s-1 for line emission at the center of the bandpass. ALEXIS is designed to be flown on a small autonomous payload carrier (a minisat) that could be launched from either a Shuttle Get-Away-Special Can or from an expendable launch vehicle. The experiment weighs 100 pounds, draws 40 watts, and produces 10 kbps of data. It can be flown in any low Earth orbit. Onboard data storage allows operation and tracking from a single ground station at Los Alamos.