THE ALEXIS PROJECT AND THE LOCAL INTERSTELLAR MEDIUM

Abstract

The ALEXIS satellite will be a valuable tool for studying the diffuse soft X-ray background (SXRB). The SXRB emission (0.07--0.25 keV) is thought to originate in a plasma at about 10/sup 6/K that exists in the local interstellar medium. Emission models of such a plasma (Raymond and Smith 1977, 1987) with normal cosmic elemental abundances predict that about 34% of the atomic cooling power comes from a set of closely-spaced lines around 70 eV from Fe VII through XII. Two passbands of the ALEXIS telescopes will be centered at 66 and 72 eV to include the Fe line emission. The flux from these lines in the SXRB could produce a signal of as much as 150 counts per second for ALEXIS telescopes with these passbands. With the narrow spectral response of the ALEXIS telescopes, an unambiguous measurement of the flux from the lines will be possible. When combined with other data sets, the intensities of the SXRB in the 66 and 72 eV bands allow distinction between models of emission from hot gas having normal and depleted abundances. ALEXIS' spatial resolution and estimated year-long operating lifetime will allow the generation of all-sky maps of the SXRB in these Fe linesmore » with about one degree resolution and considerable sensitivity. These maps will be more sensitive to absorption features due to nearby low-column-density clouds than maps done in higher energy bands. 13 refs., 3 figs., 4 tabs.« less