LIMITS ON THE DENSITY OF NEUTRAL GAS WITHIN 100 PARSECS FROM OBSERVATIONS OF THE SOFT-X-RAY BACKGROUND

We present observations in two soft X-ray bands, the Be band (0.077-0.11 1 keV) and the B' band (0.105-0.188 keV), for nine directions in the sky. The ratio of count rates in these two bands remains constant as the rates vary by a factor of 3, even though the effective interstellar absorption cross sections in the bands differ by a factor of approximately 3.5. For a model in which the bulk of the observed soft X-ray emission originates in a uniform low-density region surrounding the Sun, the constant ratio between the band rates places an upper limit on the amount of neutral material that can be homogeneously mixed with the X-ray-emitting gas. The 2-sigma upper limit on the H I column density over an average path through the local emitting region is 6.6 x 10(18) cm-2. If the average path length is approximately 100 pc, then clouds similar to the one in which the Sun is embedded (n(H) approximately 0.1 cm-3) could still have a filling factor as large as 25%.