Properties of interstellar grains derived from x‐ray eclipse observation

By a Monte Carlo computation we have estimated the spectrum of x‐rays scattered into the eclipse of 4U1538‐52 by the atmosphere of its supergiant companion. The estimate fails to account for a soft component below 4.5 keV which amounts to approximately 1.4% of the average uneclipsed flux in the same energy range. The intensity of the soft component exhibits an initial downward trend following eclipse ingress as expected of a component scattered by interstellar dust grains. Comparing the intensity with the optical extinction of QV Nor, we derive an upper limit on a quantity RXV(E) which we call the scattering/extinction ratio of interstellar dust grains and define as (E/1 keV)2 times the ratio of optical depth for scattering x‐rays of energy E to the total optical extinction. In the Rayleigh‐Gans approximation to the x‐ray scattering efficiency, this quantity is independent of energy. Our upper limit on RXV is 0.06 mag−1, which implies that the x‐ray scattering efficiency of interstellar dust is less than ...