PROPERTIES OF A BO I-STELLAR WIND AND INTERSTELLAR GRAINS DERIVED FROM GINGA OBSERVATIONS OF THE BINARY-X-RAY PULSAR 4U 1538-52

From measurements of the X-ray eclipse phenomena of the binary X-ray p ulsar 4U 1538-52, we derive properties of the stellar wind of its B0 I companion, QV Nor, and a constraint on models of interstellar grains. Estimates of the wind density as a function of the distance from the center of QV Nor are obtained from an analysis of the variation of X-r ay attenuation during an eclipse egress. The analysis takes account of the effects of X-ray ionization on the photoelectric absorption cross sections and yields a particle-number density described by the functi on Psi{1+(r/r(1))(2) exp [-(r-r(1))/h]}/(4 pi r(2) mu) with Psi = 6.7 x 10(-10) M. yr(-)1 km(-1) s h = 4.3 x 10(10) cm, and r(1) = 1.2 x 10( 12) cm, where mu = 1.34m(H) is the average atomic mass per hydrogen at om. A Monte Carlo computation of the absorption and scattering of X-ra ys in the X-ray-ionized wind accounts for approximately two-thirds of the spectrum of X-rays with energies above 4.5 keV observed during the eclipse. Addition of density enhancements, like those predicted by a numerical computation of the hydrodynamic disturbance caused by passag e of the neutron star through the wind, brings the predicted eclipse s pectrum into agreement with the observed spectrum above 4.5 keV. Below 4.5 keV there is a component of soft X-rays above the Monte Carlo pre diction with a total photon flux amounting to approximately 1.4% of th e average uneclipsed flux in the same energy range. The intensity of t he soft component exhibits an initial downward trend following eclipse ingress as expected of a component scattered by interstellar dust gra ins. Thermal emission from uneclipsed, shock-heated circumstellar matt er is probably also present in the soft component. Taking the entire s oft component as an upper limit on the intensity of the grain-scattere d X-rays, and comparing this limit with the optical extinction of QV N or, we derive an upper limit on a quantity R(XV)(E) which we call the scattering/extinction ratio of interstellar dust grains and define as (E/1 keV)(2) times the ratio of the optical depth for scattering X-ray s of energy E to the total optical extinction. In the Rayleigh-Gans ap proximation to the X-ray scattering efficiency, this quantity is indep endent of energy. Our upper limit on R(XV), is 0.06 mag(-1), which imp lies that the X-ray scattering efficiency of interstellar dust is less than expected for solid grains with a size distribution of the form n (g)(a)similar to a(-3.5) in the range from 0.005 to 0.25 mu m and comp osed of silicate (R(XV) = 0.22 mag(-1)) or a silicate-graphite mixture (R(XV) = O.11 mag(-1)) as derived from the calculations of Martin & R ouleau (1991). This lends support to the idea (Mathis & Whiffen 1989) that interstellar grains are ''fluffy'' aggregates with an average bul k density less than that of their constitutent particles. Such aggrega tes would have a smaller ratio of X-ray scattering efficiency to optic al extinction efficiency compared with solid grains of the same materi al.