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 pulsar 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-ray 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 function 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 atom. A Monte Carlo computation of the absorption and scattering of X-rays 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 passage of the neutron star through the wind, brings the predicted eclipse spectrum 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 prediction with a total photon flux amounting 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. Thermal emission from uneclipsed, shock-heated circumstellar matter is probably also present in the soft component. Taking the entire soft component as an upper limit on the intensity of the grain-scattered X-rays, and comparing this limit with the optical extinction of QV Nor, 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-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 R(XV), is 0.06 mag(-1), which implies 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 composed 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 & Rouleau (1991). This lends support to the idea (Mathis & Whiffen 1989) that interstellar grains are ''fluffy'' aggregates with an average bulk density less than that of their constitutent particles. Such aggregates would have a smaller ratio of X-ray scattering efficiency to optical extinction efficiency compared with solid grains of the same material.