Astronomy and Astrophysics – Astronomy
Scientific paper
May 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...180.4410w&link_type=abstract
American Astronomical Society, 180th AAS Meeting, #44.10; Bulletin of the American Astronomical Society, Vol. 24, p.801
Astronomy and Astrophysics
Astronomy
Scientific paper
From measurements of the X-ray eclipse phenomena of the binary X-ray pulsar 4U1538-52 we derive properties of the stellar wind of its B0 I companion, QV Nor, and information about the size distribution of interstellar dust grains. The X-ray spectra observed during an eclipse egress are fit by computed spectra of X-rays attenuated in passage through the X-ray ionized radiation-driven stellar wind which is described by an exponential function of radius with a scale height of 4.5*E(10) cm joined smoothly at r=1.2r_{*} to a function of the form n(r)=(dot {M}/4pi vt)(1-r_{*}/r)(-beta ) with beta =0.85, dot {M}=1.1*E(-6) M_&sun; yr(-1) and vt=2000 km s(-1) . The deep eclipse spectrum above 4.4 keV is matched by a Monte Carlo-computed spectrum of X-rays Compton-scattered from the pulsar into the eclipse shadow by circumstellar matter. Below 4.4 keV there is an excess of soft X-rays above the Compton scattered spectrum with a flux amounting to approximately 1.6\ the uneclipsd X-ray flux in the same energy range. Some of the soft excess is attributable to a persistent dust-scattered halo, and some to thermal emission from shock-heated plasma in the flow disturbances caused by passage of the neutron star through the stellar wind. Its low intensity coupled with the reported high optical extinction (AV \sim6.5-7.1) implies an upper limit of 1.4\times10^{-3} (\microns)2$ on the ratio of the fourth to the second moments of the grain size distribution. This is substantially less than the ratio of moments of the grain size distributions that have been used to account for the halos measured in various imaging observations, and implies a greater preponderance of small grains than has been indicated by those interpretations.
Clark George W.
Nagase Fumiaki
Woo Jonathan Wonha
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