MEASUREMENTS OF THE GRAIN-SCATTERED X-RAY HALOS OF ECLIPSING MASSIVE X-RAY BINARIES - VELA X-1 AND CENTAURUS X-3

We have measured the decaying dust-scattered X-ray halo of Cen X-3 dur ing its binary eclipse with the ASCA solid-state imaging spectrometer (SIS). The surface brightness profile (SBP) of the image in the low-en ergy band (0.5-3 keV) lies substantially above the point-spread functi on (PSF) of the X-ray telescope, while the SBP in the high-energy band (5-10 keV) exhibits no significant deviation. By contrast, the SBPs o f Vela X-l during its eclipse are consistent with the PSF in both the low- and high-energy bands-strong evidence that a dust halo is indeed present in Cen X-3. Accordingly, we modeled the SBP of Cen X-3 taken f rom six consecutive time segments under the principal assumptions that the dust is distributed uniformly along a segment of the line of sigh t, the grains have a power-law size distribution, and the low-energy s ource flux was the same function of orbital phase before as during our observation. The best-fit set of parameters included a grain density value of 1.3 g cm(-3), substantially less than the density of ''astron omical silicate.'' This result supports the idea that interstellar gra ins are ''fluffy'' aggregates of smaller solid particles. We attribute the failure to detect a halo of Vela X-1 during its eclipse phase to extended strong circumsource absorption that probably occurred before the eclipse and allowed the halo to decay away before the observation began.