M ar 2 00 3 A New Method to Use Chandra Data to Resolve the X - Ray Halos Around Point Sources and Its Application to Cygnus X - 1

The excellent angular resolution, good energy resolution and broad energy band make Chandra ACIS the best instrument for studying the X-ray halos around some galactic X-ray point sources caused by the dust scattering of X-rays in the interstellar medium, but the direct images of bright sources obtained with ACIS usually suffer from severe pileup. Making use of the fact that an isotropic image could be reconstructed from its projection on any direction, we can reconstruct the images of the X-ray halos from the data obtained with the regular HETGS and/or the CC mode. These data are nearly or totally pileup free and enable us to take full advantage of the excellent angular resolution of Chandra. With the reconstructed high resolution images, we can probe the X-ray halos as close as 1 ′′ to their associated point sources. Applying this method to Cygnus X-1 observed with Chandra HETGS in CC mode, we derive an energy dependent radial halo flux distribution and conclude that, for a 1 ′ region around the point source: (1) relative to the total intensity, the fractional halo intensity in Cygnus X-1 is about 35% at about 1 keV and drops to about 10% at around 6 keV; (2) about 50% of the halo photons is within less than 30 ′′ region; and (3) the spectrum of the point source is distorted by the halo contamination, especially in the soft energy band below 3 keV.