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

With excellent angular resolution, good energy resolution and broad energy band, the Chandra ACIS is 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. However, the direct images of bright sources obtained with ACIS usually suffer from severe pile-up. Making use of the fact that an isotropic image could be reconstructed from its projection into any direction, we can reconstruct the images of the X-ray halos from the data obtained with the HETGS and/or in CC mode. These data have no or less serious pile-up 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 derived an energy dependent radial halo flux distribution and concluded that, in a circular region (2$'$ in radius) centered at the point source: (1) relative to the total intensity, the fractional halo intensity (FHI) is about 15% at $\sim$1 keV and drops to about 5% at $\sim$6 keV; (2) about 50% of the halo photons are within the region of a radius less than 40$''$; and (3) the spectrum of the point source is slightly distorted by the halo contamination.