X-ray photons coming from an X-ray point source not only arrive at the
detector directly, but also can be strongly forward-scattered by the
interstellar dust along the line of sight (LOS), leading to a detectable
diffuse halo around the X-ray point source. The geometry of small angle X-ray
scattering is straightforward, namely, the scattered photons travel longer
paths and thus arrive later than the unscattered ones; thus the delay time of
X-ray scattered halo photons can reveal information of the distances of the
interstellar dust and the point source. Here we present a study of the X-ray
scattered around IGR J17544-2619, which is one of the so-called supergiant fast
X-ray transients. IGR J17544-2619 underwent a striking outburst when observed
with Chandra on 2004 July 3, providing a near delta-function lightcurve. We
find that the X-ray scattered halo around IGR J17544-2619 is produced by two
interstellar dust clouds along the LOS. The one which is closer to the observer
gives the X-ray scattered at larger observational angles; whereas the farther
one, which is in the vicinity of the point source, explains the halo with a
smaller angular size. By comparing the observational angle of the scattered
halo photons with that predicted by different dust grain models, we are able to
determine the normalized dust distance. With the delay times of the scattered
halo photons, we can determine the point source distance, given a dust grain
model. Alternatively we can discriminate between the dust grain models, given
the point source distance.Comment: Accepted for publication in ApJ. 25 pages, 9 figures, 6 table
