Chandra observation of GX 5-1 halo:
a deeper understanding of interstellar dust properties

E. Costantini, P. Predehl , V. Burwitz
(Max-Planck-Institut fuer extraterrestrische Physik,Giessenbachstrasse,85748 Garching, Germany)


We present ACIS and ACIS-HETG preliminary results of the bright X-ray binary GX 5-1 and the diffuse X-ray scattering halo which surrounds it. The measured halo contribution is 25% at 2 keV, consistent to what previously found by ROSAT. In addition, thanks to the broader energy band and better spatial/spectral resolution of Chandra, the energy dependence of the halo brightness could be modeled. In particular the high quality of the data allows us to discuss the validity of the classical scattering model, based on the Raleigh-Gans theory.
The study of the halo shape and intensity depends crucially on the determination of the PSF. The fine tuning needed to study extended and relatively weak emission around bright pointlike sources in presence of pileup, as the case of scattering haloes, is also described. The spectrum of GX 5-1 is satisfactorly modeled by black body curves plus photoelectric absorption, in agreement with previous studies. The ACIS and HETG observations were carried out $\sim$20 days apart. During this time interval no spectral or flux variation was registered.



Himel Ghosh