Abstract
X-ray photons, emitted by X-ray sources, are absorbed and scattered by dust grains when they travel through interstellar medium. The scattered photons within small angles result in a diffuse X-ray “halo”. Therefore, the scattered X-ray photons carry information about various properties of the dust. We study the light curves of the dust-scattered X-ray halo of Cyg X-1. Significant time lags are found between the light curves of the point source and its halo. This time lag increases with the angular distance. This can be caused by a dust concentration at 1.75 kpc from the Earth along the line of sight (LOS), with a width of ΔL = 33 pc. The time lag of Cyg X-3 also reveals a dust concentration between the point source and the Earth, and the distance of Cyg X-3 is estimated to be about 7.2 kpc. The origin of these dust concentrations is still not clearly known. Along with recent results on the dust halos of Gamma Ray Bursts (GRBs), we conclude that the dust distribution is quite nonuniform. We also study dust properties with an expanding X-ray ring from the observation of a distant GRB. The distribution of the halo photons can be used to determine the dust-radius model. Our preliminary results show that dust grain geometries may vary significantly across the Galaxy. We also propose to use the scattered X-ray halo to study the composition of the dust grains. The applicability of this method needs to be tested with advanced instruments in the future.
Highlights
The existence of a dust-scattered X-ray halo was first proposed by Overbeck (1965) theoretically
We study the light curves of the dust-scattered X-ray halo of Cyg X-1
Besides the time-lag property between the light curve of the X-ray source and that of the halo, we find that the profile of the light curves of the halo is quite similar to the light curve of the X-ray source
Summary
The existence of a dust-scattered X-ray halo was first proposed by Overbeck (1965) theoretically. Rolf (1983) first observed the halo phenomenon by analyzing the data of GX 339-4 with the IPC instrument onboard the Einstein Observatory. Predehl and Schmitt (1995) analyzed the observational data of the ROSAT Observatory and found most of those sources have X-ray halos; the halo fractions varied from a few percent to tens of percent. They found that the effective scattering optical depth is proportional to the visual extinction. With the ACIS-I instrument onboard the Chandra Observatory, Yao et al (2003) resolved the halo surface brightness to even less than 1 arc-
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