Calibration of X-ray absorption in our Galaxy
  Prediction of the soft X-ray absorption along lines of sight through our
Galaxy is crucial for understanding the spectra of extragalactic sources, but
requires a good estimate of the foreground column density of photoelectric
absorbing species. Assuming uniform elemental abundances this reduces to having
a good estimate of the total hydrogen column density, N(Htot)=N(HI)+2N(H2). The
atomic component, N(HI), is reliably provided using the mapped 21 cm radio
emission but estimating the molecular hydrogen column density, N(H2), expected
for any particular direction, is difficult. The X-ray afterglows of GRBs are
ideal sources to probe X-ray absorption in our Galaxy because they are
extragalactic, numerous, bright, have simple spectra and occur randomly across
the entire sky. We describe an empirical method, utilizing 493 afterglows
detected by the Swift XRT, to determine N(Htot) through the Milky Way which
provides an improved estimate of the X-ray absorption in our Galaxy and thereby
leads to more reliable measurements of the intrinsic X-ray absorption and,
potentially, other spectral parameters, for extragalactic X-ray sources. We
derive a simple function, dependent on the product of the atomic hydrogen
column density, N(HI), and dust extinction, E(B-V), which describes the
variation of the molecular hydrogen column density, N(H2), of our Galaxy, over
the sky. Using the resulting N(Htot) we show that the dust-to-hydrogen ratio is
correlated with the carbon monoxide emission and use this ratio to estimate the
fraction of material which forms interstellar dust grains. Our resulting recipe
represents a significant revision in Galactic absorption compared to previous
standard methods, particularly at low Galactic latitudes.
