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Title: Calibration of X-ray absorption in our Galaxy
Authors: Willingale, Richard
Starling, R. L. C.
Beardmore, Andrew P.
Tanvir, Nial R.
O'Brien, Paul T.
First Published: 28-Feb-2013
Publisher: Oxford University Press on behalf of the Royal Astronomical Society
Citation: Monthly Notices of the Royal Astronomical Society, 2013, 431 (1), pp. 394-404
Abstract: 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[subscript Htot] = N[subscript HI] + 2N[subscript H2]. The atomic component, N[subscript HI], is reliably provided using the mapped 21 cm radio emission but estimating the molecular hydrogen column density, N[subscript H2], expected for any particular direction, is difficult. The X-ray afterglows of Gamma Ray Bursts (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 X-ray Telescope, to determine N[subscript 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[subscript HI], and dust extinction, E(B − V), which describes the variation of the molecular hydrogen column density, N[subscript H2], of our Galaxy, over the sky. Using the resulting N[subscript 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.
DOI Link: 10.1093/mnras/stt175
ISSN: 0035-8711
eISSN: 1365-2966
Version: Publisher Version
Status: Peer-reviewed
Type: Journal Article
Rights: © 2013, The Authors. Deposited with reference to the publisher’s archiving policy available on the SHERPA/RoMEO website.
Description: This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Appears in Collections:Published Articles, Dept. of Physics and Astronomy

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