Calibration of X-ray absorption in our Galaxy
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TLDR
In this paper, a simple function, dependent on the product of the atomic hydrogen column density, N(HI), and dust extinction, E(B-V), was derived to estimate the variation of the molecular hydrogen column densities over the sky.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(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.read more
Citations
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The Swift Gamma-Ray Burst Mission
Neil Gehrels,G. Chincarini,G. Chincarini,Paolo Giommi,Keith O. Mason,John A. Nousek,Alan A. Wells,Nicholas E. White,S. D. Barthelmy,David N. Burrows,L. R. Cominsky,Kevin Hurley,F. E. Marshall,Peter Mészáros,Peter W. A. Roming,Lorella Angelini,Lorella Angelini,L. M. Barbier,Tomaso Belloni,Sergio Campana,P. A. Caraveo,M. M. Chester,O. Citterio,T. L. Cline,Mark Cropper,Jay Cummings,Jay Cummings,A. J. Dean,Eric D. Feigelson,E. E. Fenimore,Dale A. Frail,A. S. Fruchter,Gordon P. Garmire,Keith C. Gendreau,Gabriele Ghisellini,Jochen Greiner,Joanne E. Hill,S. D. Hunsberger,Hans A. Krimm,Hans A. Krimm,Shrinivas R. Kulkarni,Pawan Kumar,F. Lebrun,Nicole M. Lloyd-Ronning,Craig B. Markwardt,Craig B. Markwardt,Barbara J. Mattson,Barbara J. Mattson,Richard Mushotzky,Jay P. Norris,J. P. Osborne,Bohdan Paczynski,David Palmer,H.-S. Park,A. M. Parsons,J. A. Paul,Martin J. Rees,Christopher S. Reynolds,James E. Rhoads,T. P. Sasseen,Bradley E. Schaefer,A. Short,Alan P. Smale,Alan P. Smale,Ian Smith,Luigi Stella,Gianpiero Tagliaferri,Tadayuki Takahashi,Makoto Tashiro,Leisa K. Townsley,Jack Tueller,Martin J. L. Turner,M. Vietri,Wolfgang Voges,Martin Ward,Richard Willingale,F. M. Zerbi,W. W. Zhang +77 more
TL;DR: The Swift mission as discussed by the authors is a multi-wavelength observatory for gamma-ray burst (GRB) astronomy, which is a first-of-its-kind autonomous rapid-slewing satellite for transient astronomy and pioneers the way for future rapid-reaction and multiwavelength missions.
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