Calibration of X-ray absorption in our Galaxy

By R. Willingale, R. Starling, A. Beardmore, N. Tanvir, P. O’Brien. 2013

517 citations recorded by the source corpus.

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.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/0f9d4c071efdcad902cf36f0994377dbf7209183

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In Astronomy and Astrophysics

1. Bing Zhang
2. E. Berger
3. B. Metzger
4. W. Fong
5. Raffaella Margutti
6. B. Margalit
7. Ryan Chornock
8. M. Nicholl

Citations in the corpus

• An Embedded X-Ray Source Shines through the Aspherical AT 2018cow: Revealing the Inner Workings of the Most Luminous Fast-evolving Optical Transients (2018, methodology): “20sh shD ~ » – ( ) We measure the properties of the X-ray flares in AT 2018cow following the same procedure as is used for long GRBs, adopting a Norris et al. (2005) profile.”
• 1SXPS: A DEEP SWIFT X-RAY TELESCOPE POINT SOURCE CATALOG WITH LIGHT CURVES AND SPECTRA (2013, methodology): “…temperature of 1 keV; the absorption was fixed at the Galactic value in 12 The uncertainty in the ECF was not propagated into the error on the flux; this was simply the count-rate error multiplied by the ECF. the direction of the source, determined using the nhtot tool of Willingale et al. (2013).”
• Swift J1112.2−8238: a candidate relativistic tidal disruption flare (2015, background): “The combined spectrum of all available PC-mode data is well fit by an absorbed power-law of photon index Γph = 1.33 ± 0.08 and NH (int) = 2.4+1.8−1.6 × 10 21 cm−2 consistent with the Galactic value of 1.8 × 1021cm−2 (Willingale et al. 2013).”
• A DECADE OF SHORT-DURATION GAMMA-RAY BURST BROADBAND AFTERGLOWS: ENERGETICS, CIRCUMBURST DENSITIES, AND JET OPENING ANGLES (2015, background): “…the X-ray spectrum for each burst to an absorbed power law model characterized by photon index,Γ, and the intrinsic neutral hydrogen absorption column,NH,int, in excess of the Galactic column density in the direction of the burst (Kalberla et al. 2005; Wakker et al. 2011; Willingale et al. 2013).”
• The SRG/eROSITA all-sky survey (2024, unknown): “Considering the nonuniform Galactic absorption, we assume a power-law spectral model with Γ = 2 and the total Galactic absorption column density (Willingale et al. 2013) at each source position, and calculate two versions of ECF for each source, one correcting the 0.5–2 keV flux for absorption and…”