NH Help
- Introduction
- Description
- Cone Radius Parameter
- Map Parameter
- HI Map Comparison
- How does NH relate to X-ray and extreme-ultraviolet (EUV) spectra?
Introduction
NH is set up as a World-Wide Web interface to the command-line version of the 'nh' FTOOL. As such, it incorporates some of the features and all of the limitations of 'nh'. It returns hydrogen column density for a given radius around an astronomical position (right ascension and declination).
Description
This program returns, for a specified right ascension and declination, a value for the galactic hydrogen column density, NH. This value is derived from the HI map by Kalberla et al. 2005, Astronomy & Astrophysics, 440, 775, known as the Leiden/Argentine/Bonn survey (hereafter, the LAB map). The LAB map was obtained by merging two surveys covering HI radial velocities from -400 km/s to +400 km/s – the Leiden/Dwingeloo Survey (Hartmann & Burton 1997), and the Instituto Argentino de Radioastronomía Survey (Arnal et al. 2000 and Bajaja et al. 2005). See the LAB Galactic HI Survey site for details and other information. The map used within NH was integrated over all velocities. The resulting velocity integrated map had a resolution of approximately 0.5°, and was resampled onto 0.675° × 0.675° bins in L and B.Alternatively, the NH value can be derived from the HI map by Dickey & Lockman 1990, Ann. Rev. Ast. Astr. 28, 215 (hereafter, the DL map). The DL map was obtained by merging several surveys (see Dickey & Lockman 1990) and averaged into 1° × 1° bins in L and B. See HI Maps Comparison for details on the two maps.
The NH values are in units of hydrogen atoms × cm-2. The software calculates an average NH using values within N° from the request position (N is the Cone Radius parameter). Two NH average values are output for the requested map (see Map Parameter): a simple average and an average weighted by the inverse of the distance from the request position. By setting the Map parameter to "Both" the average and weighted values are calculated for both maps.
NOTE: The maximum NH value from the DL map is 2.58×1022 cm-2 at RA (2000)=15h 59m 29.383s Dec (2000)=-53d 04m 40.04s corresponding to (l, b) = (329.0, 0.0). In Dickey & Lockman (1990), this is instead printed as (l, b) = (339.0, 0.0), which is assumed to be a typo.
NOTE 2: Previous versions of this program use only the DL map to calculate NH.
Cone Radius Parameter
Only NH values within N° are used in the calculation, where N is the value entered in the Cone Radius box. The default is to use all points within 1°.NOTE: Increasing the radius may lead to erroneous values for NH. Decreasing the search radius may not allow the program to find any data points.
Map Parameter
This option lets one choose which map to use for the NH calculation. Selecting "Both" will perform the calculation for both the LAB and DL maps, displaying the results from each. See the Description section for information about each map, and see HI Maps Comparison for a comparison of the two.
HI Maps Comparison
The resolution of the LAB and DL maps differ - the DL map is lower resolution. There are also differences in the two maps, besides resolution. In particular, the number of observational artifacts is reduced in the LAB map, and the Large and Small Magellanic Clouds are both visible in the LAB map. Figure 1 shows these differences.
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Figure 1. top: Leiden/Argentine/Bonn HI map with the positions of test sources (see below) marked with blue stars. bottom: Dickey & Lockman HI map also with the positions of test sources marked with blue stars. |
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To quantify the differences between these two HI maps, ∼250 positions were randomly chosen on the sky. The NH values were calculated with the FTOOL 'nh' for each location with both the LAB and DL maps. The Cone Radius used for each case was 1°. Figure 2 shows a plot of the NH calculated with the LAB map versus that calculated with the DL map, along with the best fit line to the data. The relation between the two maps is linear, as expected, but the DL map tends to give higher values for NH.
How does NH relate to X-ray and extreme-ultraviolet (EUV) spectra?
If large enough, the column density of gas between an X-ray/EUV source and the Earth will absorb radiation (particularly at the softer energies) and consequently alter the overall spectral shape. This effect can be calculated using the standard formula, tau = sigma x NH, where tau is the optical depth and sigma is the cross-section. Since sigma is a steep function of energy E (or, equivalently, wavelength w), so is tau. Some typical values of sigma for a gas with properties and abundances similar to the nearby interstellar medium are 6 x 10-20 cm2 at 0.07 keV (175 Angstroms), 4 x 10-21 cm2 at 0.25 keV (50 Angstroms), 2 x 10-22 cm2 at 1 keV (12 Angstroms), and 1 x 10-23 cm2 at 10 keV (1.2 Angstroms). These cross-sections imply that a column density of >= 8 x 1019 cm-2 will absorb most (EUV) emission with energies below 0.07 keV (w > 175 A), NH >= 1 x 1021 cm-2 most soft X-rays and EUV emission with E < 0.25 keV (w > 50 A), NH >= 2 x 1022 cm-2 most emission below 1 keV (w > 12 A), and NH >= 5 x 1023 cm-2 will essentially suppress almost all of the emission in the Chandra and XMM-Newton bands, i.e., all energies below 10 keV (w > 1.2 A). (For these estimates, we have quoted the NH values equivalent to an optical depth of 4.5, i.e., an attenuation factor e-tau of 1%, at the stated energies.)Thus, for example, heavily absorbed X-ray sources with NH > ~1023 cm-2 (equivalent to an optical reddening E(B-V) > ~20 magnitudes, using the relation NH = 5 x 1021 x E(B-V) valid for a standard gas to dust ratio) were typically not detectable by the ROSAT X-ray detectors as these latter were only sensitive to X-rays with energies E < 2.5 keV.
This file was last modified on Thursday, 17-May-2007 12:20:35 EDT
NH is a Web interface to the NH FTOOL. NH was developed by
Lorella Angelini at the HEASARC. The Web interface is maintained by
Edward J. Sabol of the HEASARC.
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