Galactic NH (nhtot)

Warning

This page is a work in progress. See jinwu.core.utils.nhtot() for the complete API.

The nhtot function queries the Swift UKSSDC nhtot web service to obtain the total Galactic hydrogen column density using the method of Willingale et al. (2013, MNRAS, 431, 394).

This includes: - NHI — atomic hydrogen (from 21-cm surveys) - NH₂ — molecular hydrogen (estimated from dust reddening E(B-V)) - NH,tot = NHI + 2×NH₂

Why nhtot?

HEASoft’s nh tool returns NHI only (from the HI4PI map). At low Galactic latitudes (|b| < 20°), molecular hydrogen contributes significantly, and using NHI-only values in tbabs (which assumes 20% molecular fraction) systematically underestimates Galactic absorption.

The Swift community (including Valan et al. 2023) uses nhtot for the Galactic component of X-ray spectral fitting.

Usage

from jinwu.core.utils import nhtot

# Decimal degrees
result = nhtot(ra=159.386, dec=56.171)
print(f"N_HI       = {result['nhi_weighted']:.2e} cm⁻²")
print(f"N_H₂       = {result['nh2_weighted']:.2e} cm⁻²")
print(f"N_H,tot    = {result['nhtot_weighted']:.2e} cm⁻²")
print(f"E(B-V)     = {result['ebv_weighted']:.3f} mag")

# Sexagesimal coordinates also accepted
result = nhtot("10:37:32.6", "+56:10:15.6")

# For XSPEC tbabs:
nh_gal = result["nhtot_weighted"]  # Use this, not nh!

Reference

Willingale, R., Starling, R. L. C., Beardmore, A. P., Tanvir, N. R., & O’Brien, P. T. 2013, MNRAS, 431, 394 (arXiv:1303.0843)