The column density towards LMC X-1

TL;DR

This study measures the neutral hydrogen absorption towards LMC X-1 using multiple X-ray spectra, revealing phase-dependent variations that explain observed flux modulation.

Contribution

It provides the first consistent measurement of the neutral absorption towards LMC X-1 accounting for LMC metallicity and orbital phase.

Findings

Equivalent hydrogen column densities of (1.0-1.3)×10^22 cm^-2

Absorption variation explains orbital flux modulation

Powerlaw models may overestimate NH

Abstract

We measure the neutral absorption towards the black hole X-ray binary system LMC X-1 from six archival soft X-ray spectra obtained with the gratings and/or CCD detectors on Chandra, XMM-Newton, and Swift. Four spectral models for the soft continuum have been investigated. While the powerlaw model may overestimate NH considerably, the others give consistent results. Taking the lower metalicity of the Large Magellanic Cloud into account, we find equivalent hydrogen column densities of N_H = (1.0-1.3)*10^22 cm^-2, with a systematic dependence on the orbital phase. This variation of the neutral absorption can nearly explain the orbital modulation of the soft X-ray flux recently detected with the All Sky Monitor (ASM) on the Rossi X-ray Timing Explorer (RXTE).