[Paper Review] Column Densities Towards Three Bursting Low-Mass X-ray Binaries from High Resolution X-ray Spectroscopy

Patricia Wroblewski, Tolga Güver+1 more|ArXiv.org|Sep 30, 2008

Astrophysical Phenomena and Observations1 references3 citations

TL;DR

This study measures hydrogen column densities toward three bursting low-mass X-ray binaries—GX 17+2, 4U 1705-44, and 4U 1728-34—using high-resolution X-ray spectra from Chandra’s HETG. By modeling Mg and Si absorption edges, it finds $N_{ m H} = (2.38 /pm 0.12) imes 10^{22}$ cm$^{-2}$, $(2.44 /pm 0.09) imes 10^{22}$ cm$^{-2}$, and $(2.49 /pm 0.14) imes 10^{22}$ cm$^{-2}$, respectively, with Mg-edge values preferred due to instrumental systematics at the Si edge.

ABSTRACT

We measured the galactic hydrogen column densities to the neutron-star binaries GX 17+2, 4U 1705-44, and 4U 1728-34 by modeling the Mg and Si absorption edges found in high-resolution X-ray spectra obtained by the Chandra X-ray Observatory. We found for GX 17+2, N_H = (2.38 +/- 0.12) x 10^22 cm^-2, for 4U 1705-44, N_H = (2.44 +/- 0.09) x 10^22 cm^-2, and for 4U 1728-34, N_H = (2.49 +/- 0.14) x 10^22 cm^-2. These values are in reasonable agreement with the hydrogen column densities inferred earlier from modeling of the continuum spectra of the sources. Our results can be used to constrain the uncertainties of model parameters of the X-ray spectra of these sources that are correlated to the uncertainties of the hydrogen column density. In the case of continuum spectra obtained during thermonuclear X-ray bursts, they will significantly reduce the uncertainties in the spectroscopically measured masses and radii of the neutron stars.

Motivation & Objective

• To determine precise hydrogen column densities toward three bursting low-mass X-ray binaries using high-resolution X-ray spectroscopy.
• To resolve discrepancies in column density measurements arising from instrumental systematics, particularly at the silicon absorption edge.
• To provide model-independent column density constraints that reduce uncertainties in neutron star mass and radius measurements from X-ray burst spectroscopy.
• To validate and compare results with previous continuum-based estimates from ROSAT, BeppoSAX, and RXTE data.
• To support future distance measurements using red clump stars by providing accurate interstellar extinction values.

Proposed method

• Acquired high-resolution X-ray spectra using the Chandra High Energy Transmission Grating (HETG) across multiple archival observations of GX 17+2, 4U 1705-44, and 4U 1728-34.
• Fitted the spectra using photoelectric absorption models to measure the depths and energies of Mg and Si absorption edges, which directly constrain $N_{ m H}$.
• Assessed systematic uncertainties by comparing $N_{ m H}$ values derived from Mg and Si edges, identifying count-rate-dependent discrepancies due to uncalibrated detector response features.
• Preferred Mg-edge-derived $N_{ m H}$ values due to their consistency and lower sensitivity to instrumental artifacts compared to Si-edge measurements.
• Cross-validated results against previous continuum spectral fits from ROSAT, BeppoSAX, RXTE, and Chandra, using different spectral models (blackbody, power-law, Comptonized blackbody).
• Used the observed column densities to assess their impact on neutron star radius measurements, given the $R \propto T_{\rm eff}^{-2}$ scaling and strong correlation between $T_{\rm eff}$ and $N_{\rm H}$.

Experimental results

Research questions

• RQ1 What are the precise hydrogen column densities toward GX 17+2, 4U 1705-44, and 4U 1728-34 as measured from high-resolution X-ray absorption edges?
• RQ2 How do column density measurements from Mg and Si edges compare, and what systematic effects might explain discrepancies?
• RQ3 To what extent do instrumental response inaccuracies at the Si edge affect $N_{ m H}$ estimates, and how does this vary with source count rate?
• RQ4 How do the model-independent $N_{ m H}$ values from grating spectra compare with previous continuum-based estimates from other observatories?
• RQ5 How do these improved $N_{ m H}$ values reduce uncertainties in neutron star mass and radius measurements from X-ray burst spectroscopy?

Key findings

• The hydrogen column density toward GX 17+2 is $N_{ m H} = (2.38 /pm 0.12) imes 10^{22}$ cm$^{-2}$, derived from the Mg absorption edge.
• For 4U 1705-44, the column density is $N_{ m H} = (2.44 /pm 0.09) imes 10^{22}$ cm$^{-2}$, with consistent Mg-edge results and a significant discrepancy from Si-edge measurements at low count rates.
• The column density toward 4U 1728-34 is $N_{ m H} = (2.49 /pm 0.14) imes 10^{22}$ cm$^{-2}$, derived from the Mg edge, and is consistent with BeppoSAX and Chandra HETG continuum fits.
• Discrepancies between Mg- and Si-edge-derived $N_{ m H}$ values are strongly correlated with source count rate, indicating instrumental systematics at the Si edge, especially at low flux levels.
• The Mg-edge-based $N_{ m H}$ values are preferred due to their stability and lower sensitivity to detector response uncertainties.
• The results are consistent with previous continuum-based estimates from RXTE and BeppoSAX, and support the use of high-resolution grating spectroscopy for accurate $N_{ m H}$ determination in neutron star studies.