Wavelength Measurements of K Transitions of Oxygen, Neon, and Magnesium with X-ray Absorption Lines

TL;DR

This study improves the accuracy of wavelength measurements for K transitions of oxygen, neon, and magnesium using X-ray absorption lines in interstellar medium spectra, providing new data for atomic physics and astronomy.

Contribution

It presents a novel analysis of Chandra data with Bayesian methods to reduce uncertainties and biases in atomic transition wavelength measurements, including first-time detections of weak lines.

Findings

Wavelength accuracy improved by a factor of 2-5.

Systematic uncertainties and biases significantly reduced.

First detection of several weak transitions in the interstellar medium.

Abstract

Accurate atomic transition data are important in many astronomical research areas especially in line spectroscopy study. Whereas transition data of He-like and H-like ions (i.e., ions at high-charge states) are accurately calculated, that of K transitions of neutral or low-ionized metal elements are still very uncertain. Spectroscopy of absorption lines produced in the interstellar medium (ISM) has been proven to be an effective way to measure the central wavelengths of these atomic transitions. In this work we analyze 36 Chandra High Energy Transmission Grating observations and search for and measure the ISM absorption lines along sight lines of 11 low-mass X-ray binaries. We correct the Galactic rotation velocity to the rest frame for every observation and then use two different methods to merge all the corrected spectra to a co-added spectrum. However the co-added spectra obtained by these methods exhibit biases, either to the observations of high counts or high signal-to-noise ratios of the lines. We make Bayesian analysis to several significantly detected lines to obtain the systematic uncertainty and the bias correction of other lines. Compared to previous studies (e.g., Yao et al. 2009), our results improve the accuracy of wavelengths by a factor from two to five and significantly reduce the systematic uncertainties and biases. Several weak transitions (e.g., 1s--2p of Mg IV and Mg V; 1s--3p of Mg III and Mg V) are also detected for the first time, albeit with low significance; future observations with improved accuracy are required to confirm these detections.