Chandra/LETGS Studies of the Collisional Plasma in 4U 1626-67

TL;DR

This study analyzes Chandra/LETGS observations of the ultracompact X-ray binary 4U 1626-67, revealing details about its accreted plasma, system inclination, and elemental abundances, especially neon enrichment.

Contribution

It provides the first detailed spectral modeling of the emission lines and plasma parameters, constraining system geometry and elemental ratios in 4U 1626-67.

Findings

Constrained system inclination to 25-60 degrees.

Estimated the inner disk radius at approximately 1500 gravitational radii.

Confirmed high neon abundance with a Ne/O ratio of 0.47.

Abstract

We present an analysis of \chandra/LETGS observations of the ultracompact X-ray binary (UCXB) 4U 1626$-$67, continuing our project to analyze the existing Chandra gratings data of this interesting source. The extremely low mass, hydrogen-depleted donor star provides a unique opportunity to study the properties and structure of the metal-rich accreted plasma. There are strong, double-peaked emission features of OVII-VIII and Ne IX-X, but no other identified emission lines are detected. Our spectral fit simultaneously models the emission line profiles and the plasma parameters, using a two-temperature collisionally-ionized plasma. Based on our line profile fitting, we constrain the inclination of the system to 25--60$^{\circ}$ and the inner disk radius to $\sim$1500 gravitational radii, in turn constraining the donor mass to $\lesssim$0.026 M_sun, while our plasma modeling confirms previous reports of high neon abundance in the source, establishing a Ne/O ratio in the system of $0.47 \pm 0.04$, while simultaneously estimating a very low Fe/O ratio of $0.0042 \pm 0.0008$ and limiting the Mg/O ratio to less than 1% by number. We discuss these results in light of previous work.