Implications of X-Ray Line Variations for 4U1822-371

TL;DR

This study uses Chandra X-ray observations of 4U 1822-371 to analyze line variations, revealing details about the accretion disk, corona, and the nature of the X-ray emission, with implications for understanding high-inclination binary systems.

Contribution

It provides new insights into the structure and emission regions of 4U 1822-371 through detailed spectral and line analysis, updating orbital parameters and constraining physical properties.

Findings

Photo-ionized line emission originates from the outer accretion disk edge.

The illuminating luminosity exceeds observed X-ray flux, indicating an obscured central source.

Spectral analysis suggests a flat power-law continuum with partial covering absorption.

Abstract

4U 1822-371 is one of the proto-type accretion disk coronal sources with an orbital period of about 5.6 hours. The binary is viewed almost edge-on at a high inclination angle of 83 degrees, which makes it a unique candidate to study binary orbital and accretion disk dynamics in high powered X-ray sources. We observed the X-ray source in 4U 1822-371 with the Chandra High Energy Transmission Grating Spectrometer (HETGS) for almost nine binary orbits. X-ray eclipse times provide an update of the orbital ephemeris. We find that our result follows the quadratic function implied by previous observations; however, it suggests a flatter trend. Detailed line dynamics also confirm a previous suggestion that the observed photo-ionized line emission originates from a confined region in the outer edge of the accretion disk near the hot spot. Line properties allow us to impose limits on the size of accretion disk, the central corona, and the emission region. The photo-ionized plasma is consistent with ionization parameters of log(xi) > 2, and when combined with disk size and reasonable assumptions for the plasma density, this suggests illuminating disk luminosities which are over an order of magnitude higher than what is actually observed. That is, we do not directly observe the central emitting X-ray source. The spectral continua are best fit by a flat power law with a high energy cut-off and partial covering absorption (N_H ranging from 5.4-6.3x10^{22} cm^{-2}) with a covering fraction of about 50%. We discuss some implications of our findings with respect to the photo-ionized line emission for the basic properties of the X-ray source.