Implications of X-ray Line Variations for 4u1822-371

Abstract

4U 1822–371 is one of the prototype accretion disk coronal sources with an orbital period of about 5.6 hr. The binary is viewed almost edge-on at a high inclination angle of 83°, 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 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. From the orbital line dynamics, we confirm the previous suggestion that recombination emission is located at the impact bulge of the accretion stream. Our observations show that the recombining plasma is confined to this region, and prove that it has to be a very thin layer on top of the accretion disk. Fe XXVI emission is detected at all phases and likely originates from the hot central corona which has optical depth <<1. The implied ionization parameters from both regions strongly suggest that the illuminating source is obscured with respect to the observer and is orders of magnitude brighter than what is actually observed. This is independently confirmed by the best fit of a flat power law with a high-energy cutoff and partial covering absorption with a covering fraction of about 50%. We discuss the implications of our findings with respect to the photoionized line emission and the spectral continuum on basic properties of the X-ray source.