Discrete X‐Ray Signatures of a Photoionized Plasma above the Accretion Disk of the Neutron Star EXO 0748−676

Abstract

During the disk-mediated accretion phase, the high-resolution X-ray spectrum of the low-mass X-ray binary system EXO 0748À676 reveals a photoionized plasma that is orbiting the neutron star. Our observa-tions with the Chandra High Energy Transmission Grating Spectrometer constrain the structure of the upper layers of the accretion disk by means of the recombination emission lines from the H-like and He-like ions of O, Ne, and Mg, which have a mean velocity broadening v $ 750 AE 120 km s À1 . The Mg xi emission region has density n e e10 12 cm À3 and is located within 7 Â 10 9 cm < r < 6 Â 10 10 cm of the neutron star, while the temperature of the Ne x region is kTd20 eV. These lines favor a vertically stratified distribution of ions in the disk. The spectra show that the line region is spatially extended and unabsorbed, while the continuum region is compact and heavily absorbed. The absorber has variable column density and is composed of both neutral and ionized gas, which can explain the stochastic and periodic X-ray intensity dips, the X-ray continuum evolution, and the O vii and Mg xi K-shell absorption edges. The absorber is located 8 –15 above the disk midplane, inclusive of two bulges near the disk edge. This outer disk gas may participate in the outflow of ionized plasma that was previously identified in XMM-Newton grating spectra obtained during type I bursts. The thickened photoionized region above the disk can be produced by heating from the neutron star X-rays and by the impact of the accretion stream.