During the disk-mediated accretion phase , the high-resolution X-ray spectrum of the low-mass X-ray binary system EXO 0748-676 reveals a photoionized plasma which is orbiting the neutron star .
Our observations with the Chandra High Energy Transmission Grating Spectrometer ( HETGS ) 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 \sigma _ { v } \sim 750 \pm 120 { \thinspace km } { \thinspace s } ^ { -1 } .
The Mg xi emission region has density n _ { e } \gtrsim 10 ^ { 12 } { \thinspace cm } ^ { -3 } and is located within 7 \times 10 ^ { 9 } < r < 6 \times 10 ^ { 10 } cm of the neutron star , while the temperature of the Ne x region is kT \lesssim 20 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 ^ { \circ } –15 ^ { \circ } above the disk midplane , inclusive of two bulges near the disk edge .
This outer disk gas may participate in the outflow of ionized plasma which 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 .