X‐Ray Line Emission from Evaporating and Condensing Accretion Disk Atmospheres

We model the X-rays reprocessed by an accretion disk in a fiducial low-massX-ray binary system with a neutron star primary. An atmosphere, or theintermediate region between the optically thick disk and a Compton-temperaturecorona, is photoionized by the neutron star continuum. X-ray lines from therecombination of electrons with ions dominate the atmosphere emission andshould be observable with the Chandra and XMM-Newton high-resolutionspectrometers. The self-consistent disk geometry agrees well with opticalobservations of these systems, with the atmosphere shielding the companion fromthe neutron star. At a critical depth range, the disk gas has one thermallyunstable and two stable solutions. A clear difference between the model spectraexists between evaporating and condensing disk atmospheres. This differenceshould be observable in high-inclination X-ray binaries, or whenever thecentral continuum is blocked by absorbing material and the extended diskemission is not.Comment: 5 pages, 3 figures; accepted for publication in Ap