1 research outputs found
Modelling the disk atmosphere of the low mass X-ray binary EXO 0748-676
Low mass X-ray binaries exhibit ionized emission from an extended disk
atmosphere that surrounds the accretion disk. However, its nature and geometry
is still unclear. In this work we present a spectral analysis of the extended
atmosphere of EXO 0748-676 using high-resolution spectra from archival
XMM-Newton observations. We model the RGS spectrum that is obtained during the
eclipses. This enables us to model the emission lines that come only from the
extended atmosphere of the source, and study its physical structure and
properties. The RGS spectrum reveals a series of emission lines consistent with
transitions of O VIII, O VII, Ne IX and N VII. We perform both Gaussian line
fitting and photoionization modelling. Our results suggest that there are two
photoionization gas components, out of pressure equilibrium with respect to
each other. One with ionization parameter of 2.5 and a large opening angle, and
one with 1.3. The second component is possibly covering a smaller fraction of
the source. From the density diagnostics of the O vii triplet using
photoionization modelling, we detect a rather high density plasma of > 10^13
cm^-3 for the lower ionization component. This latter component also displays
an inflow velocity. We propose a scenario where the high ionization component
constitutes an extended upper atmosphere of the accretion disk. The lower
ionization component may instead be a clumpy gas created from the impact of the
accretion stream with the disk.Comment: A&A accepted, 10 pages, 9 figure