Disk emission and absorption lines in LMXB. Note on the physical conditions of an absorbing material

TL;DR

This paper models the X-ray spectrum of LMXB 4U 1630-472 using accretion disk atmosphere models, showing that iron absorption lines can originate from the disk atmosphere itself, offering an alternative to wind-based explanations.

Contribution

It introduces detailed radiative transfer modeling of accretion disk atmospheres to explain iron absorption lines, challenging the wind-origin hypothesis in LMXBs.

Findings

Absorption lines can originate from the disk atmosphere, not just winds.

Disk atmosphere models fit high-resolution spectra well.

Wind density and location match the upper disk atmosphere conditions.

Abstract

We show that the continuum X-ray spectrum of 4U 1630-472 with iron absorption lines can be satisfactorily modeled by the spectrum from an accretion disk atmosphere. We performed full radiative transfer calculations using our code ATM21 to model the emission from an accretion disk surface that is seen at different viewing angles. Computed models are then fitted to the high-resolution X-ray spectra of 4U 1630-472 obtained by {\it Suzaku} satellite. Absorption lines of highly ionized iron originating in a hot accretion-disk atmospheres are important part of the observed line profile, and can be an alternative or complementary explanation to the wind model usually favored for this type of sources. Next, assuming that absorption lines originate from the wind illuminated by X-ray central source in LMXBs, we can put constrains on the wind location only if we know the volume density number of the absorbing material. There are a few derivations of the distance to the wind in X-ray binaries. We show here, that the density number and the wind location agree with the density of an upper disk atmosphere at optical depth of two-thirds, at the same distance from the black hole. This comparison is done assuming optically thick, geometrically thin standard accretion disk model. Nevertheless, it shows that the wind physical conditions are the same as in thermalized disk gas, and we only have to figure out how the wind is blowing?