The appearance of non-spherical systems. Application to LMXB

TL;DR

This paper models the UV/X-ray spectra of neutron star accretion systems considering non-spherical geometry, explaining observed spectral features and fitting data from XTE J1709-267, though some predicted features remain unobserved.

Contribution

It introduces a non-spherical geometric model for neutron star accretion systems that predicts double bump spectral features, enhancing understanding of LMXB spectra.

Findings

Double bumps in UV/X-ray spectra are caused by non-spherical geometry.

Model explains high-energy broadening in disk spectra.

Predicted features are not observed in current data for XTE J1709-267.

Abstract

We study the appearance of the neutron star - accretion disk system as seen by a distant observer in the UV/X-ray domain. The observed intensity spectra are computed assuming non-spherical geometry of the whole system, in which outgoing spectrum is not represented by the flux spectrum, the latter being valid for spherically symmetric objects. Intensity spectra of our model display double bumps in UV/X-ray energy domains. Such structure is caused by the fact that the the source is not spherically symmetric, and the proper integration of intensity over emitted area is needed to reproduce observed spectral shape. Relative normalization of double bump is self consistently computed by our model. X-ray spectra of such a type were often observed in LMXB with accretion disk, ultra luminous X-ray sources, and accreting black hole systems with hot inner compact corona. Our model naturally explains high energy broadening of the disk spectrum observed in some binaries. We attempted to fit our model to X-ray data of XTE~J1709-267 from {\it XMM-Newton}. Unfortunately, the double intensity bump predicted by our model for LMXB is located in soft X-ray domain, uncovered by existing data for this source.