Progressive covering in dipping and Comptonization in the spectrum of XB1916-053 from the BeppoSAX observation

TL;DR

This study analyzes BeppoSAX observations of the LMXB XB 1916-053, revealing spectral changes during dips and identifying the origins of different emission components, including evidence of high-energy dipping and scattering effects.

Contribution

It provides a detailed spectral analysis of XB 1916-053, demonstrating the progressive covering model and the distinct origins of blackbody and power-law emissions in dipping behavior.

Findings

Blackbody component is rapidly absorbed, indicating a point-like source.

Extended emission likely from an accretion disk corona.

Dipping observed up to ~40 keV with evidence of electron scattering.

Abstract

We report results of a BeppoSAX observation of the low-mass X-ray binary (LMXB) dipping source XB 1916-053. The source joins the small group of LMXB detected at energies ~100 keV. The non-dip spectrum is well fitted by an absorbed blackbody with a temperature of 1.62+/-0.05 keV and an absorbed cut-off power law with a photon index of 1.61+/- 0.01 and a cut-off energy of 80+/-10 keV. Below 10 keV, where photoelectric absorption is dominant, the dramatic spectral changes observed during dips can be simply modelled by progressive covering of the blackbody and cut-off power law components. The blackbody component is very rapidly absorbed during dips, consistent with it being point-like, while the cutoff power law is more gradually absorbed, consistent with it being extended. The most likely locations for the blackbody component are the surface of the neutron star or the boundary layer between the neutron star and the accretion disk. The extended emission most probably originates in an accretion disk corona. Above 10 keV, dipping is detected up to \~40 keV, and there is some evidence for an energy-independent reduction in intensity of up to 15%. This reduction could be caused by electron scattering or obscuration. In the first case, the change is consistent with an electron column density of ~2.9e23cm^-2, several times smaller than the average hydrogen column measured simultaneously.