Spectral evolution of bright NS LMXBs

TL;DR

This paper introduces a new spectral model, CompTB, for neutron-star LMXBs that accounts for thermal and bulk Comptonization, and applies it to INTEGRAL data, notably analyzing the transient hard X-ray tail in GX 5-1.

Contribution

The paper develops and tests a novel spectral fitting model, CompTB, incorporating both thermal and bulk Comptonization effects for neutron-star LMXBs.

Findings

Successful modeling of broad band spectra with CompTB

Interpretation of the hard X-ray tail via bulk motion Comptonization

Insights into the physical conditions near the neutron star

Abstract

Theoretical and observational support suggests that the spectral evolution of neutron-star LMXBs, including transient hard X-ray tails, may be explained by the interplay between thermal and bulk motion Comptonization. In this framework, we developed a new model for the X-ray spectral fitting XSPEC package which takes into account the effects of both thermal and dynamical (i.e. bulk) Comptonization, CompTB. Using data from the INTEGRAL satellite, we tested our model on broad band spectra of a sample of persistently low magnetic field bright neutron star Low Mass X-ray Binaries, covering different spectral states. The case of the bright source GX 5-1 is presented here. Particular attention is given to the transient powerlaw-like hard X-ray (above 30 keV) tail that we interpret in the framework of the bulk motion Comptonization process, qualitatively describing the physical conditions of the environment in the innermost part of the system.