[Paper Review] BeppoSAX Observations of the Atoll X-Ray Binary 4U0614+091

S. Piraino, A. Santangelo+2 more|arXiv (Cornell University)|Oct 6, 1999

Astrophysical Phenomena and Observations3 citations

TL;DR

This study presents the first simultaneous broad-band X-ray spectrum (0.3–150 keV) of the neutron star X-ray binary 4U0614+091 using BeppoSAX. It identifies a hard X-ray tail best modeled by thermal Comptonization with electron temperatures >220 keV or a non-thermal power law, and detects a spectral feature consistent with Compton reflection from an optically thick disk, showing a correlation between the power-law photon index and reflected fraction similar to that seen in black hole binaries and Seyfert galaxies.

ABSTRACT

We report the first simultaneous measurement of the broad band X-ray (0.3-150 keV) spectrum of the neutron star x-ray binary 4U0614+091. Our data confirm the presence of a hard x-ray tail that can be modeled as thermal Comptonization of low-energy photons on electrons having a very high temperature, greater than 220 keV, or as a non-thermal powerlaw. We detected a spectral feature that can be interpreted as reprocessing, via Compton reflection, of the direct emission by an optically-thick disk and found a correlation between the photon index of the power-law tail and the fraction of radiation reflected which is similar to the correlation found for black hole candidate x-ray binaries and Seyfert galaxies.

Motivation & Objective

• To obtain the first simultaneous broad-band X-ray spectrum of the atoll-type X-ray binary 4U0614+091 across 0.3–150 keV.
• To determine the physical origin of the hard X-ray tail observed in 4U0614+091, distinguishing between thermal Comptonization and non-thermal power-law emission.
• To investigate the presence and properties of Compton reflection features in the source's spectrum.
• To test whether the correlation between the power-law photon index and the fraction of reflected radiation, previously observed in black hole X-ray binaries and Seyfert galaxies, also holds in neutron star systems.

Proposed method

• Acquisition of simultaneous X-ray data from the BeppoSAX satellite covering the energy range 0.3–150 keV.
• Spectral fitting using a model combining a multicolor disk blackbody, a power-law component, and a Compton reflection hump to describe the observed continuum.
• Modeling of the hard X-ray tail via thermal Comptonization with a high-temperature electron population (>220 keV).
• Alternative modeling of the tail using a non-thermal power-law component to assess spectral consistency.
• Quantitative analysis of the correlation between the photon index of the power-law component and the fraction of reflected radiation.
• Comparison of spectral behavior in 4U0614+091 with that of black hole candidate X-ray binaries and Seyfert galaxies to assess universality of spectral correlations.

Experimental results

Research questions

• RQ1 What is the physical origin of the hard X-ray tail in the X-ray spectrum of 4U0614+091?
• RQ2 Can the observed spectral features in 4U0614+091 be explained by Compton reflection from an optically thick accretion disk?
• RQ3 Is there a correlation between the photon index of the power-law component and the fraction of radiation reflected in this neutron star system?
• RQ4 How does the spectral behavior of 4U0614+091 compare to that of black hole X-ray binaries and Seyfert galaxies in terms of reflection and hard tail properties?

Key findings

• The broad-band X-ray spectrum of 4U0614+091 reveals a hard X-ray tail that is best described by thermal Comptonization with electron temperatures exceeding 220 keV.
• An alternative fit using a non-thermal power-law component also provides a good description of the hard tail, indicating spectral ambiguity between thermal and non-thermal origins.
• A spectral feature consistent with Compton reflection is detected, indicating reprocessing of direct emission by an optically thick accretion disk.
• A correlation is found between the photon index of the power-law tail and the fraction of radiation reflected, mirroring a previously observed trend in black hole X-ray binaries and Seyfert galaxies.
• The spectral characteristics of 4U0614+091 suggest that reflection-based spectral correlations may be a universal feature across different types of X-ray sources, including neutron star systems.