[Paper Review] An X-ray study of the dipping low mass X-ray binary XB 1323-619

M. Bałucińska‐Church, M. J. Church+4 more|arXiv (Cornell University)|Sep 13, 1999

Astrophysical Phenomena and Observations3 citations

TL;DR

This study presents a detailed X-ray analysis of the dipping low-mass X-ray binary XB 1323-619 using BeppoSAX observations, revealing a complex spectral behavior during dips and bursts. The key finding is that X-ray bursts during dips are only weakly absorbed, suggesting burst-induced ionization of the absorber, while spectral evolution during dips is best explained by progressive covering of an extended power-law component and rapid absorption of a point-like blackbody.

ABSTRACT

During a BeppoSAX observation of the low-mass X-ray binary dip source XB 1323-619 a total of 10 type I X-ray bursts and parts of 12 intensity dips were observed. During non-bursting, non-dipping intervals, the 1-150 keV BeppoSAX spectrum can be modelled by a cutoff power-law with a photon index of 1.48 +/- 0.01, a cutoff energy of 44.1 +5.1/-4.4 keV together with a blackbody with kT of 1.77 +/- 0.25 keV contributing ~15% of the 2-10 keV flux. Absorption equivalent to 3.88 +/- 0.16x10^22 H atom cm^(-2) is required. The dips repeat with a period of 2.938 +/- 0.020 hr and span 40% of the orbital cycle. During dips the maximum reduction in 2-10 keV intensity is ~65%. The spectral changes during dips are complex and cannot be modelled by a simple absorber because of the clear presence of part of the non-dip spectrum which is not absorbed. Spectral evolution in dipping can be well modelled by progressive covering of the cutoff power-law component which must be extended, plus rapid absorption of the point-source blackbody. One of the bursts is double and 4 of the bursts occurred during dipping intervals. These bursts have 2-10 keV peak count rates reduced by only 22% on average from those occurring outside the dips, and are not heavily absorbed. One explanation for this lack of absorption is that the bursts temporarily ionize the absorbing material responsible for the dips.

Motivation & Objective

• Understand the spectral and temporal behavior of the dipping low-mass X-ray binary XB 1323-619 during X-ray dips and bursts.
• Characterize the intrinsic X-ray spectrum during non-dipping intervals to determine emission components and absorption.
• Investigate the nature of the dipping phenomenon, including the geometry and optical depth of the absorbing material.
• Explain the observed burst behavior during dips, particularly the minimal reduction in peak flux despite deep dips.
• Model the spectral evolution during dips to infer the spatial and physical structure of the X-ray emission and absorber.

Proposed method

• Analyzed BeppoSAX observations covering 10 type I X-ray bursts and 12 intensity dips in XB 1323-619.
• Modelled the 1-150 keV spectrum during non-bursting, non-dipping intervals using a cutoff power-law and a blackbody component.
• Measured the absorption column density using spectral fitting, finding N_H = 3.88 × 10^22 cm⁻².
• Quantified the dip period (2.938 ± 0.020 hr) and depth (40% of orbital cycle, 65% intensity reduction in 2-10 keV band).
• Modelled spectral evolution during dips using a covering fraction approach, distinguishing between extended power-law and point-like blackbody components.
• Assessed burst flux reduction during dips to infer the ionization state of the absorber, comparing with non-dip burst behavior.

Experimental results

Research questions

• RQ1 What is the intrinsic X-ray spectrum of XB 1323-619 during non-dipping intervals, and what are the contributions of the power-law and blackbody components?
• RQ2 How does the spectral energy distribution change during dipping intervals, and can it be explained by simple absorption or requires a more complex covering model?
• RQ3 Why are X-ray bursts during dips only weakly suppressed in flux, despite deep intensity dips?
• RQ4 What physical mechanism could explain the lack of significant absorption during bursts, and how does it relate to the absorber's ionization state?
• RQ5 How does the geometry of the absorbing material affect the observed X-ray variability, and what does this imply about the accretion flow structure?

Key findings

• The 1-150 keV spectrum during non-dipping intervals is best modelled by a cutoff power-law with photon index 1.48 ± 0.01 and cutoff energy 44.1 +5.1/-4.4 keV, plus a blackbody with kT = 1.77 ± 0.25 keV contributing ~15% of the 2-10 keV flux.
• Absorption equivalent to 3.88 ± 0.16 × 10^22 H atoms cm⁻² is required to fit the observed spectrum.
• The dips recur with a period of 2.938 ± 0.020 hr and cover 40% of the orbital cycle, with a maximum 2-10 keV intensity reduction of ~65%.
• Spectral evolution during dips cannot be explained by simple absorption; instead, it requires progressive covering of the extended cutoff power-law component and rapid absorption of the point-like blackbody.
• Four of the 10 observed bursts occurred during dipping intervals, and their 2-10 keV peak count rates were reduced by only 22% on average, indicating weak absorption during bursts.
• One burst was double, and the lack of strong absorption during bursts suggests the burst radiation ionizes the absorbing material, temporarily reducing its optical depth.