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Time lags and their association with the Boundary Layer structure in a Z source GX 349+2

Abhishek M. V. R., Sriram K, Gouse SD

TL;DR

GX 349+2, a Sco-like Z-source neutron star X-ray binary, is analyzed to reveal how soft (0.8–2.0 keV) and hard (2.0–10 keV) X-ray emission co-vary over the source's HID branches. Using XMM-Newton EPIC-pn timing data, the authors compute cross-correlation functions across HB, NB, and FB segments and during transition windows, finding branch-dependent asymmetry and lags of hundreds of seconds on the HB but near-zero lag on NB/FB. Lag significance is established via 10,000 Monte Carlo realizations and supplemented by time-resolved spectral fits with two continuum models, plus MCMC to probe parameter covariances. Interpreting the lags as slow boundary-layer/corona readjustment within a viscous inner flow, they infer very low effective viscosity and estimate the BL radius to be in the ~13–34 km range depending on the spectral model, consistent with an extended inner accretion structure. The work links timing signatures to the BL/ADC physics and jet–disk coupling, and it motivates coordinated X-ray–radio campaigns to further elucidate jet formation and inner-disk geometry.

Abstract

Studying the cross-correlation function between the soft and hard X-ray emission in Neutron Star Low Mass X-ray Binaries provides crucial insight into the structure and dynamics of the innermost accretion regions. In this work, we investigate the CCF of the Z-source GX 349+2 using an XMM-Newton observation. We noted that asymmetric CCFs with lags of a few hundred secondsbetween soft and hard band LCs in the horizontal branch, whereas CCFs remained symmetric in normal and flaring branches. We also performed a CCF study during the flux transition duration and observed lags of the order of a few tens to hundreds of seconds. Monte Carlo simulations were performed to assess the robustness of these CCFs, confirming their significance at a 95% confidence level. Spectral analysis during the flux transitions further suggests that the inner accretion disk extends close to the last stable orbit. We propose that the observed hard lags arise from the readjustment of the boundary layer/coronal region located near the inner edge of the accretion disk. From the measured lags, we estimate the characteristic size of the boundary layer. We show that the observed lags could also be associated with the depletion timescale of the boundary layer with low viscosity.

Time lags and their association with the Boundary Layer structure in a Z source GX 349+2

TL;DR

GX 349+2, a Sco-like Z-source neutron star X-ray binary, is analyzed to reveal how soft (0.8–2.0 keV) and hard (2.0–10 keV) X-ray emission co-vary over the source's HID branches. Using XMM-Newton EPIC-pn timing data, the authors compute cross-correlation functions across HB, NB, and FB segments and during transition windows, finding branch-dependent asymmetry and lags of hundreds of seconds on the HB but near-zero lag on NB/FB. Lag significance is established via 10,000 Monte Carlo realizations and supplemented by time-resolved spectral fits with two continuum models, plus MCMC to probe parameter covariances. Interpreting the lags as slow boundary-layer/corona readjustment within a viscous inner flow, they infer very low effective viscosity and estimate the BL radius to be in the ~13–34 km range depending on the spectral model, consistent with an extended inner accretion structure. The work links timing signatures to the BL/ADC physics and jet–disk coupling, and it motivates coordinated X-ray–radio campaigns to further elucidate jet formation and inner-disk geometry.

Abstract

Studying the cross-correlation function between the soft and hard X-ray emission in Neutron Star Low Mass X-ray Binaries provides crucial insight into the structure and dynamics of the innermost accretion regions. In this work, we investigate the CCF of the Z-source GX 349+2 using an XMM-Newton observation. We noted that asymmetric CCFs with lags of a few hundred secondsbetween soft and hard band LCs in the horizontal branch, whereas CCFs remained symmetric in normal and flaring branches. We also performed a CCF study during the flux transition duration and observed lags of the order of a few tens to hundreds of seconds. Monte Carlo simulations were performed to assess the robustness of these CCFs, confirming their significance at a 95% confidence level. Spectral analysis during the flux transitions further suggests that the inner accretion disk extends close to the last stable orbit. We propose that the observed hard lags arise from the readjustment of the boundary layer/coronal region located near the inner edge of the accretion disk. From the measured lags, we estimate the characteristic size of the boundary layer. We show that the observed lags could also be associated with the depletion timescale of the boundary layer with low viscosity.
Paper Structure (9 sections, 5 equations, 10 figures, 4 tables)

This paper contains 9 sections, 5 equations, 10 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Observations
  3. Data Reduction and Analysis
  4. Validation of Lags
  5. Cross Correlation Confidence Interval
  6. Spectrum Analysis
  7. Discussion
  8. Relation between lags and BL
  9. Conclusion

Figures (10)

  • Figure 1: The left panel is a plot of the LC in the 0.8-10 keV band, with the sections where lags are found (horizontal branch) highlighted; the right panel is a plot of the Hardness Intensity Diagram of the observation.
  • Figure 2: First two sub-plots in each row are the soft and hard LCs, respectively, and the third is the CCF plot. HB1 and HB2.
  • Figure 3: The first two plots in each row are the soft and hard LCs, respectively, and the third is the CCF. All rows belong to the FB.
  • Figure 4: Continued.. The top 2 rows display the LCs and CCFs of the FB, and the bottom 2 panels show the NB section.
  • Figure 5: continued.. NB section LCs and CCF.
  • ...and 5 more figures