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Detection of a Type-C QPO during the soft-to-hard transition in Swift J1727.8-1613

Maïmouna Brigitte, Noel Castro Segura, Federico García, Jiří Svoboda, María Díaz Trigo, Mariano Méndez, Federico Vincentelli, Douglas J. K. Buisson, Diego Altamirano

Abstract

Timing analysis of accreting systems is key to probe the structure and dynamics around compact objects. In Black-Hole Low-Mass X-ray Binaries (BH LMXBs), the compact object accretes matter from a low-mass companion star via Roche Lobe overflow, forming an accretion disk, and occasionally exhibiting bright eruptions. The BH LMXB Swift J1727.8-1613 (hereafter J1727), recently underwent one of the brightest outbursts ever recorded in X-rays, in August 2023. This analysis aims to study the timing properties of J1727, in the decaying phase of its outburst, using high-time resolution XMM-Newton data. We analyzed J1727's power spectrum (PS) and cross spectrum (CS), which we modeled with Lorentzians. The PS reveals how the source's power is distributed across frequencies, and the Real and Imaginary parts of the CS compare the displacement of the light curves in different energy bands across the observations. Finally, we simultaneously derived the phase lags and the coherence, using a constant phase lag model. While the first (soft-state) observation does not show any strong variability, the two harder observations exhibit quasi-periodic oscillations (QPOs). Because the QPO is more significantly detected in the Imaginary part of the CS than in the PS, we refer to it as the 'Imaginary QPO'. The QPO is more prominent in the soft 0.3-2 keV band than in the hard 2-12 keV band. As the source evolves towards the hard state, the Imaginary QPO shifts to lower frequencies, the broadband fractional rms amplitude in the 0.3-2 keV energy band increases, while the rms covariance of the Imaginary QPO decreases. Simultaneously, the phase lags increase and the coherence function drops at the Imaginary QPO frequency. In the elusive soft-to-hard transition of J1727, the first XMM-Newton observations of the source reveal an Imaginary QPO also detected in the PS, exhibiting the properties of a type-C QPO.

Detection of a Type-C QPO during the soft-to-hard transition in Swift J1727.8-1613

Abstract

Timing analysis of accreting systems is key to probe the structure and dynamics around compact objects. In Black-Hole Low-Mass X-ray Binaries (BH LMXBs), the compact object accretes matter from a low-mass companion star via Roche Lobe overflow, forming an accretion disk, and occasionally exhibiting bright eruptions. The BH LMXB Swift J1727.8-1613 (hereafter J1727), recently underwent one of the brightest outbursts ever recorded in X-rays, in August 2023. This analysis aims to study the timing properties of J1727, in the decaying phase of its outburst, using high-time resolution XMM-Newton data. We analyzed J1727's power spectrum (PS) and cross spectrum (CS), which we modeled with Lorentzians. The PS reveals how the source's power is distributed across frequencies, and the Real and Imaginary parts of the CS compare the displacement of the light curves in different energy bands across the observations. Finally, we simultaneously derived the phase lags and the coherence, using a constant phase lag model. While the first (soft-state) observation does not show any strong variability, the two harder observations exhibit quasi-periodic oscillations (QPOs). Because the QPO is more significantly detected in the Imaginary part of the CS than in the PS, we refer to it as the 'Imaginary QPO'. The QPO is more prominent in the soft 0.3-2 keV band than in the hard 2-12 keV band. As the source evolves towards the hard state, the Imaginary QPO shifts to lower frequencies, the broadband fractional rms amplitude in the 0.3-2 keV energy band increases, while the rms covariance of the Imaginary QPO decreases. Simultaneously, the phase lags increase and the coherence function drops at the Imaginary QPO frequency. In the elusive soft-to-hard transition of J1727, the first XMM-Newton observations of the source reveal an Imaginary QPO also detected in the PS, exhibiting the properties of a type-C QPO.
Paper Structure (15 sections, 4 equations, 5 figures, 2 tables)

This paper contains 15 sections, 4 equations, 5 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Data Sets
  3. Analysis and results
  4. The power spectra
  5. The real and imaginary parts of the cross spectrum
  6. The phase lags and coherence
  7. The energy dependence of the QPO lags and rms amplitude
  8. Discussion and conclusions
  9. The type-C QPO
  10. Phase-lag jump
  11. Coherence drop
  12. QPO energy dependence
  13. Summary
  14. Observation log and light curve
  15. Hardness intensity diagram

Figures (5)

  • Figure 1: Left panel: Power spectra of the second (top) and third (bottom) EPIC-PN observations of Swift J1727.8--1613 in the 0.3--2 keV (black) and 2--12 keV (orange) energy bands. Right panel: real (green) and imaginary (pink) parts of the cross spectra of the same two observations using the 0.3--2 keV and 2--12 keV energy bands, rotated by 45° (see Sect. \ref{['real and imaginary parts of the CS']} for more details). The vertical red line shows the position of the central frequency $\nu_0$ of the imaginary QPO. The total model is shown with the continuous line, and the individual Lorentzians are shown as dotted lines. The residuals defined as $\mathrm{\Delta \chi=}$ (data--model)/error are plotted below each fit.
  • Figure 2: Phase lags in radian (top) and the coherence function (bottom) for the second (left) and third (right) EPIC-PN observation of Swift J1727.8--1613. The continuous lines show the predicted model from the fits of the PS and CS represented in Fig. \ref{['fig: PS fit']}. The bottom panels show the residuals defined as (data-model)/error. The dotted vertical red line indicates the central frequency of the imaginary QPO for each observation.
  • Figure 3: Energy dependence of the fractional rms (top) and the phase lags (bottom) of the imaginary QPO for the second (blue circles) and third (purple squares) EPIC-PN observations of Swift J1727.8--1613.
  • Figure 4: Zoomed XMM EPIC-PN light curves of Swift J1727.8--1613 in the 0.2--12 keV energy range, with a bin size of 0.1 s for the second observation (epoch XMM2). The shaded regions delimit the telemetry gaps and the colored data points are the good time intervals (GTIs).
  • Figure 5: Hardness intensity diagram (HID) of Swift J1727.8--1613 from the 2023 outburst. The photon fluxes are derived from the MAXI/GSC instrument with a bin size of 1 day from the "on-demand process" archives. The hardness ratio is defined as the photon flux ratio in the 4--10 keV over the photon flux in the 2--4 keV energy band.