[Paper Review] Detection of a broad iron emission line and sub-millisecond quasiperiodic oscillations from the type I X-ray burster 4U1728-34 in a high state

S. Piraino, A. Santangelo+1 more|arXiv (Cornell University)|Aug 9, 2000

Astrophysical Phenomena and Observations1 references4 citations

TL;DR

This study reports the detection of a broad iron Kα emission line (FWHM ≈ 0.8 keV) and a sub-millisecond quasiperiodic oscillation (QPO) at 1284 ± 6 Hz—the highest frequency ever observed in 4U 1728-34—during simultaneous RXTE and BeppoSAX observations in a high-luminosity state (L_X ≈ 0.1 L_Edd). The results suggest the QPO may trace Keplerian motion near the inner accretion disk, while the broad iron line likely arises from relativistically broadened emission in the ionized inner disk, offering a pathway to measure the neutron star's mass via simultaneous spectral-timing analysis.

ABSTRACT

We report results from simultaneous RossiXTE and BeppoSAX observations of the neutron-star x-ray binary and type I X-ray burster 4U1728-34. The source was found in a high luminosity state, L_X near 0.1 L_Edd, and quasiperiodic oscillations were detected at 1284 +/- 6 Hz, the highest frequency ever observed from this source. The x-ray spectrum shows a broad, FWHM = 0.8 keV, iron K-alpha fluorescence line. We discuss interpretations of the broad line and the quasiperiodic oscillations and how future simultaneous spectral and timing observations can be used to test these interpretations and, potentially, to estimate the mass of the compact object.

Motivation & Objective

• To investigate the nature of high-frequency QPOs and broad iron emission lines in the neutron star X-ray binary 4U 1728-34 during a high-luminosity state.
• To test whether the observed QPO frequency and broad iron line can be linked to relativistic effects in the inner accretion disk.
• To explore the potential of simultaneous spectral and timing observations for measuring the mass of the neutron star in accreting systems.
• To determine the origin of the broad iron line—whether from the accretion disk or a compact corona—using ionization and broadening constraints.

Proposed method

• Simultaneous observations using RXTE (for high-time-resolution timing) and BeppoSAX (for high-energy-resolution spectroscopy) during a 51.3 ks BeppoSAX and 48 ks RXTE observation.
• Analysis of 122 μs-resolution PCA data to search for kHz QPOs in 400–2000 Hz range, with energy filtering above 4 keV to enhance signal-to-noise.
• Spectral fitting of the X-ray continuum and emission features, focusing on the Fe Kα line profile to assess its width, energy, and ionization state.
• Use of relativistic disk models to interpret the broad iron line as a result of Doppler and gravitational redshift effects near the inner disk radius.
• Application of Kepler’s third law (ν² ∝ r⁻³) to relate QPO frequency to orbital radius, enabling mass estimation if disk radius is independently constrained.
• Comparison of QPO frequency evolution with spectral state (color-color diagram) to infer accretion rate and disk structure.

Experimental results

Research questions

• RQ1 Can the highest-frequency QPO ever detected in 4U 1728-34 (1284 ± 6 Hz) be interpreted as the Keplerian orbital frequency at the inner edge of the accretion disk?
• RQ2 Is the observed broad iron Kα emission line (FWHM ≈ 0.8 keV) consistent with relativistic broadening from the ionized inner accretion disk?
• RQ3 Does the line originate from the disk or from a compact corona, and how can future high-resolution spectral data distinguish between these origins?
• RQ4 Can a correlation between QPO frequency and relativistically broadened Fe line profile provide a direct measurement of the neutron star mass?
• RQ5 What constraints do the observed ionization state (ξ ≈ 500–1200 erg cm⁻¹ s⁻¹) and Comptonization effects place on the emission region geometry?

Key findings

• A sub-millisecond QPO at 1284 ± 6 Hz was detected with a 4.3σ significance, marking the highest frequency QPO ever observed from 4U 1728-34.
• The QPO has a width of 32 ± 12 Hz and an rms amplitude of 3.9% ± 0.5% above 4 keV, indicating a coherent, high-frequency oscillation in the persistent emission.
• A broad iron Kα emission line with a full width at half maximum of approximately 0.8 keV was detected, consistent with relativistic broadening from the inner accretion disk.
• The ionization state of the emitting material (ξ ≈ 500–1200 erg cm⁻¹ s⁻¹) supports emission from the inner disk rather than the neutron star surface, as it allows for significant Fe XXV–XXVI line flux.
• The source was in a high mass accretion rate state (L_X ≈ 0.1 L_Edd), as inferred from its position in the color-color diagram, which correlates with increasing QPO frequency.
• The results suggest that future simultaneous spectral and timing observations could correlate QPO frequency with disk radius (via Fe line profile) to measure the neutron star mass via Kepler’s law.