X-ray spectral and temporal evolution of atoll source 4U 1820-30 with AstroSat: detection of high frequency quasi-periodic oscillation

Abstract: AstroSat/LAXPC and SXT observed the persistent neutron star low-mass X-ray binary 4U 1820-30 between 2016 and 2022. During these observations, the hardness-intensity diagram (HID) and color-color diagram (CCD) indicated that the source was in the banana state. We divided the CCD into 11 segments for spectral and timing analyses. For each segment in the CCD, we modeled the spectral data using two distinct approaches over the 0.7-20.0 keV band. A combination of a multi-color-disk component with an inner disk temperature of around 0.6 keV and Comptonized emission from the boundary layer (BL)/ hot corona provided the best description of the X-ray spectral data of this source. The truncation radius was found to be in the range of $\sim$ 19-40 km. The Comptonized component has an optical depth in the range of $\sim 7 - 13$ with electron temperature in the range of $\sim 2.5 - 3.8$ keV. The optical depth of the corona varies significantly along the position on the CCD, while $\sim$ 80\% of the X-ray flux comes from the Comptonized component. We discuss possible physical scenarios to explain the relationship between the spectral evolution and motion of the source along the CCD. The timing analysis revealed kHz QPOs peaks at $\sim 710$ Hz and $\sim 740$ Hz in the lower left banana branch. An energy-dependent study indicates that these QPOs are stronger in the high-energy band.

• The paper identifies disk truncation in 4U 1820-30 with an inner radius of 19–40 km, suggesting magnetospheric effects or boundary layer formation.
• It employs joint SXT and LAXPC data and multi-component spectral models to trace the source’s evolution along the banana state, with ~80% flux from the Comptonized corona.
• The study robustly detects high-frequency QPOs (~710–740 Hz) in the lower banana branch, linking them to oscillations in the boundary layer via the relativistic precession model.

X-ray Spectral and Temporal Evolution of Atoll Source 4U 1820-30 with AstroSat: Detection of High Frequency Quasi-Periodic Oscillation

Context and Goals

This study delivers a comprehensive spectral and timing investigation of the persistent neutron star LMXB 4U 1820-30 utilizing AstroSat observations from 2016 to 2022. The primary focus is on characterizing spectral evolution across the banana state, probing disk truncation mechanisms, and detecting high-frequency kHz QPOs with robust statistical significance. The analysis leverages SXT and LAXPC data, covering 0.7–20 keV, parsing the source’s color-color diagram (CCD) into 11 segments, and combining physically motivated multi-component spectral models with high-resolution timing.

Observational Approach and Methodology

AstroSat’s SXT (0.3–8 keV) and LAXPC (3–80 keV; LAXPC20 only) were employed for joint spectral and timing analysis. Data were divided across three instrument gain epochs, further segmented via CCD position to isolate correlated temporal and spectral evolution. The HID and CCD confirmed the source repeatedly occupied the banana state, exclusively probing high mass-accretion (0.3–0.5 $L_{Edd}$) branches (2603.25036).

Spectral modeling combined the multicolor disk (diskbb) with thermal Comptonization components (nthcomp/comptb). The latter enables constraints on electron temperature and optical depth in the corona/BL. Dead-time-corrected power density spectra (PDS) were generated across energy bands (3–50 keV), identifying noise components and QPO candidates at high statistical significance.

Spectral Properties and Disk Truncation

The spectra are best fitted by a combination of MCD ($kT_\mathrm{in} \sim 0.6$ keV) plus a thermal Comptonized corona/BL component ($kT_e \sim 2.5-3.8$ keV, $\tau \sim 7-13$). The inner disk radius is found to span $\sim 19-40$ km, significantly exceeding the expected NS surface radius ($\sim 9$ km), indicating disk truncation. As the animal source moves along the BS, the corona remains optically thick, and the electron cloud stays cool ($kT_e < 4$ keV).

A strong numerical result is the dominance of the Comptonized component: approximately 80% of the total flux in the 0.7–20 keV band originates from the corona. The disk temperature remains relatively constant, with the truncation radius and optical depth evolving in tandem with the source’s CCD position.

The inferred disk truncation is interpreted as resulting from either magnetospheric interaction or formation of a boundary layer (BL) between the disk and NS. Magnetic dipole moment and field strength are estimated, and the radial extent/thickness of the BL ($R_{max}-R_{*}$, $R_{max}$) are modeled, matching the inner disk radius in several segments.

Timing Analysis and kHz QPOs

PDS analysis identifies VLFN, LFN, HFN, and prominent kHz QPOs, particularly in the lower banana branch. Statistically robust kHz QPO peaks at $\sim 710$ Hz and $kT_\mathrm{in} \sim 0.6$0 Hz (significance $kT_\mathrm{in} \sim 0.6$1, $kT_\mathrm{in} \sim 0.6$2–17) are detected in BB1 and BB2 segments. These QPOs are energy-dependent, with rms amplitude increasing in the hard (5–50 keV) band. Their occurrence correlates with the lowest mass accretion rates (around $kT_\mathrm{in} \sim 0.6$3 g cm$kT_\mathrm{in} \sim 0.6$4 s$kT_\mathrm{in} \sim 0.6$5), and the calculated radius for QPO origin via the relativistic precession model matches the BL extent ($kT_\mathrm{in} \sim 0.6$616 km), strongly linking the emission region to the BL/corona.

No transient X-ray bursts (Type I) were detected in the considered epochs.

Physical Interpretation

The observations support the scenario in which disk truncation is due to either a magnetospheric effect or formation of an optically thick boundary layer. As the disk approaches the NS surface, the corona’s optical depth increases, maximizing Comptonized emission. The cold, optically thick corona/BL is dominant in the banana state. The detected kHz QPOs, located within the BL, suggest oscillatory behavior within this region as the underlying driver for high-frequency QPOs, consistent with the relativistic precession model [Stella1998].

Contradictory claims regarding Fe emission lines, reflection features, and Compton humps are refuted; none were detected in any segment.

Implications and Future Directions

Practically, this analysis enables constraints on accretion geometry and magnetic field strengths in persistent NS LMXBs, advancing understanding of disk truncation scenarios and boundary layer dynamics. The precise correlation between BL extent and QPO origin strengthens the case for BL-driven oscillations in atoll sources. The dominance of thermal Comptonization in bolometric output, coupled with disk truncation, has implications for models of spectral state transitions and feedback to NS surface/burst mechanisms.

Theoretically, these results reinforce the BL/corona paradigm for spectral-timing coupling. The high-quality detection and localization of kHz QPOs offer avenues for future investigations of vertical structure, magnetohydrodynamic instabilities, and angular momentum dissipation at the disk-NS interface. Further polarimetric and broadband X-ray studies may resolve ambiguities concerning outflows and central corona geometry.

Conclusion

The spectral and timing properties of 4U 1820-30, as resolved by AstroSat, are characterized by a truncated accretion disk with soft thermal emission, a dominant optically thick Comptonizing corona/boundary layer, and high-frequency QPOs linked to this interface. The primary spectral evolution is driven by corona/BL properties, not disk geometry, as the source migrates along the banana branch. The detected kHz QPOs demonstrate an intimate association with BL extent, supporting precessional or boundary-induced oscillatory origins. These findings refine models for accretion flow in persistent neutron star LMXBs and inform future observational strategies for multi-messenger studies of compact accretors (2603.25036).