Discovery of Evolving Low-Frequency Quasi-Periodic Oscillations in Hard X-rays Observed in Black Hole Swift J1727.8−1613 with AstroSat
This study presents the pioneering detection of evolving low-frequency quasi-periodic oscillations (LFQPO) in hard X-rays up to 100 keV using the AstroSat/LAXPC instrument during an unconventional outburst phase of the black hole Swift J1727.8−1613 in the hard intermediate state (HIMS). The research identifies the first instance of evolving LFQPO features in Swift J1727.8−1613 above 40 keV during the HIMS, making it the third known black hole X-ray binary to exhibit significant LFQPO (>1 Hz) detection at higher energies (∼100 keV). The detailed analysis sheds light on the dynamic behavior of LFQPOs in the high-energy spectrum, opening new avenues for understanding the accretion processes in black hole systems.
Introduction:
The investigation at hand unveils a remarkable discovery in black hole astrophysics, elucidating the evolution of low-frequency quasi-periodic oscillations (LFQPO) in hard X-rays around the black hole Swift J1727.8−1613. Through observations with the AstroSat/LAXPC instrument, this study advances our understanding of the behavior of high-energy emission in black hole X-ray binaries, particularly during the unusual outburst phase in the hard intermediate state.
Observations and Analysis:
The study is anchored on data acquired from the AstroSat/LAXPC instrument, which enabled the unprecedented detection of evolving LFQPO features in hard X-rays (up to 100 keV) during the HIMS of Swift J1727.8−1613. The rigorous analysis of the observed LFQPO frequencies and their evolution provides valuable insights into the accretion processes and variability mechanisms operating in the vicinity of black hole systems.
Findings and Implications:
The identification of evolving LFQPOs in the high-energy spectrum of Swift J1727.8−1613 not only expands our knowledge of the behavior of black hole X-ray binaries but also holds implications for our comprehension of accretion-driven phenomena in these systems. The findings pave the way for further explorations into the dynamics of high-energy emission from black hole accretion disks and the underlying physical processes governing such manifestations.
In closing, this study heralds a significant milestone in the realm of black hole astrophysics by unveiling the first detection of evolving low-frequency quasi-periodic oscillations in hard X-rays in the black hole Swift J1727.8−1613. The discovery not only showcases the capabilities of the AstroSat/LAXPC instrument but also underscores the potential for unraveling the intricacies of high-energy phenomena associated with black hole accretion.
References:
The findings of this study draw upon data from the AstroSat mission of the Indian Space Research Organisation (ISRO), archived at the Indian Space Science Data Centre (ISSDC). The study also utilizes data from the Monitor of All-sky X-ray Image (MAXI), provided by the Institute of Physical and Chemical Research (RIKEN), Japan Aerospace Exploration Agency (JAXA), and the MAXI team. The usage of software from the High Energy Astrophysics Science Archive Research Center (HEASARC) and NASA’s Astrophysics Data System Bibliographic Services is also acknowledged. Lastly, the study makes use of data from the Neutron Star Interior Composition Explorer (NICER) missions, archived at the HEASARC data center.
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The following summary will further help our readers to encapsulate the profound significance of the discovery of evolving low-frequency quasi-periodic oscillations in hard X-rays observed in the black hole Swift J1727.8−1613 with AstroSat, showcasing the intricate interplay of high-energy phenomena in black hole X-ray binaries:
Discovery of Evolving Low-Frequency QPOs in Hard X-rays:
– The first detection of evolving low-frequency quasi-periodic oscillation (LFQPO) frequencies in hard X-rays up to 100 keV with AstroSat/LAXPC during the ‘unusual’ outburst phase of Swift J1727.8−1613 in hard intermediate state (HIMS).
– Type-C QPOs (1.09–2.6 Hz) are found to evolve monotonically during HIMS of the outburst with clear detection in hard X-rays (80−100 keV).
Spectral Analysis and Correlation:
– The observed LFQPO in the 20–100 keV range has a centroid hertz, a coherence factor Q = 7.14, and an amplitude with significance σ = 5.46.
– Correlation is seen between low- (2–20 keV) and high-energy (15–50 keV) flux, where the frequency () anticorrelates with low-energy flux and correlates with high-energy flux.
Energy Spectrum and Luminosity:
– The wide-band (0.7−40 keV) energy spectrum of NICER/XTI and AstroSat/LAXPC is satisfactorily described by the dominant thermal Comptonization contribution (∼88%) in the presence of a weak signature of disc emissions (kTin ∼ 0.36 keV), indicating a harder spectral distribution.
– The unabsorbed bolometric luminosity, considering source mass and distance (1.5 < d (kpc) < 5), is estimated.
Context and Implications:
– Black hole X-ray binaries (BH-XRBs) serve as cosmic laboratories to understand the underlying physical mechanisms governing accretion dynamics around compact objects.
– LFQPOs are considered effective diagnostic tools to examine accretion scenarios and the nature of the central source, offering insights into accretion dynamics around black hole X-ray binaries.
Discovery of HIMS in Swift J1727.8−1613:
– LFQPO signatures observed at higher energies in a rare phenomenon.
– Monitoring with MAXI/GSC reveals a rapid increase in X-ray flux reaching peak value ∼7 Crab.
Radio Emission Observations:
– Radio counterpart consistent with optical position, VLA and ATA observe increase in radio flux post-discovery.
– VLITE continuously monitors radio frequency at 338 MHz.
QPOs and X-ray Polarimetry:
– Strong QPOs in the frequency range 0.44–0.88 Hz observed by NICER and Swift/BAT.
– IXPE detects polarized emission in HIMS with predicted inclination and distance of i ∼ 30°–60° and 1.5 kpc.
Energy Resolved LFQPO:
– Energy resolved LFQPO detected at frequencies Hz in hard X-rays.
– AstroSat/LAXPC enables study of LFQPO variability at high energies.
Observation Details:
– AstroSat observes Swift J1727.8−1613 in slew mode and as part of Target of Opportunity campaign.
– LAXPC10 data used for temporal analysis and spectral modelling.
NICER Observations:
– Analysis of NICER observations quasi-simultaneous with AstroSat data.
– Extraction of spectral products using NICER data analysis software.
Outburst Profile:
– Swift J1727.8−1613 exhibits canonical outburst profile with sudden rise and slow decay.
– Radio emission variations observed, including a significant increase and subsequent strong radio flare.
Hardness-Intensity Diagram:
– Study of outburst profile using data from MAXI, BAT, and VLITE.
– Evolution of detected Type-C QPO frequency during the onset phase.
Long-term Monitoring:
– Variation of count rates in different energy ranges
– Evolution of source detected from different instruments
Hardness-Intensity Diagram:
– Shows different spectral states during outburst progression
– Source likely in decay phase of SIMS with specific HR and flux level
Detection of LFQPO in Hard X-Rays:
– Examining power density spectra during outburst phase
– Using Lorentzians and power-law components for PDS modelling
Modeling Approach:
– Describing PDS continuum and QPO features in specific energy range
– Fitting the PDS with various Lorentzians and power-law components
Best-Fitting PDS:
– Model components used for fitting PDS in energy band
– Strong LFQPO feature observed at specific frequency
Model Parameters Obtained from Best-Fitting PDS:
– The obtained model parameters from the best-fitting Power Density Spectrum of epoch AS2 (MJD 60195) in various energy bands.
– Parameters include power-law index, normalization (αPL, normPL), and Lorentzian components (Li where i = 1 to 9) used in the fitting.
Significance of LFQPO Features:
– The significance of LFQPO features is verified by comparing the chi-square statistics before and after including respective Lorentzian components.
– Significance is indicated by the improvement in χ2 per degree of freedom presented in Table 1.
Estimation of σ and Q factor for LFQPO Features:
– σ and Q factor for fundamental LFQPO are estimated as 5.46σ (AS2) and 22.97σ (AS7) along with Q factors of 7.14 and 5.02, respectively.
– Percentage rms amplitude (RMS tot) of detected LFQPO is also computed for the respective epochs.
Best-Fitting QPO and Harmonic Characteristics:
– Characteristics obtained from different observations with LAXPC20 (AS1) and LAXPC10 (AS2, AS7) of AstroSat are presented.
– Fundamental and harmonic characteristics for LFQPO features in different energy ranges are tabulated in Table 2.
Evolution of LFQPO Frequency:
– The evolution of LFQPO frequency during the AstroSat campaign is shown, including NICER and Swift/BAT detections.
– Details are provided in Fig. 1(e) along with frequency range observations.
Fundamental QPO AS13-100:
– Frequency range: 21.8-23.2 Hz
– Q factor: 1.00
Harmonic AS13-100:
– Frequency range: 22.3-24.6 Hz
– Q factor: 1.02
Fundamental QPO AS220-100:
– Frequency range: 7.14-8.43 Hz
– Q factor: 1.12
Harmonic AS220-100:
– Frequency range: 19.17-21.88 Hz
– Q factor: 1.10
Fundamental QPO AS720-100:
– Frequency range: 5.02-6.46 Hz
– Q factor: 1.13
Harmonic AS720-100:
– Frequency range: 17.55-22.95 Hz
– Q factor: 1.37
Important Note:
– Higher frequency observed due to excess residuals
– No additional Lorentzian used
Table Reference:
– Best-fitting QPO and harmonic characteristics from AstroSat
– Information provided for AS1, AS2, and AS7
Detection of LFQPO Features:
– Strong Type-C LFQPO features are detected in energy bands 20–40, 40–60, 60–80, and 80–100 keV.
– Centroid frequency remains independent of X-ray photon energy.
Detection Significance Confirmation:
– Simulation analysis using simftest confirms the significance of LFQPO detection.
– Real observed data aligns outside the simulated distribution.
Variation of LFQPO Amplitude:
– Energy-resolved rms amplitude variation with Swift/BAT and MAXI/GSC fluxes is studied.
– Anticorrelation with Swift/BAT flux and positive correlation with MAXI/GSC flux are observed.
Energy Fitting and Parameters:
– Energy dependent model fitting is performed for Swift J1727.8−1613.
– Parameters and fit statistics are tabulated in Table 2.
Spectral Energy Distribution Analysis:
– Spectral properties of Swift J1727.8−1613 studied using combined NICER/XTI and AstroSat/LAXPC20 observations.
– Model combination const*Tbabs*(Gaussian + diskbb + smedge*nthcomp) used for spectral modeling.
Model Fitting Results:
– Acceptable fit achieved with spectral model, indicating presence of weak disc signature and hard Comptonized spectral tail.
– Model parameters estimated and tabulated in Table 3.
Flux Estimation and Parameters:
– Flux calculation for different spectral components and total bolometric flux performed.
– Optical depth (τ) and Compton y-parameter estimated following specific methods.
Best-fitting Spectra Visualization:
– Best-fitting energy spectra of Swift J1727.8−1613 shown with NICER and AstroSat observations.
– Residual variations illustrated for different model combinations.
LFQPO Features Discovery:
– Evolving LFQPO features (1.4−2.6 Hz) detected at hard X-rays (∼100 keV) with AstroSat during the ‘unusual’ outburst phase of Swift J1727.8−1613.
– Significant detection of energy-dependent LFQPO features above 40 keV, particularly in the range of 80–100 keV with Q > 6 and σ > 3, observed in the presence of HIMS.
Spectral Analysis:
– Source showed strong LFQPO (Hz) at energies <100 keV during different outburst phases, indicating a potential connection to Lense–Thirring precession of the inner hot flow.
– Unfolded energy spectra indicated dominance of non-thermal emission with electron temperature kTe ∼ 4–7 keV and spectral index Γ ∼ 1.6−1.9.
LFQPO Origin Models:
– Investigations into Type-C LFQPOs using LT precession of a small-scale jet suggested a possible explanation for the observed modulation of flux.
– Limitations in the jet precession model noted in explaining the evolution of LFQPO features at high energies >40 keV in Swift J1727.8−1613.
Implications of Observations:
– Challenges identified in accounting for LFQPO evolution based on jet precession models in low-inclination sources.
– Model limitations highlighted concerning the increase in LFQPO frequency with decreasing jet height in the context of the observed source behavior.
Observations of Swift J1727.8−1613:
– The Type-C LFQPO (low-frequency quasi-periodic oscillation) features of Swift J1727.8−1613 have been detected above 40 keV in the hard intermediate spectral state (HIMS) observed with AstroSat.
– This detection makes Swift J1727.8−1613 the third known black hole X-ray binary (BH-XRB) to exhibit a significant detection of LFQPO (>1 Hz) at higher energies (~100 keV).
Characteristics of LFQPOs in BH-XRBs:
– The Type-C LFQPOs are observed in BH-XRBs with a time-scale of 5–20 days, which remains challenging to explain.
– Type-B QPOs are generally observed during the spectral state transition from HIMS to SIMS and in SIMS.
Potential Explanations for LFQPOs:
– It is suggested that local inhomogeneity in the accretion disc yields time-varying modulation of the inner ‘hot’ flow, contributing to the observed LFQPOs in Swift J1727.8−1613.
– The conjecture resembles the corona oscillation scenario caused by the undulation of the ‘hot’ and ‘dense’ downstream flow near black holes.
Factors Affecting LFQPOs:
– The dynamics of the downstream flow can be regulated by tuning the accretion parameters, namely accretion rate and viscosity.
– When the soft photons from the upstream are upscattered at the inner ‘hot’ flow, high-energy X-ray emissions are produced due to the inverse-Comptonization process.
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For original article published in ‘Monthly Notices of the Royal Astronomical Society’ click this link:
https://academic.oup.com/mnras/article/531/1/1149/7666767
–Rashmi Kumari
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