X-ray flares from the stellar tidal disruption by a candidate supermassive black hole binary

Supermassive black holes (SMBHs) reside at the centers of most massive galaxies. While often quiescent, they can be revealed through stellar tidal disruption events (TDEs), where a star passing too close is torn apart by the SMBH's tidal forces. The resulting accretion of stellar material produces a luminous flare, primarily observed in the ultraviolet (UV) and optical wavelengths. Early theoretical models predicted bright soft X-ray emission from the newly formed accretion disk, but most observed TDEs show surprisingly weak or absent X-ray emission. This discrepancy has led to investigations into alternative emission mechanisms, such as shocks from stream self-collisions or thermal reprocessing of accretion power by a surrounding gas layer. X-ray observations in the initial months are crucial for distinguishing between these scenarios, yet such data remain scarce. This study focuses on the TDE OGLE16aaa, aiming to shed light on the emission mechanisms and potential implications for SMBH binary systems.

Previous research on TDEs has highlighted the inconsistencies between theoretical predictions and observations. While theoretical models often predict strong X-ray emission, many observed TDEs show only weak or no X-ray counterparts. Several explanations have been proposed, including delayed onset of accretion disk formation, obscuration by the ejecta, or patchy obscuration. Existing studies have focused on analyzing the optical and UV light curves, but have lacked comprehensive X-ray data, particularly in the crucial early stages of the event. This study builds upon previous work by providing a detailed analysis of the X-ray light curve of OGLE16aaa, combined with UV and optical data, aiming to reconcile theoretical predictions with observational data and explore the possibility of SMBHB involvement.

The authors analyzed publicly available data from the Swift and XMM-Newton observatories, focusing on OGLE16aaa. This included optical I-band data from the Optical Gravitational Lensing Experiment (OGLE-IV), and UV data from Swift's Ultraviolet/Optical Telescope (UVOT). X-ray data were extracted from Swift's X-ray Telescope (XRT) and XMM-Newton's European Photon Imaging Camera (EPIC). Data reduction involved standard procedures, including background subtraction, correction for Galactic extinction, and spectral fitting using XSPEC. The analysis included fitting blackbody and power-law models to the X-ray spectra to determine temperature, luminosity, and other parameters. The UV and optical data were fitted with blackbody models to determine temperature and radius evolution. The authors also compared the optical to X-ray luminosity ratio with other X-ray bright TDEs. In addition, a supermassive black hole binary (SMBHB) model was used to simulate the X-ray light curve for comparison.

The key findings of this study are as follows: 1. **Delayed X-ray Brightening:** OGLE16aaa exhibited a significant X-ray brightening approximately 140 days after the optical peak, with a subsequent decline interrupted by several flux dips and flares. This delayed brightening and variability are not typical for standard TDEs. 2. **Unusual X-ray Light Curve:** The X-ray light curve shows a rapid rise to a peak within about two weeks and then a rapid decay punctuated by several significant dips and flares. This is significantly different from the typical behaviour in other TDEs, where the X-ray emission follows a smoother decline. 3. **Soft X-ray Spectra:** The X-ray spectra, particularly those from XMM-Newton, are characterized by being quasi-soft, lacking emission above ~2 keV. This is common in other TDEs but adds to the unusual nature of the OGLE16aaa lightcurve. 4. **UV/Optical and X-ray Discrepancy:** The UV/optical emission of OGLE16aaa was well-fitted with a blackbody, exhibiting a temperature around 16,000-25,000 K and radius around 2 × 10¹⁵ cm, but this model could not explain the quasi-simultaneous X-ray emission. The X-ray emitting region shows significantly higher temperatures and much smaller radii, suggesting separate physical origins. 5. **SMBHB Model Consistency:** The authors found the X-ray light curve, especially the presence of dips and flares, consistent with models of tidal disruption by an SMBHB, suggesting a possible explanation for OGLE16aaa's unusual behavior. They estimated parameters such as eccentricity, penetration factor, mass ratio and orbital period of the SMBHB. 6. **Reprocessing Scenario:** The early X-ray non-detections might be explained by obscuration from a dense, expanding outflow, with reprocessing of X-rays into UV/optical radiation. However, the late-time X-ray variability is challenging to fully account for with a reprocessing scenario alone.

The unusual X-ray properties of OGLE16aaa challenge conventional models of TDEs. The delayed brightening and subsequent dips are inconsistent with simple scenarios of delayed accretion disk formation. The authors explore two main interpretations: the presence of an SMBHB or patchy obscuration. The SMBHB model successfully reproduces the overall X-ray light curve and provides constraints on SMBHB parameters, offering a plausible explanation for the observed flux interruptions. However, the authors also acknowledge that while the reprocessing scenario can explain the initial lack of X-ray emission, it has difficulties in explaining the late-time variations. The presence of an additional X-ray spectral component, also seen in other TDEs, is addressed as likely originating from a transient corona synchronized with accretion disk formation. The authors conclude that the SMBHB interpretation is the more likely scenario, while the reprocessing model may still play a part in the initial phases.

This study presents compelling evidence for OGLE16aaa as a potential stellar tidal disruption event by a supermassive black hole binary. The unique combination of delayed X-ray brightening and multiple flux dips during the decay phase, coupled with spectral analysis and modeling, strongly supports the SMBHB scenario. The results highlight the importance of X-ray observations in revealing such systems and the need for further research to understand the interplay between accretion, reprocessing, and the dynamics of SMBHBs. Future surveys with increased sensitivity and coverage will be crucial for discovering and studying similar events, providing valuable insights into the nature and evolution of SMBHBs.

The primary limitation is the relatively sparse X-ray coverage, particularly during the early phases of the event. Additional observations would have improved the accuracy of the spectral and temporal analysis. Moreover, the SMBHB model has multiple free parameters, which introduces uncertainties in the estimated orbital parameters of the binary. A more detailed investigation into the nature of the additional X-ray spectral component is also needed.