Lense-Thirring precession after a supermassive black hole disrupts a star

The study investigates the phenomenon of a supermassive black hole (SMBH) disrupting a star, leading to the formation of a misaligned accretion disk. This misalignment induces relativistic torques (Lense-Thirring effect) causing the disk to precess. At early times, precession is expected to be significant, while at later times, the disk should align with the black hole, halting precession. The research aims to observe and characterize this precession using high-cadence X-ray monitoring of a TDE. Understanding SMBH spin is crucial as it affects various astrophysical processes, and this study offers a unique method to constrain this parameter. The importance of this research lies in its potential to provide an independent measurement of SMBH spin, a parameter otherwise difficult to obtain. The use of high-cadence X-ray monitoring offers a novel approach to detect subtle, regular processes amidst the chaotic aftermath of a stellar disruption, potentially revolutionizing the way we study SMBHs.

Previous studies have theoretically predicted the observable consequences of Lense-Thirring precession in TDEs, suggesting that it could manifest as periodic variations in X-ray emission. Stone and Loeb (2012) and Franchini et al. (2016) explored this phenomenon through theoretical modeling. Other mechanisms, like radiation-pressure instability (RPI), have also been proposed to explain variability in accretion disks around SMBHs (Lightman & Eardley, 1974; Janiuk et al., 2000). These mechanisms are considered as alternative explanations for the observed periodicities. The paper also reviews existing work on the properties of TDEs, including their multi-wavelength behavior and disk formation processes (Bonnerot & Lu, 2020; Andalman et al., 2022; Cufari et al., 2022). Studies on X-ray QPOs in stellar-mass black holes are referenced to provide context for the observed timescale (Ingram & Done, 2010; Motta et al., 2014; Nixon & Salvesen, 2014). The paper also cites research on the quasi-periodic behavior seen in some X-ray binaries, where Lense-Thirring precession is considered a potential cause (Belloni et al., 1997; Muno et al., 1999; Altamirano et al., 2011; Belloni et al., 1997).

The study utilized high-cadence X-ray monitoring data from the Neutron Star Interior Composition Explorer (NICER), supplemented by data from XMM-Newton and Swift (for X-ray and UV observations) and the Zwicky Transient Facility (ZTF) and Sloan Digital Sky Survey (SDSS) for optical data. The NICER data, covering the first 130 days post-discovery, revealed quasi-periodic X-ray flares approximately 15 days apart. Data reduction involved standard procedures for each instrument, including background subtraction and the identification and removal of hot detector pixels in the NICER data. A Lomb-Scargle periodogram (LSP) was used to analyze the X-ray light curve, quantifying the quasi-periodic variability. The statistical significance of the 15-day periodicity was rigorously assessed using Monte Carlo simulations, considering various noise continuum models and searching across a range of frequencies and coherence values. Time-resolved X-ray spectral analysis of the NICER data revealed two thermal components (a cool and a warm component), both exhibiting quasi-periodic modulations with a period of roughly 15 days. The black hole mass was estimated from the host galaxy's stellar velocity dispersion using SDSS data and an established M-σ relation. Different models for the observed variability were considered, including repeating partial TDEs, repeated debris stream self-interactions, and neutral column density changes, which were ruled out based on the data. Radiation-pressure instability (RPI) was also considered but deemed unlikely due to its predicted modulation amplitude. The authors then focused on the Lense-Thirring precession model, considering two modes: precession of a geometrically thick disk (rigid body precession) and precession of a thin disk tearing into discrete annuli. The primary analysis focused on the rigid body precession mode, given the likely high accretion rate after the TDE.

The main finding is the discovery of strong, quasi-periodic X-ray flux and temperature modulations in the TDE AT2020ocn with a period of approximately 15 days, lasting about 130 days. The significance of this periodicity was determined to be highly statistically significant, with a false alarm probability of less than 1 in 10,000. The X-ray spectral analysis revealed two thermal components (cool and warm) both exhibiting the same quasi-periodicity. Alternative explanations, such as repeating partial TDEs, repeated debris stream self-interactions, neutral column density changes and RPI, were examined and ruled out. The Lense-Thirring precession model provided the best explanation, particularly under the assumption of a rigid body precession of the accretion disk. Based on this model and standard TDE parameters, the dimensionless spin parameter (a) of the SMBH was constrained to be 0.05 ≤ a ≤ 0.5. The observed asymmetry in the early X-ray flares suggests a warped, rather than a planar disk. The persistence of the X-ray modulations for 130 days is consistent with theoretical predictions for rigid body precession in SMBHs of this mass range. The lack of similar modulations in the optical and UV bands supports the precession model, as optical/UV emission is not expected to be directly from the inner accretion disk.

The observed 15-day quasi-periodic X-ray modulations, strongly supported by statistical analysis, are best explained by the Lense-Thirring precession of the accretion disk formed after the SMBH disrupted the star. The constraints on the SMBH's spin parameter (0.05 ≤ a ≤ 0.5) represent a significant finding, as measuring SMBH spin is a challenging task. This study demonstrates the potential of high-cadence X-ray monitoring to reveal subtle, regular processes even in the chaotic environment of a TDE. This method provides an independent way to constrain SMBH spins, which complements existing techniques. The findings suggest that future, all-sky surveys could reveal hundreds of similar events, potentially providing a comprehensive statistical measure of SMBH spin distribution in the local universe.

The study successfully detected and characterized quasi-periodic X-ray modulations in a TDE consistent with Lense-Thirring precession. This provided an independent constraint on the SMBH's spin parameter. High-cadence X-ray monitoring is established as a powerful tool to study relativistic processes near SMBHs. Future work should focus on analyzing more TDEs with similar monitoring capabilities to strengthen statistical analyses of SMBH spin distribution. The methodology developed here could be applied to other high-cadence time-series data in astrophysics.

The study relies on the assumption of a rigid-body precession of the accretion disk, which may not hold true in all scenarios. The constraint on the SMBH spin parameter is based on a specific model and specific assumptions about TDE parameters. Further research is needed to assess the robustness of this constraint under different assumptions. The analysis focused on the first 130 days of the TDE; long-term monitoring could provide additional insights. The interpretation could be affected by uncertainties in the black hole mass estimation and potential systematic errors in the adopted M-σ relation.