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

Abstract

An accretion disk formed around a supermassive black hole after it disrupts a star is expected to be initially misaligned with respect to the equatorial plane of the black hole. This misalignment induces relativistic torques (the Lense–Thirring effect) on the disk, causing the disk to precess at early times, whereas at late times the disk aligns with the black hole and precession terminates1,2. Here we report, using high-cadence X-ray monitoring observations of a tidal disruption event (TDE), the discovery of strong, quasi-periodic X-ray flux and temperature modulations. These X-ray modulations are separated by roughly 15 days and persist for about 130 days during the early phase of the TDE. Lense–Thirring precession of the accretion flow can produce this X-ray variability, but other physical mechanisms, such as the radiation-pressure instability3,4, cannot be ruled out. Assuming typical TDE parameters, that is, a solar-like star with the resulting disk extending at most to the so-called circularization radius, and that the disk precesses as a rigid body, we constrain the disrupting dimensionless spin parameter of the black hole to be 0.05 ≲ ∣a∣ ≲ 0.5.

Tidal Disruption Event AT2020ocn: Early Time X-Ray Flares Caused by a Possible Disk Alignment Process

“…Such timescales for AT2020ocn are in agreement with the calculations (a duration of ∼1 − 10 days for individual flares, and the source stops flaring 200 days after the first Swift detection of the event). Pasham et al (2024) discovered a 17.0 2.4 1.2 -+ day quasiperiodicity in NICER data in the early time period, using an energy band of 0.3-1.0 keV that is slightly different from that in our study (0.3-1.1 keV). Without assuming their best-fit period, we fit the early time NICER spectra and find an evolution of the θ and the IC strength.…”

“…An LSP is designed to search for (quasi)periodic signals in unevenly sampled time series. In the frequency domain, we find that LSPs of the count rate in 0.3-1.1 keV and the kT e as a function of time show periodicity peaks that are consistent with those found in Pasham et al (2024), while the LSP of the θ as a function of time only shows peaks at  3σ significance level assuming a white-noise background (Figure A8). It is possible that, as the IC component dominates the spectrum when the inclination decreases, most of the periodicity is imprinted in the IC component while the periodicity in θ is less pronounced.…”

“…Without assuming their best-fit period, we fit the early time NICER spectra and find an evolution of the θ and the IC strength. We investigated if the already-found periodicity in the light-curve data (Pasham et al 2024) can also be found in a Lomb-Scargle periodogram (LSP; Lomb 1976; Scargle 1982) of our best-fit θ or kT e parameter values as a function of time. An LSP is designed to search for (quasi)periodic signals in unevenly sampled time series.…”

“…Neutron star Interior Composition ExploreR (NICER) started monitoring the source on 2020 July 11 (MJD 59041). Recently, Pasham et al (2024) discovered a ∼17 day quasiperiodicity modulating the X-ray flux of AT2020ocn as observed by NICER over the first ∼130 days. Various mechanisms can lead to this phenomenon, including a precessing accretion disk, as suggested by those authors.…”

“…It is located at the center of an otherwise quiescent, early type galaxy SDSS J135353.80 +535949.7 at a redshift of z = 0.0705. The M-σ * relation suggests a BH mass of ∼10 6.4±0.6 solar mass (Pasham et al 2024). Subsequent observations by the Neil Gehrels Swift satellite revealed the source to be bright in the UV and the X-ray band (Gezari et al 2020;Miller & Reynolds 2020).…”