Large-amplitude modulations and hours-timescale variability in the early X-ray light curve of a tidal disruption flare

Abstract

We present new X-ray, optical, and UV observations of the tidal disruption event candidate eRASSt J234402.9-352640, (hereafter J2344). Between 50 and 60 days after peak optical brightness, J2344 exhibited large-amplitude modulations in its 0.2-2 keV emission, when the flux repeatedly dimmed and re-brightened by a factor of ~6, over a ~3-day timescale. These modulations exhibited harder-when-brighter behaviour but were not detected in high-cadence observations obtained 60-70 days and 170-200 days after peak optical brightness, when the system instead exhibited stochastic X-ray variability over timescales of hours. We discuss the different physical mechanisms responsible for such exotic X-ray variability and explore the possibility that the modulations in J2344 were caused by the Lense-Thirring precession of the inner accretion flow around the disrupting black hole.

Figures (14)

• Figure 1: Multi-wavelength light curve evolution of J2344 , with the unabsorbed 0.2--2 keV fluxes (top panel) and the optical and UV fluxes (middle). Triangular data points denote 3$\sigma$ upper limits on the flux. The horizontal dashed red line in the X-ray light curve panel denotes the 3$\sigma$ upper limit on the 0.2--2 keV flux inferred from the non-detection of J2344 in eRASS1 homan_discovery_2023, approximately 200 days before optical peak. The radio evolution (bottom panel) is well described by an expanding synchrotron-emitting region from a single ejection of material, consistent with an outflow launched by a non-relativistic TDE goodwin_radio_2024.
• Figure 2: Zoomed-in view of the X-ray evolution during the high-cadence NICER observations. The two panels have the same $y$-axis and cover the same amount of time on the $x$-axis. The different shaded backgrounds denote windows W1, W2, and W3 for the NICER observations (Sect. \ref{['sec:spec_nicer']}), taken 50--60 days, 60--70 days, and 165--210 days after optical peak.
• Figure 3: Joint evolution of the observed 0.2--2 keV flux (top panel) and the photon index (bottom panel) over time. The dashed red line marks the 3$\sigma$ flux upper limit inferred from the non-detection in eRASS1. The X-ray spectrum remains ultra-soft ($\Gamma \gtrsim 4$) over the $\sim$760 days of X-ray monitoring ($\sim 800$ days after optical peak; homan_discovery_2023).
• Figure 4: Top panel: Evolution of the 0.2--2 keV flux, $F_{\mathrm{X}}$, over time during the high-cadence NICER observations at early times ($\sim$50 days after optical peak). Second panel: spectral hardness estimated as $\log [A_1 / A_2]$, with $A_1$ and $A_2$ the normalisation of the softer and harder blackbody components, respectively. Since smaller $\log [A_1 / A_2]$ values correspond to harder spectra, J2344 exhibits a harder-when-brighter behaviour. Third and fourth panels: log$A_1$ and log$A_2$ in units of $L_{39}/[D_{10}(1+z)]^2$, with $L_{39}$ the luminosity in $10^{39}$ erg s$^{-1}$ and $D_{10}$ the distance to the source in units of 10 kpc. Bottom panel: Evolution of the blackbody temperatures for each model component. The $kT_1$ (orange) and $kT_2$ (blue) were constrained to the 35-55 eV and 100-140 eV ranges during fitting.
• Figure 5: Power-law decay (red) and exponential decay (blue) models fitted to the 0.2--2 keV X-ray light curve of J2344 (excluding the early-time NICER data that exhibit modulations). The late-time flattening in the X-ray fluxes is better fitted by the power-law model here.