Time-Dependent obscuration of a tidal disruption event candidate in the active galactic nucleus CSS100217

TL;DR

The authors develop a time-dependent obscuration model for TDEs in AGNs, describing the observed flux as $F_\lambda^{obs}(t)=F_\lambda(t)\,b_\lambda(t)$ with $b_\lambda(t)=10^{-0.4k(\lambda)E(B-V)(t)}$ and a Weibull extinction law $E(B-V)(t)=A\cdot\left(\frac{a}{s}\right)\left(\frac{t-t_0}{s}\right)^{a-1}\exp\left[-\left(\frac{t-t_0}{s}\right)^a\right]$. Applying this to CSS100217, fitted via MOSFIT/MCMC, yields $M_{\rm BH}\approx 3.3\times10^7\,M_\odot$, $M_*\approx 4.6\,M_\odot$, total energy $7.23\times10^{53}$ erg, and debris accreted $\sim1.38\,M_\odot$. The pre-flare luminosity is AGN-dominated, and the post-flare dimming arises from line-of-sight obscuration by unbound debris or moving nuclear clouds, peaking near $\sim$1000 days and predicted to recover to pre-flare levels after several thousand days. This obscured-TDE framework offers a viable explanation for CSS100217 and suggests obscuration could be a common factor in other TDE-like AGN variability.

Abstract

CSS100217 is considered a peculiar tidal disruption event (TDE) candidate occurring in an active galactic nucleus (AGN). Unlike typical TDEs, where the post-flare luminosity is equal to that pre-flare, CSS100217 decayed to $\sim$ 0.4 magnitudes fainter than its pre-flare V band level. In this manuscript, we propose an obscured TDE model to explain the light curve of CSS100217. Assuming that the time-dependent obscuration, caused by the TDE unbound stellar debris, or by nuclear clouds moving around the supermassive black hole (SMBH), follows a Weibull distribution, we find that the light curve of CSS100217 can be described by the tidal disruption of a $4.6_{-0.9}^{+0.9}{\rm M_\odot}$ main-sequence star by a $3.3_{-0.3}^{+0.3}\times10^7{\rm M_\odot}$ black hole. The total energy of the event derived from our fit is $7.23\times10^{53}$ ergs and about 1.38 ${\rm M_\odot}$ of debris mass is accreted by the central SMBH. The model indicates that the contribution of the host galaxy to the observed pre-flare optical luminosity is not-significant compared to that of the AGN, which is consistent with the results of the spectral analysis. These results suggest that obscuration may play an important role in explaining the unusual TDE-like variability observed in CSS100217.

Figures (5)

• Figure 1: Top: Raw single-exposure CSS V-band data are plotted in pink. The binned data are plotted in blue and grouped in sets of 15 consecutive points. The best description by the obscured TDE model is shown as a solid pink line, with the corresponding confidence bands (shaded pink area) determined by uncertainties of the model parameters. For comparison, the solid green line displays the intrinsic TDE light curve recovered by removing the obscured component, while the dashed purple line shows the conventional theoretical TDE model prediction assuming no obscuration. The dashed black line and the gray band show the mean and standard deviation of the pre‑flare magnitudes, respectively, while the dash-dotted gray shows the post‑flare mean magnitude,$\sim$ 0.35 mag fainter (red arrow). Bottom: Time evolution of the color excess E(B–V) determined by the best-fit parameters of the obscured TDE model.
• Figure A.1: Posterior distributions of the TDE with obscured model parameters derived from the MCMC technique. In each panel, the three circles from outer to inner represent $3\sigma$, $2\sigma$, and $1\sigma$ confidence levels and the blue dot in the center of each contour marks the position of the best-fit parameter.
• Figure B.1: Left: Rest frame spectrum of CSS100217. Right: Spectrum in the range 3900$\sim$4100 Å. In both panels, the dark green components indicate the spectrum. In the right panel, the vertical blue lines mark the center wavelengths of Ca ii H & K absorption lines.
• Figure C.1: Same as Fig. \ref{['lc']} but showing follow-up observations from other projects, as indicated in the legend at the top right. All data points are shown after 15-day binning. The dashed line and corresponding shaded region of the same color indicate the mean value and standard deviations of the light curve. For clarity, the ATLAS data points, as well as the corresponding mean and standard deviation, have been offset by $-1$ mag.
• Figure D.1: Time evolution of the photospheric radius of the TDE and the radius of the unbound stellar debris.