Tidal Disruption Events

A tidal disruption event (TDE) occurs when a star passes close enough to a (typically super-massive) black hole that the hole’s tidal field exceeds the star’s self-gravity, unbinding and disrupting it; roughly half of the stellar debris remains bound and eventually falls back onto the black hole, powering a luminous, months-long flare. Optically/UV-selected TDEs, the class UVEX is sensitive to, are observed to radiate as blue, roughly constant-temperature thermal sources with characteristic blackbody temperatures of a few \(\times10^4\) K [1], and this SED’s default priors are built from the empirical rise/decline/temperature statistics of that work’s homogeneously-analyzed sample of 39 optical/UV TDEs.

This population is implemented by TidalDisruptionEvent, pairing VanVelzenTDESED with the rate/duration metadata described below.

Quick Facts

Quantity	Value	Source	Notes
Rate	\(3.1^{+0.6}_{-1.0}\times10^{-7}\ \mathrm{Mpc^{-3}\,yr^{-1}}\) (constant in \(z\))	Yao et al.[2]	Maximum-volume-corrected demographic rate from 33 spectroscopically-confirmed TDEs from three years of the Zwicky Transient Facility. Taken as constant in \(z\), since its evolution remains actively debated – Karmen et al.[3] show the observed redshift-dependent TDE rate is highly sensitive to the poorly-constrained evolution of the supermassive black hole mass function itself. The quoted \(+0.6/-1.0\) bounds are Yao et al.[2]’s own 90% confidence interval and are carried through as RATE_CI (see Rate Uncertainty and All-Sky Yield).
Redshift limit	\(z = 2\)	–	Generous relative to the timescales and luminosities of the observed optical/UV TDE population.
Duration	200 days	–	Generous relative to the timescales and luminosities of the observed optical/UV TDE population.

SED Model

There are two TDE SED models which are available:

• VanVelzenTDESED implements the Gaussian-rise/exponential-decline parameterization of van Velzen et al.[1], with a constant-temperature blackbody photosphere. This is the simplest model that reproduces the observed optical/UV TDE population.

• AlushStoneTDESED implements a more complex model that includes a late-time component, representing a magnetized accretion disk.

By default, the transient class TidalDisruptionEvent uses the former, simpler model, a constant-temperature blackbody modulated by a Gaussian-rise, exponential-decay light curve:

\[\begin{split}L_\nu(\nu, t) = L_0 \cdot \ell(t) \cdot \frac{\pi B_\nu(\nu, T)}{\sigma_\mathrm{SB} T^4}, \qquad \ell(t) = \begin{cases} \exp\left[-\dfrac{(t-t_\mathrm{peak})^2}{2\sigma^2}\right] & t < t_\mathrm{peak} \\[6pt] \exp\left[-\dfrac{t-t_\mathrm{peak}}{\tau}\right] & t \ge t_\mathrm{peak} \end{cases},\end{split}\]

with \(t_\mathrm{peak}=5\sigma\). This is the simplest model that reproduces the observed optical/UV TDE population and is sufficient for most population-level UVEX forecasting, where the late-time disk plateau discussed below is faint and rarely detected.

Parameter	Symbol	Prior	Notes / Source
amplitude	\(L_0\)	LogNormal(\(\log_{10}(L_0/\mathrm{erg\,s^{-1}})\); mean=43.8, \(\sigma\)=0.3)	Peak bolometric luminosity, \(L_0=L_\mathrm{bol}(t_\mathrm{peak})\) [1].
temperature	\(T\)	LogNormal(\(\log_{10}(T/\mathrm{K})\); mean=4.3, \(\sigma\)=0.1)	Photospheric temperature, \(\approx2\times10^4\) K [1].
sigma_rise	\(\sigma\)	LogNormal(\(\log_{10}(\sigma/\mathrm{d})\); mean=0.91, \(\sigma\)=0.25)	Gaussian width of the pre-peak rise [1].
tau_decline	\(\tau\)	LogNormal(\(\log_{10}(\tau/\mathrm{d})\); mean=1.7, \(\sigma\)=0.2)	Exponential decline timescale after peak [1].

Plateau visibility: AlushStoneTDESED

AlushStoneTDESED should be used instead of the default model whenever a study specifically cares about the visibility of the late-time disk plateau (e.g. forecasting how often UVEX would detect the plateau itself, or how it biases late-time photometry), rather than population-level early-time behavior. It adds a second, separately-normalized blackbody component on top of the same early-time photosphere used by VanVelzenTDESED, one that smoothly softens from a flat plateau to a power-law decline:

\[L_\nu(\nu, t) = L_\mathrm{bol}^\mathrm{early}(t)\, S(\nu, T) + L_\mathrm{bol}^\mathrm{plat}(t)\, S(\nu, T_\mathrm{p}), \qquad L_\mathrm{bol}^\mathrm{plat}(t) = L_\mathrm{p} \left(1 + \frac{t - t_\mathrm{peak}}{\tau_\mathrm{p}}\right)^{-\alpha_\mathrm{p}},\]

where \(S(\nu, T)\) is the normalized blackbody shape and \(t_\mathrm{peak}=5\sigma\) is the early component’s own peak time. The two components are summed in linear luminosity space, not switched between, since a real disk plateau does not sharply replace the fading early-time emission. The plateau’s functional form and its default decline index, \(\alpha_\mathrm{p}\sim5/6\), follow the magnetized-disk model of Alush and Stone[4], which predicts an \(L_\mathrm{UV}\propto t^{-5/6}\) late-time decline persisting for decades to centuries.

Parameter	Symbol	Prior	Notes / Source
amplitude	\(L_0\)	LogNormal(\(\log_{10}(L_0/\mathrm{erg\,s^{-1}})\); mean=43.8, \(\sigma\)=0.3)	Early-time peak bolometric luminosity [1].
temperature	\(T\)	LogNormal(\(\log_{10}(T/\mathrm{K})\); mean=4.3, \(\sigma\)=0.1)	Early-time photospheric temperature, \(\approx2\times10^4\) K [1].
sigma_rise	\(\sigma\)	LogNormal(\(\log_{10}(\sigma/\mathrm{d})\); mean=0.91, \(\sigma\)=0.25)	Gaussian width of the pre-peak rise [1].
tau_decline	\(\tau\)	LogNormal(\(\log_{10}(\tau/\mathrm{d})\); mean=1.8, \(\sigma\)=0.2)	Exponential decline timescale after peak [1].
plateau_temperature	\(T_\mathrm{p}\)	LogNormal(\(\log_{10}(T_\mathrm{p}/\mathrm{K})\); mean=4.0, \(\sigma\)=0.3)	Late-time disk-plateau blackbody temperature; fiducial scale, not yet calibrated.
plateau_amplitude	\(L_\mathrm{p}\)	LogNormal(\(\log_{10}(L_\mathrm{p}/\mathrm{erg\,s^{-1}})\); mean=41.5, \(\sigma\)=0.2)	Plateau bolometric luminosity at \(t_\mathrm{peak}\); fiducial scale, not yet calibrated.
plateau_timescale	\(\tau_\mathrm{p}\)	LogNormal(\(\log_{10}(\tau_\mathrm{p}/\mathrm{d})\); mean=2.3, \(\sigma\)=0.3)	Timescale over which the plateau softens into its power-law decline.
plateau_decline	\(\alpha_\mathrm{p}\)	Uniform(0, 2)	Late-time decline index, centered near the magnetized-disk prediction \(5/6\) [4][5].

Simulated Light Curves

The plot below draws 1000 random parameter realizations of the default VanVelzenTDESED model from the priors above and shows the resulting bolometric light curves, recentered on each realization’s own peak time to match the peak-relative observed bolometric light curves of five optical/UV TDEs from van Velzen et al.[1].

(Source code, png, hires.png, pdf)

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Observability Summary

Below are the redshifts \(z\) and corresponding bandpass calculated peak apparent AB magnitudes \(m_\mathrm{AB}\) of 3000 simulated TDEs drawn from the priors above, with the UVEX 1 Dwell limit of \(m<24.5\) overplotted. Findings here justify our confidence in a \(z=2\) redshift limit for this population.

(Source code, png, hires.png, pdf)

The anticipated rate of TDEs detectable by UVEX at these limits is as follows assuming that any event above the \(m<24.5\) limit is detectable, and that the population is isotropic and homogeneous in comoving volume out to \(z=2\):

(Source code, png, hires.png, pdf)

References

[1] (1,2,3,4,5,6,7,8,9,10,11)

Sjoert van Velzen, Suvi Gezari, Erica Hammerstein, Nathaniel Roth, Sara Frederick, Charlotte Ward, Tiara Hung, S. Bradley Cenko, Robert Stein, Daniel A. Perley, Kirsty Taggart, Jesper Sollerman, Igor Andreoni, Eric C. Bellm, Kishalay De, Richard Dekany, Ryan J. Foley, Christoffer Fremling, Anna Y. Q. Ho, Mansi M. Kasliwal, and Shrinivas R. Kulkarni. Seventeen tidal disruption events from the first half of ZTF survey observations: entering a new era of population studies. The Astrophysical Journal, 908(1):4, 2021. arXiv:2001.01409, doi:10.3847/1538-4357/abc258.

[2] (1,2)

Yuhan Yao, Vikram Ravi, Suvi Gezari, Sjoert van Velzen, Wenbin Lu, Steve Schulze, Jean J. Somalwar, S. R. Kulkarni, Erica Hammerstein, Matt Nicholl, Matthew J. Graham, Daniel A. Perley, S. Bradley Cenko, and Robert Stein. Tidal disruption event demographics with the zwicky transient facility: volumetric rates, luminosity function, and implications for the local black hole mass function. The Astrophysical Journal Letters, 955(1):L6, 2023. arXiv:2303.06523.

[3]

Mitchell Karmen, Suvi Gezari, Colin Norman, and Muryel Guolo. Tidal disruption event rates across cosmic time: forecasts for LSST, Roman, and JWST and their constraints on the supermassive black hole mass function. The Astrophysical Journal, 1006:20, 2026. arXiv:2602.04947, doi:10.3847/1538-4357/ae7a49.

[4] (1,2)

Yael Alush and Nicholas C. Stone. Late-time evolution of magnetized disks in tidal disruption events. arXiv e-prints, 2025. arXiv:2503.03811.

[5]

Yael Alush, Nicholas C. Stone, and Sjoert van Velzen. How flat is a plateau? evolution of late-time tde disks. arXiv e-prints, 2025. arXiv:2510.24696.