Optical Tidal Disruption Events

• Optical TDEs are transient events occurring when a star is disrupted by a supermassive black hole, identified by blue thermal spectra and distinctive light curve signatures.
• They are discovered through time-domain surveys with criteria like spatial association to galactic centers, significant flux increases, and unique broad emission lines, especially in hydrogen and helium.
• Studying optical TDEs provides insights into supermassive black hole demographics, accretion physics, and multi-wavelength emission processes, informing future survey strategies.

An optical tidal disruption event (TDE) is a transient phenomenon that arises when a star is gravitationally disrupted and accreted by a supermassive black hole (SMBH), with the bulk of the radiative output observed in the optical or ultraviolet bands. These events display a rich set of photometric and spectroscopic properties that distinguish them from supernovae (SNe), active galactic nucleus (AGN) flares, and other variable astrophysical phenomena. The paper of optical TDEs provides a powerful probe of quiescent SMBHs, the physical conditions in galactic nuclei, and the fundamental physics of extreme accretion and debris circularization.

1. Discovery, Identification, and Classification

The systematic discovery of optical TDEs originated with time-domain surveys such as SDSS Stripe 82 and subsequently expanded through operations of facilities like iPTF, ASAS-SN, Pan-STARRS, and ZTF. Early searches (Velzen et al., 2010) required both a spatial and temporal association with galaxy centers (using stringent cuts on host-flare separation, e.g., $d/\sigma_d < 2$ and $d < 0.2''$ for nucleation) to discriminate against off-nuclear SNe.

Candidates are filtered by:

• Photometric variability: e.g., a $>10\%$ flux increase at $\geq7\sigma$ significance over baseline.
• Spatial association with the galaxy nucleus, as determined via point-spread function (PSF) fitting and astrometric consistency.
• Host galaxy spectral and photometric exclusion of AGN/QSO loci (using emission-line diagnostics or color-color cuts).
• Long-term monitoring to exclude hosts with persistent variability or broad-lined AGN spectra.

The modern optical TDE catalog (Langis et al., 5 Jun 2025) now comprises over one hundred events, classified spectroscopically into four main categories:

• TDE-H+He: broad hydrogen + helium emission (dominant; $\approx$60%)
• TDE-H: hydrogen-dominated emission
• TDE-He: pure helium emission
• TDE-featureless: lacking broad emission lines, possibly associated with the most luminous or jetted events

Event rates are measured at roughly $10^{-4}$ per galaxy per year (Velzen et al., 2020), though the luminosity function suggests faint TDEs are undercounted due to flux-limited selection (Zhu et al., 2023).

2. Optical and Ultraviolet Properties

Optical TDEs are characterized by:

• Blue, thermal continuum spectra described by blackbody fits with $T_{\rm BB} \sim (1-3)\times10^4$ K (e.g., $T_{\rm BB} \simeq 2.4\times10^4$ K for TDE1, $1.8\times10^4$ K for TDE2 (Velzen et al., 2010), $\sim3\times10^4$ K for iPTF16axa (Hung et al., 2017)).
• Light curves exhibiting a rapid rise (few weeks), with the peak magnitude spanning $M_g \approx -17$ to $-21$ and $\nu L_\nu \sim 10^{42}-10^{44}$ erg s$^{-1}$.
• Power-law decay post-maximum, with canonical slopes near $t^{-5/3}$ (fallback rate; observed directly in both $g$, $r$ bands) and blackbody temperature that remains nearly constant with minimal color evolution over timescales of months (Hung et al., 2017).

The peak luminosities of optical TDEs cluster in a relatively narrow range ($\log L_{\rm peak} [{\rm erg\,s}^{-1}] = 43.4$–$44.4$ (Hung et al., 2017)), but the temperature and radius evolution (blackbody radius decreasing from several $10^{14}$ to $10^{13}$ cm over months) are consistent across the class.

Ultraviolet observations, primarily from GALEX and Swift/UVOT, confirm strong far- and near-UV emission persisting for hundreds to thousands of days past the optical flare (Velzen et al., 2010), reinforcing the dissimilarity to SNe where the UV emission fades rapidly.

3. Spectroscopic Signatures and Emission Line Physics

Optical TDEs display broad emission lines:

• Balmer (H$\alpha$, H$\beta$) and He II $\lambda4686$, with full-width at half-maximum velocities extending to $\sim10^4$–$1.5\times10^4$ km s$^{-1}$ (Velzen et al., 2010, Zhu et al., 2023).
• The ratio $L({\rm He\,II})/L({\rm H}\alpha)$ frequently exceeds the nebular Case B value of 0.32 due to high-density conditions ($n\gtrsim10^{10} \,{\rm cm}^{-3}$), implying that Balmer lines are suppressed by optical depth while He II remains strong (Hung et al., 2017). The formal relation, $$L({\rm He II})/L({\rm H}\alpha)\simeq3.98\,n({\rm He}^{++})/n_p$$, encapsulates this breakdown of simple recombination physics.

Bowen fluorescence features (N III, O III) may appear, especially in the TDE-H+He class, requiring a high EUV flux and indicating a compact, hot line-emitting region. Featureless spectra with minimal line emission are commonly associated with the most luminous or jetted events (Hammerstein et al., 9 Jun 2025).

Event-to-event diversity is observed in the spectral evolution, composition, and line ratios, which remain areas of active investigation (Velzen et al., 2020).

4. Multi-Wavelength Evolution and Physical Interpretation

Optical TDEs are multi-component phenomena:

• The prompt optical/UV emission has been attributed to reprocessing of X-ray/EUV photons from the nascent accretion disk by an optically thick, extended photosphere, often requiring a radius $\sim10^{14}$–$10^{15}$ cm (Leloudas et al., 2022).
• Polarimetric studies find wavelength-independent continuum polarization ($\sim$0.7–2.1%) and partial depolarization across emission lines, supporting scattering in an extended, mildly aspherical electron-scattering envelope of $\sim$1000 gravitational radii.
• In some cases (e.g., AT 2020mot) a high ($\sim$25%) optical polarization is observed, best explained as synchrotron emission from colliding debris stream shocks during disk formation rather than from a relativistic jet or standard disk atmosphere (Liodakis et al., 2022).
• Radio emission, when present, is often delayed and may indicate late outflow launching linked to a transition in the accretion state (Horesh et al., 2021, Zhang et al., 2023).
• X-ray detection is highly variable, with some events showing prompt, ultra-soft X-ray flares that subsequently fade as the disk is veiled by the optically thick envelope formed during circularization (e.g., the steep X-ray drop concurrent with optical brightening in AT 2022dsb (Malyali et al., 2023) and OGLE16aaa (Shu et al., 2020)).

Notably, detailed modeling demonstrates that for the canonical reprocessing-outflow scenario, inferred outflow masses exceed the disrupted stellar mass for plausible velocities (Matsumoto et al., 2020), challenging the simplest interpretations and motivating alternative models, e.g., quasi-static envelopes or outer-shock powered optical emission.

5. Theoretical Underpinnings: Dynamics and Emission Processes

The physical basis for optical TDEs is set by the process of stellar disruption:

• Tidal radius: $R_t \simeq R_* (M_{\rm BH}/M_*)^{1/3}$
• Debris fallback rate: $\dot{M}_{\rm fb}\propto (t-t_D)^{-5/3}$, under the impulse approximation after first pericenter passage (Rossi et al., 2020, Gezari, 2021).
• The fate of debris depends on the spread in specific energies frozen in during pericenter passage ($\Delta E\sim GM_{\rm BH} R_*/R_t^2$).
• Circularization efficiency and the geometry of intersection of debris streams determine both the rate and nature of disk formation.
• Reprocessing of high-energy photons through an optically thick outflow/envelope (formed from super-Eddington fallback, wind, or extended bound debris) is invoked to shift emission from soft X-rays to the optical/UV.

Hydrodynamic simulations, as well as analytic affine models, support the complex interplay between strong shocks, energy redistribution, and the formation of an extended emitting region (Rossi et al., 2020).

6. Multi-Epoch Evolution, Diversity, and Population Insights

Recent large-sample studies (Langis et al., 5 Jun 2025) reveal:

• Light curve durations (typically $\sim$350 days), rise times (mean $\sim$84 days), and decay times (mean $\sim$250 days) are log-normally distributed, with $t_{\rm rise}/t_{\rm decay}$ ratios also log-normal, indicative of multiplicative physical processes.
• Repeating TDEs are rare but documented, with secondary optical flares exhibiting similar shapes to primaries, mostly within the TDE-H+He class.
• Cross-band emission is coordinated: IR-detected TDEs (18 so far) are typically X-ray bright, suggesting a connection, likely due to both dust reprocessing and high-energy photon production near the SMBH.
• The faintest and closest optical TDEs (e.g., AT 2023clx) prove that a significant fraction of the TDE luminosity function remains unexplored, likely to be revealed by deeper ongoing surveys (Zhu et al., 2023).

7. Broader Astrophysical Context, Host Environments, and Future Prospects

Optical TDEs modulate their host galaxies far beyond the nuclear region:

• The long-lived, blue accretion disks of TDEs are efficient UV/EUV ionizing sources capable of producing galaxy-scale extended emission line regions (EELRs) with radial extents up to $10^4$ light years (Mummery et al., 18 Mar 2025).
• Dense circumnuclear gas illuminated by the early, hard spectrum disk can produce transient "coronal" forbidden lines ([Fe X], [Fe XIV]); these phenomena may be overrepresented in TDE hosts.
• Time-propagating light echoes in the optical and infrared map the recent history of TDE flares, and the presence of EELRs in non-AGN galaxies is now interpreted as a signature of past TDEs rather than classical AGN activity.

The development of comprehensive catalogues (TDECat (Langis et al., 5 Jun 2025)) and comparative modeling tools (e.g., TiDE (Kovács-Stermeczky et al., 2023), MOSFiT) enables robust population analyses, constraining rates, luminosity functions, and correlations across multi-wavelength regimes. Upcoming deep, high-cadence surveys (e.g., LSST, WFST) are expected to uncover hundreds to thousands of new optical TDEs, especially at low luminosities.

In summary, optical TDEs constitute a class of luminous, nuclear flares with distinct photometric and spectroscopic properties, slow color evolution, persistent late-time ultraviolet emission, and strong links to nuclear black hole and host galaxy demographics. Their paper informs relativistic accretion physics, extends the accessible black hole mass function to quiescent hosts, and reveals both the multi-scale physical impact and diagnostic potential of stellar disruptions in galactic centers.