Tidal Disruption Events

TL;DR

This paper reviews tidal disruption events (TDEs) as probes of supermassive black holes and galactic nuclei, detailing the disruption physics, debris fallback, and emission mechanisms. Central to the picture is the fallback rate, which scales as $\dot{M} \propto T^{-5/3}$ for full disruptions and can evolve as $T^{-9/4}$ in partial disruptions, driving luminosity via shocks and subsequent disk accretion with typical efficiencies around $\varepsilon \sim 0.1$. The chapter discusses how optical/UV light curves arise from circularization, reprocessing, and disk winds, while X-ray emission traces the inner accretion region; late-time plateaus are attributed to viscous disk spreading. Host-galaxy properties, especially high central densities and post-starburst/green-valley classifications, are linked to enhanced TDE rates, offering a path to constrain the low-mass end of the SMBH mass function and SMBH occupation fractions. Many open questions remain, including the exact roles of stream collisions, nozzle shocks, wind geometries, and the diversity of spectroscopic outcomes across viewing angles and environments.

Abstract

Stars that orbit too close to a black hole can be ripped apart by strong tides, producing a type of luminous transient event called a ``tidal disruption event" (TDE). Tidal disruption events of stars by supermassive black holes (SMBHs) provide windows into the nuclei of galaxies at size scales that are difficult to observe directly outside our own galactic neighborhood. They provide a unique opportunity to study these supermassive black holes under feeding conditions that change dramatically over ~week-month timescales, and that regularly reach super-Eddington mass inflow rates. Their light curves are dependent on the properties of the disrupting black hole, and can be used to help constrain the lower mass end of the SMBH mass function -- a region of parameter space that is difficult to access with classic dynamical mass measurements.

Figures (17)

• Figure 1: The loss cone of a star depicted at a point in its orbit around a SMBH. The key takeaway is that the angle of the loss cone $\theta_{\rm lc}$ is defined by the velocity and radial distance vectors of the star. The star will be tidally disrupted if its velocity vector falls into the loss cone. The periapsis shift shows how the orbit will precess under general relativity, which can affect the circularization process of debris post-disruption (discussed later in Section\ref{['sec:circularization']}), but does not change the loss cone calculation for a given passage. From Amaro-Seoane2018.
• Figure 2: Left: The rest-frame $u-r$ color versus host stellar mass of optically selected TDE host galaxies from the ZTF survey (colored points) compared to a volume-limited sample of SDSS galaxies (background contours). The TDE host galaxies are colored by the spectroscopic sub-type of the TDE (see Section \ref{['sec: reprocess-spec']} for more). It is often found that $>50\%$ of TDE hosts lie within the two dashed green lines, a region of parameter space known as the green valley. This is in contrast to the much smaller fraction of the general galaxy population ($\sim 13\%$) that resides there. Modified from Hammerstein2023. Right: The Lick H$\delta$ absorption index versus the H$\alpha$ emission equivalent width for a sample of optically selected TDE host galaxies (colored points) compared to a volume-limited background sample of SDSS galaxies (contours). The median uncertainties for the TDE hosts are shown in the top left. The solid boxed region indicates the E+A classification region, whereas the extended dashed region indicates the more loosely defined QBS region. TDE host points are colored by the spectroscopic type of the corresponding TDE. While only two galaxies fall within the E+A region, this corresponds to an E+A overrepresentation factor of $\sim$22 compared to the general galaxy population. This has now been observed in other samples of TDEs discovered in the optical and X-ray. Reproduced with permission from Hammerstein2021 (their Figure 4).
• Figure 3: A star of mass $M_{\star}$ and radius $R_{\star}$ approaches a black hole of mass $M_{\bullet}$; the distance between the center of the star and the black hole is $r$.
• Figure 4: The debris stream produced from the partial and complete disruption of a $5/3$ polytropic star. Figure adapted from mainetti17.
• Figure 5: Left: one of the first SPH simulations of a TDE, performed by evans89, who used $4\times 10^{4}$ SPH particles and who also performed grid-based simulations. The star is modeled as a 5/3-polytrope and enters from the top-right, the point of closest approach has $r_{\rm p} = r_{\rm t}$, and the circle indicates the Schwarzschild radius of the SMBH that has a mass of $10^6M_{\odot}$ (note that evans89 increased the spatial scale by a factor of 15 to aid in the presentability of the results). Right: the same simulation as in the left in terms of physical setup (i.e., $5/3$-polytrope, $10^6 M_{\odot}$ SMBH, $\beta = 1$) but with $10^6$ particles, showing the same tidally disrupted debris stream has formed as a byproduct of the encounter; adapted from bonnerot17.