Optical Flares from the Tidal Disruption of Stars by Massive Black Holes

Abstract: A star that wanders too close to a massive black hole (BH) is shredded by the BH's tidal gravity. Stellar gas falls back to the BH, releasing a flare of energy. In anticipation of upcoming transient surveys, we predict the light curves and spectra of tidal flares as a function of time, highlighting the unique signatures of tidal flares in the optical and near-IR. Some of the gas initially bound to the BH is likely blown away when the fallback rate is super-Eddington at early times. This outflow produces an optical luminosity comparable to that of a supernova; such events have durations of ~10 days and may have been missed in supernova searches that exclude the nuclear regions of galaxies. When the fallback rate subsides below Eddington, the gas accretes onto the BH via a thin disk whose emission peaks in the UV to soft X-rays. Some of this emission is reprocessed by the unbound stellar debris, producing a spectrum of very broad emission lines (with no corresponding narrow forbidden lines). These lines are strongest for BHs with MBH ~ 10^5 - 10^6 Msun and thus optical surveys are particularly sensitive to the lowest mass BHs in galactic nuclei. Calibrating our models to ROSAT and GALEX observations, we predict detection rates for Pan-STARRS, PTF, and LSST and highlight observational challenges in the optical. Pan-STARRS should detect at least several events per year--many more if current theoretical models of super-Eddington outflows are correct. These surveys will significantly improve our knowledge of stellar dynamics in galactic nuclei, the physics of super-Eddington accretion, the demography of intermediate mass BHs, and the role of tidal disruption in the growth of massive BHs.

• The paper demonstrates how tidal disruption events generate optical flares through super-Eddington accretion and detailed modeling of light curves and spectra.
• It calibrates predictions using ROSAT and GALEX data to estimate detection rates for wide-field surveys like Pan-STARRS, PTF, and LSST.
• The study provides actionable insights into probing black hole demographics and distinguishing TDEs from other galactic nuclear activities.

Optical Flares from the Tidal Disruption of Stars by Massive Black Holes

The paper by Strubbe and Quataert provides a thorough investigation into the optical emissions generated during the tidal disruption events (TDEs) of stars by massive black holes (BHs). When stars stray sufficiently close to BHs, their self-gravity is overcome by tidal forces, leading to their disruption. A fraction of the stellar debris becomes bound to the BH, forming an accretion disk, while the rest is expelled. The accretion process results in extremely high luminosity flares, observable at multiple wavelengths, notably in the optical and near-infrared (NIR).

Predictions of Light Curves and Spectra

The authors predict the specific light curves and spectra expected from TDEs, focusing on optical and NIR emissions. These predictions are made under the context of upcoming wide-field surveys like Pan-STARRS, PTF, and LSST. The initial phase is highlighted by a super-Eddington accretion phase where the fallback mass rate exceeds the Eddington limit, subsequently leading to powerful outflows. The study suggests this phase results in significant optical emissions, potentially comparable to supernovae, with durations around ten days.

As accretion rates drop below Eddington levels, the BH is fed by a thin disk, radiating primarily in the UV and soft X-ray bands. Notably, some of this emission is reprocessed by unbound stellar debris, characterized by broad emission lines but lacking narrow forbidden lines—offering a contrast that surveys could exploit for detection.

Detection Rates and Challenges

Strubbe and Quataert calibrate their model predictions using data from existing observations by ROSAT and GALEX and apply these to estimate detection rates by upcoming surveys. For instance, Pan-STARRS is projected to detect several TDEs annually, potentially more if current models for outflows during super-Eddington accretion hold accurately. Despite these promising projections, the study acknowledges observational challenges, such as managing the bright nuclear regions of galaxies and distinguishing TDEs from other variable sources or noise.

Implications for Astrophysics

The potential to detect these optical flares has significant implications for understanding stellar dynamics in galactic nuclei and the physics of super-Eddington accretion. Moreover, detecting TDEs offers a novel method for probing the populations of intermediate-mass black holes, thereby addressing their role in the evolution and growth of supermassive black holes.

Concluding Thoughts

This research provides a compelling framework to anticipate the optical and NIR consequences of TDEs, with sophisticated modeling of the associated accretion dynamics and radiation processes. While the predictions hold promise for upcoming astronomical surveys, they also stress the inherent challenges in distinguishing these transient events from the nuclear activity of their host galaxies. As such, the work of Strubbe and Quataert is an essential contribution to the field, bridging theoretical astrophysics and observational astronomy through detailed modeling and empirical calibration. The implications of future survey discoveries based on this work extend well beyond stellar astrophysics, offering insights into the nature of black holes and galactic evolution.