‘Failed’ tidal disruption events and X-ray flares from the Galactic Centre

Abstract

The process of tidal disruption of stars by a supermassive black hole (SMBH) provides luminous UV and soft X-ray flares with peak luminosities of ≈ 10 46 ergs/sec and duration of a few months. As part of a wider exploration of the effects of stellar rotation on the outcome of a TDE, we have performed hydrodynamical simulations of the disruption of a rotating star whose spin axis is opposite to the orbital axis. Such a retrograde rotation makes the star more resilient to tidal disruption, so that, even if its orbit reaches the formal tidal radius, it actually stays intact after the tidal encounter. However, the outer layers of the star are initially stripped away from the core, but then fall back onto the star itself, producing a newly formed accretion disc around the star. We estimate that the accretion rate onto the star would be strongly super-Eddington (for the star) and would result in an X-ray flare with luminosity of the order of ≈ 10 40 ergs/sec and duration of a few months. We speculate that such events might be responsible for the known X-ray flares from Sgr A* in the recent past.

What causes the fragmentation of debris streams in TDEs?

“…The star is relaxed until it reaches a new equilibrium state that we monitor through the central density and thermal energy. Usually the value of α varies during the relaxation process (Sacchi & Lodato 2019). However, the value of α in our simulation is relatively small, α = 0.2.…”

“…This occurs when the stellar rotation is sufficiently fast and its axis is parallel and opposite to the stellar orbital angular momentum, or within some tens degrees from this configuration. In this case the stream does not form or it is extremely faint (Sacchi & Lodato 2019) and therefore is not relevant to the problem at hand.…”

“…Assuming the flatness of the energy distribution, it is possible to find what is considered to be the "smoking gun" of these phenomena: a light curve that should fall as t −5/3 . Results from numerical simulations and analytical considerations, however, found that these phenomena depend on a wide range of parameters, such as the stellar internal structure (Lodato et al 2009) and spin (Golightly et al 2019;Kagaya et al 2019;Sacchi & Lodato 2019), the properties of the black hole (Hayasaki et al 2016;Tejeda et al 2017), the penetration factor β = r t /r p (Guillochon & Ramirez-Ruiz 2013) and the physics of disc formation (Hayasaki et al 2013;Bonnerot et al 2016), that can consistently modify the t −5/3 behavior. Furthermore, the TDEs' observable properties are closely connected to the stream of debris that forms after the stellar disruption.…”

Simulations of Tidal Disruption Events

“…Care should be taken in this context only for very deeply penetrating events, for which relativistic effects and thermonuclear detonation (for white dwarfs disruption) could play an important role. As a result, most of the numerical work done in this context, rather than improving the numerics, is aimed at including progressively more complex geometries, for example exploring disruption by binary black holes Vigneron et al 2018) or the role of stellar spin (Golightly et al 2019a;Sacchi and Lodato 2019), or that of realistic stellar structure (Law-Smith et al 2019).…”

“…In the description of simulation methods above, the primary goal of previous work has been to study the dynamics of the stellar gas through gravitational interactions. Generally, the stellar model is assumed to be a nonrotating polytrope (except in recent papers, such as Sacchi andGolightly et al 2019a), where the distinguishing characteristic between main sequence stars and white dwarfs, for example, is the difference in adiabatic index. Radiation has not been included in most simulations although the importance of its role is still an open question.…”