Simulating realistic disc formation in tidal disruption events

Abstract

A star coming too close to a supermassive black hole gets disrupted by the tidal force of the compact object in a tidal disruption event, or TDE. Following this encounter, the debris evolves into an elongated stream, half of which coming back to pericenter. Relativistic apsidal precession then leads to a self-crossing shock that initiates the formation of an accretion disc. We perform the first simulation of this process considering a realistic stellar trajectory and black hole mass, which has so far eluded investigations for computational reasons. This numerical issue is alleviated by using as initial conditions the outflow launched by the self-crossing shock according the local simulation of Lu & Bonnerot (2019). We find that the gas leaving the intersection point experiences numerous secondary shocks that result in the rapid formation of a thick and marginally-bound disc. The mass distribution features two overdensities identified as spiral shocks that drive slow gas inflow along the mid-plane. Inward motion primarily takes place along the funnels of the newly-formed torus, from which a fraction of the matter can get accreted. Further out, the gas moves outward forming an extended envelope completely surrounding the accretion flow. Secondary shocks heat the debris at a rate of a few times 10 44 erg s −1 with a large fraction likely participating to the bolometric luminosity. These results pave the way towards a complete understanding of the early radiation from TDEs that progressively becomes accessible from observations.

High-Energy Neutrino and Gamma-Ray Emission from Tidal Disruption Events

“…, where H = H d /R d , H d is the scale height of the disk, R d is the disk radius, α is the viscous parameter (Shakura & Sunyaev 1973), Ω K is the Keplerian angular frequency, and R S = 2GM BH /c 2 is the Schwarzschild radius. In the super-Eddington phase, the outflows also affect the disk evolution (Ohsuga & Mineshige 2011;Sadowski et al 2014;Jiang et al 2014), and the time evolution of the disk radius and mass accretion rate are under debate (see Coughlin & Begelman 2014;Shiokawa et al 2015;Bonnerot & Lu 2019, where the disk radius much larger than the classical circularization radius has been suggested).…”

“…A significant fraction of the shocked debris can be unbound as an outflow for massive SMBHs (Lu & Bonnerot 2020), which may be responsible for soft X-ray attenuation, reprocessed optical/UV emission that is observed, and radio emission by sub-relativistic flows with ∼ (0.01 − 0.1)c. The remaining fraction may form a geometrically-thick disk, whose radius is much larger than R T , and a quasi-spherical weakly-bound debris (Loeb & Ulmer 1997;Coughlin & Begelman 2014;Sadowski et al 2016;Bonnerot & Lu 2019). Such "TDE debris" is schematically depicted in Figure 1.…”

“…Details will depend on the fate of the disk especially in the inner region. In addition to disk accretion, a fraction of TDE debris would accrete onto a SMBH via the funnel (Sadowski et al 2016;Bonnerot & Lu 2019). The wind is expected to be launched from the vicinity of the SMBH by radiation from a slim or geometrically-thick disk (e.g., Strubbe & Quataert 2009;Metzger & Stone 2016), line emission (Miller 2015), or magnetic dissipation, which may further interact with the TDE debris and streams mentioned above.…”

Wind-reprocessed Transients

“…For the first example, we consider the tidal disruption event (TDE) of a solar mass star by a supermassive black hole. There remains considerable uncertainty in where the powering originates from in such events, whether it be from a small amount of material fed into the BH (Metzger & Stone 2016), dissipation of stream self-interaction (Piran et al 2015), or secondary shocks (Bonnerot & Lu 2019). In any scenario though it is generally thought that there is reprocessing of the emission from these sites because of the relatively low temperatures (∼ 10 4 K) measured from observations in comparison to what is expected from the emission regions ( 10 5 K).…”

Eccentric Tidal Disruption Event Disks around Supermassive Black Holes: Dynamics and Thermal Emission

“…A prediction from this model is the broadening of Hα and optical emission lines from non-Keplerian motion within the eccentric disk (Piran et al 2015;Liu et al 2017;Cao et al 2018). This model also avoids the problem of how the disk efficiently circularizes, since hydrodynamical simulations show the TDE disk remains extended and eccentric long after formation, because not enough orbital energy is dissipated to drive the disk's eccentricity to zero (Guillochon et al 2014;Shiokawa et al 2015;S ądowski et al 2016;Bonnerot & Lu 2019). After the optical emission is driven by shocks from the circularization process, Svirski et al (2017) argued that X-ray thermal emission from viscous heating of the disk's inner edge may be avoided if the disk remains highly eccentric.…”