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Title:RELATIVISTIC TIDAL DISRUPTIONS OF REALISTIC STARS BY SUPERMASSIVE BLACK HOLES
Authors:ID Jankovič, Taj, Univerza v Novi Gorici (Author)
ID Gomboc, Andreja, Univerza v Novi Gorici (Mentor) More about this mentor... New window
ID Bonnerot, Clément, University of Birmingham (Comentor)
Files:.pdf jankovic_thesis_final.pdf (16,29 MB)
MD5: ED89D33265A2709E67E272F40B959EA7
 
Language:English
Work type:Doctoral dissertation
Typology:2.08 - Doctoral Dissertation
Organization:FPŠ - Graduate School
Abstract:Stellar tidal disruption events (TDEs), where a star gets disrupted by strong tidal forces of a supermassive black hole (SMBH), offer a unique opportunity for studies of SMBHs and stellar dynamics in galactic nuclei and provide insights into accretion physics. Currently, there are ≈ 100 observed TDEs, however, this number is expected to increase significantly with the start of new wide-field optical surveys, e.g. with the Vera Rubin Observatory. We focus on hydrodynamic simulations of TDEs with the smoothed particle hydrodynamics code Phantom. To begin with, we simulate TDEs in a general relativistic and Newtonian description of an SMBH’s gravity. Stars, which are placed on parabolic orbits with different parameters β (to be defined here), are constructed with the stellar evolution code MESA and therefore have realistic stellar density profiles. We study the mass fallback rate of the debris Ṁ, a quantity often assumed to determine the TDE light curves, and its dependence on the β, stellar mass and age as well as the black hole’s spin and the choice of the gravity’s description. We find that relativistic disruptions at the same pericenter distance are stronger than disruptions in a Newtonian description of the SMBH’s gravity. We also determine the differences between Ṁ of realistic stars with various ages and masses. In addition, we characterize the effect of SMBH’s rotation on the Ṁ and find that it depends on the orientation of SMBH’s spin vector relative to the stellar orbital angular momentum. Encounters on prograde orbits result in narrower Ṁ curves with higher peak values, while the opposite occurs for retrograde orbits. Stellar disruption results in an elongated stream of gas that partly falls back to the pericenter. Due to apsidal precession, the returning stream may collide with itself, leading to a self-crossing shock that launches an outflow. If the black hole spins, this collision may additionally be affected by Lense-Thirring precession which can cause an offset between the two stream components. We study the impact of this effect on the outflow properties by carrying out local simulations of collisions between offset streams. As the offset increases, we find that the geometry of the outflow becomes less spherical and more collimated along the directions of the incoming streams, with less gas getting unbound by the interaction. However, even the most grazing collisions we consider significantly affect the trajectories of the colliding gas, likely promoting subsequent strong interactions near the black hole and rapid disc formation. We analytically compute the offset to stream width ratio, finding that even slowly spinning black holes can cause both strong and grazing collisions. We propose that the deviation from outflow sphericity may enhance the self-crossing shock luminosity due to a reduction of adiabatic losses, and cause significant variations of the efficiency at which X-ray radiation from the disc is reprocessed to the optical band depending on the viewing angle. These potentially observable features hold the promise of constraining the black hole spin from tidal disruption events.
Keywords:Computer modelling and simulation, hydrodynamics, black holes, infall
Place of publishing:Nova Gorica
Year of publishing:2023
PID:20.500.12556/RUNG-8359 New window
COBISS.SI-ID:162516483 New window
NUK URN:URN:SI:UNG:REP:SCM0U0LW
Publication date in RUNG:29.08.2023
Views:956
Downloads:12
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Secondary language

Language:Slovenian
Title:RELATIVISTIČNA PLIMSKA RAZTRGANJA REALISTIČNO MODELIRANIH ZVEZD V BLIŽINI SUPERMASIVNIH ČRNIH LUKENJ
Abstract:Plimska raztrganja zvezd (TDE), pri katerih močne plimske sile supermasivne črne luknje (SMBH) raztrgajo zvezdo, ponujajo edinstveno priložnost za preučevanje SMBH in dinamike zvezd v središčih galaksij ter omogočajo vpogled v fiziko akrecije snovi. Trenutno je potrjenih ≈ 100 TDE, vendar pričakujemo, da se bo to število znatno povečalo s pričetkom novih širokokotnih optičnih pregledov neba, npr. z Observatorijem Vere Rubin. Uporabili smo hidrodinamske simulacije TDE z metodo zglajenih delcev s kodo Phantom. Najprej smo simulirali TDE v splošno relativističnem in Newtonovem opisu gravitacije SMBH. Zvezde, ki jih smo postavili na parabolične orbite z različnimi vrednostmi parametra β (definiran v delu), smo naredili s kodo za razvoj zvezd MESA, zato imajo realistične gostotne profile. Proučevali smo hitrost vračanja snovi v bližino črne luknje Ṁ, količino za katero se pogosto predpostavlja, da določa svetlobne krivulje TDE, in njeno odvisnost od β, mase in starosti zvezd ter vrtenja črne luknje in izbire opisa gravitacijskega polja. Ugotovili smo, da so raztrganja v relativističnem opisu na enaki razdalji do pericentra močnejša kot raztrganja v Newtonovem opisu gravitacije SMBH. Določili smo razlike med Ṁ zvezd različnih starosti in mas. Določili smo tudi vpliv vrtenja SMBH na Ṁ in ugotovili, da je vpliv vrtenja SMBH odvisen od orientacije njene vrtilne količine glede na tirno vrtilno količino zvezde. Srečanja na progradnih orbitah imajo ožje Ṁ krivulje z višjimi maksimalnimi vrednostmi, obratno pa velja za retrogradne orbite. Podolgovat tok plina, ki nastane kot posledica raztrganja zvezde, se delno vrne v bližino SMBH. Zaradi apsidalne precesije lahko vračajoči se del snovi trči z delom preostalega toka, kar povzroči udarne valove, ki sprožijo tok snovi iz območja samo-prečkanja. Če se črna luknja vrti, lahko na to prečkanje vpliva Lense-Thirringova precesija, ki povzroči zamik med obema komponentama toka. Proučevali smo vpliv tega učinka na lastnosti toka snovi z lokalnimi simulacijami trkov med dvema zama- knjenima curkoma. Ugotovili smo, da postaja z večanjem zamika geometrija toka snovi manj sferno simetrična in da je tok snovi bolj usmerjen vzdolž smeri vpadnih curkov, kar je posledica manjše izgube energije pri delnem trku tokov. Vendar pa tudi v primeru, ko se curka le oplazita, pride do znatnih sprememb tirnic plina, kar verjetno ojača nadaljnje interakcije v bližini črne luknje in povzroči hiter nasta nek akrecijskega diska. Analitično smo izračunali razmerje med zamikom in širino vpadnega toka ter ugotovili, da lahko tudi počasi vrteče se črne luknje znatno vplivajo na zamik med tokovoma. Sklepamo, da lahko odstopanje od sferičnosti toka snovi poveča izsev iz območja samo-prečkanja zaradi zmanjšanja adiabatnih izgub in povzroči znatne spremembe učinkovitosti pretvorbe rentgenskega sevanja iz diska v optično svetlobo. Te lastnosti so potencialno opazljive in bi lahko omogočile določitev spina črnih lukenj s TDE.
Keywords:Računalniško modeliranje in simulacije, hidrodinamika, črne luknje, padanje snovi


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