1. RELATIVISTIC TIDAL DISRUPTIONS OF REALISTIC STARS BY SUPERMASSIVE BLACK HOLESTaj Jankovič, 2023, doctoral dissertation 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
Published in RUNG: 29.08.2023; Views: 172; Downloads: 2
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8. Hitrost vračanja snovi pri relativističnih plimskih raztrganjih zvezdTaj Jankovič, Andreja Gomboc, 2022, published scientific conference contribution abstract Abstract: hole - SMBH) s svojo plimsko silo raztrga zvezdo. Trenutno je z opazovanji odkritih
≈ 10 novih dogodkov na leto, a priˇcakujemo, da se bo s pregledom neba LSST na observatoriju Vere Rubin ˇstevilo poveˇcalo na ≈ 1000/leto. Teoretiˇcno TDE modeliramo
s programom Phantom v sploˇsno-relativistiˇcnem in Newtonovem opisu gravitacijskega
polja SMBH. Zvezde, ki so postavljene na paraboliˇcne orbite z razliˇcnimi parametri β
(z razliˇcnimi razdaljami pericentra), ustvarimo s programom za razvoj zvezd MESA
in imajo realistiˇcne profile gostote. Preuˇcujemo hitrost vraˇcanja snovi M˙
, koliˇcino, ki
je pogosto tesno povezana s svetlobno krivuljo TDE. Posebnost naˇse raziskave je, da
obravnavamo raztrganja zvezd z realistiˇcnim opisom profila gostote in izraˇcunamo M˙ v
sploˇsno-relativistiˇcnem gravitacijskem polju vrteˇce-se SMBH. Izraˇcunamo odvisnost M˙
od β, mase in starosti zvezd ter rezultate primerjamo s prejˇsnjimi raziskavami. Prav tako
karakteriziramo razlike, do katerih pride zaradi razliˇcnega opisa gravitacijskega polja, in
vpliv vrtenja SMBH.
Keywords: plimska raztrganja zvezd, Splošna teorija relativnosti, črne luknje
Published in RUNG: 16.11.2022; Views: 573; Downloads: 0
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9. The rise and fall of the iron-strong nuclear transient PS16dtmTaj Jankovič, 2022, published scientific conference contribution abstract Abstract: Thanks to the advent of large-scale optical surveys, a diverse set of flares from the nuclear regions of galaxies has recently been discovered. These include the disruption of stars by supermassive black holes at the centres of galaxies - nuclear transients known as tidal disruption events (TDEs). Active galactic nuclei (AGN) can show extreme changes in the brightness and emission line intensities, often referred to as changing-look AGN (CLAGN). Given the physical and observational similarities, the interpretation and distinction of nuclear transients as CLAGN or TDEs remains difficult. One of the obstacles of making progress in the field is the lack of well-sampled data of long-lived nuclear outbursts in AGN. I will present PS16dtm, a nuclear transient in a Narrow Line Seyfert 1 (NLSy1) galaxy which has been proposed to be a TDE candidate. I will show our multi-year spectroscopic and photometric study of PS16dtm, which can help us to better understand the outbursts originating in NLSy1 galaxies.
Keywords: supermassive black holes, active galactic nuclei, transients
Published in RUNG: 09.11.2022; Views: 610; Downloads: 5
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