Kesden, Michael
Permanent URI for this collectionhttps://hdl.handle.net/10735.1/4147
Michael Kesden is an Assistant Professor of Physics. He also serves as a faculty member of the UTD Cosmology, Relativity and Astrophysics Group. Dr. Kesden's research interests and areas of expertise include:
- Theoretical astrophysics and relativity
- Binary black hole formation, evolution, and merger
- Gravitational wave emission and detection
- Stellar tidal disruption by supermassive black holes
- Astrophysical probes of dark-matter dynamics
- Gravitational lensing of the cosmic microwave background
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Browsing Kesden, Michael by Author "0000-0002-5987-1471 (Kesden, M)"
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Item Stellar Tidal Disruption Events in General Relativity(Springer/Plenum Publishers, 2019-02-12) Stone, Nicholas C.; Kesden, Michael; Cheng, Roseanne M.; van Velzen, Sjoert; 0000-0002-5987-1471 (Kesden, M); Kesden, MichaelA tidal disruption event (TDE) ensues when a star passes too close to a supermassive black hole (SMBH) in a galactic center, and is ripped apart by its tidal field. The gaseous debris produced in a TDE can power a bright electromagnetic flare as it is accreted by the SMBH; so far, several dozen TDE candidates have been observed. For SMBHs with masses above approximate to 10⁷ M⊙, the tidal disruption of solar-type stars occurs within ten gravitational radii of the SMBH, implying that general relativity (GR) is needed to describe gravity. Three promising signatures of GR in TDEs are: (1) a super-exponential cutoff in the volumetric TDE rate for SMBH masses above approximate to 10⁸ M⊙ due to direct capture of tidal debris by the event horizon, (2) delays in accretion disk formation (and a consequent alteration of the early-time light curve) caused by the effects of relativistic nodal precession on stream circularization, and (3) quasi-periodic modulation of X-ray emission due to global precession of misaligned accretion disks and the jets they launch. We review theoretical models and simulations of TDEs in Newtonian gravity, then describe how relativistic modifications give rise to these proposed observational signatures, as well as more speculative effects of GR. We conclude with a brief summary of TDE observations and the extent to which they show indications of these predicted relativistic signatures.Item Unified Treatment of Tidal Disruption by Schwarzschild Black Holes(2017-04-03) Servin, Juan; Kesden, Michael; 0000-0002-5987-1471 (Kesden, M); Servin, Juan; Kesden, MichaelStars on orbits with pericenters sufficiently close to the supermassive black hole at the center of their host galaxy can be ripped apart by tidal stresses. Some of the resulting stellar debris becomes more tightly bound to the hole and can potentially produce an observable flare called a t (TDE). We provide a self-consistent, unified treatment of TDEs by nonspinning (Schwarzschild) black holes, investigating several effects of general relativity including changes to the boundary in phase space that defines the loss-cone orbits on which stars are tidally disrupted or captured. TDE rates decrease rapidly at large black hole masses due to direct stellar capture, but this effect is slightly countered by the widening of the loss cone due to the stronger tidal fields in general relativity. We provide a new mapping procedure that translates between Newtonian gravity and general relativity, allowing us to better compare predictions in both gravitational theories. Partial tidal disruptions in relativity will strip more material from the star and produce more tightly bound debris than in Newtonian gravity for a stellar orbit with the same angular momentum. However, for deep encounters leading to full disruption in both theories, the stronger tidal forces in relativity imply that the star is disrupted further from the black hole and that the debris is therefore less tightly bound, leading to a smaller peak fallback accretion rate. We also examine the capture of tidal debris by the horizon and the relativistic pericenter precession of tidal debris, finding that black holes of 10⁶ solar masses and above generate tidal debris precessing by 10° or more per orbit.Item Wide Nutation: Binary Black-Hole Spins Repeatedly Oscillating from Full Alignment to Full Anti-Alignment(Institute of Physics Publishing, 2019-04-15) Gerosa, D.; Lima, A.; Berti, E.; Sperhake, U.; Kesden, Michael; O'Shaughnessy, R.; 0000-0002-5987-1471 (Kesden, M); Kesden, MichaelWithin the framework of 2PN black-hole binary spin precession, we explore configurations where one of the two spins oscillates from being completely aligned with the orbital angular momentum to being completely anti-aligned with it during a single precession cycle. This wide nutation is the extreme limit of the generic phenomenon of spin nutation in black-hole binaries. Crucially, wide nutation happens on the short precession time scale and it is not a secular effect due to gravitational-wave radiation reaction. The spins of these binaries, therefore, flip repeatedly as one of these special configurations is entered. Binaries with total mass M, mass ratio q, and dimensionless spin X1(X2) of the more (less) massive black hole are allowed to undergo wide nutation at binary separations r ≤ rwide ≡ [(qX2 - X1)/(1 - q)]2M. Sources that are more likely to nutate widely have similar masses and effective spins close to zero. © 2019 IOP Publishing Ltd.