# 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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### Recent Submissions

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 Scaffolded Training Environment for Physics Programming (STEPP)(Association for Computing Machinery, Inc, 2019-06) Kitagawa, Midori; Fishwick, Paul Anthony; Kesden, Michael; Urquhart, Mary; Guadagno, R.; Jin, Rong; Tran, Ngoc M.; Omogbehin, Erik; Prakash, Aditya; Awaraddi, Priyanka; Hale, Baily; Suura, Ken; Raj, A.; Stanfield, J.; Vo, H.; Kitagawa, Midori; Fishwick, Paul Anthony; Kesden, Michael; Urquhart, Mary; Jin, Rong; Tran, Ngoc M.; Omogbehin, Erik; Prakash, Aditya; Awaraddi, Priyanka; Hale, Baily; Suura, KenWe are a year into the development of a software tool for modeling and simulation (M&S) of 1D and 2D kinematics consistent with Newton’s laws of motion. Our goal has been to introduce modeling and computational thinking into learning high-school physics. There are two main contributions from an M&S perspective: (1) the use of conceptual modeling, and (2) the application of Finite State Machines (FSMs) to model physical behavior. Both of these techniques have been used by the M&S community to model high-level “soft systems” and discrete events. However, they have not been used to teach physics and represent ways in which M&S can improve physics education. We introduce the NSF-sponsored STEPP project along with its hypothesis and goals. We also describe the development of the three STEPP modules, the server architecture, the assessment plan, and the expected outcomes. ©2019 Association of Computing Machinery.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.Item Explaining LIGO's Observations via Isolated Binary Evolution with Natal Kicks(Amer Physical Soc) Wysocki, Daniel; Gerosa, Davide; O'Shaughnessy, Richard; Belczynski, Krzysztof; Gladysz, Wojciech; Berti, Emanuele; Kesden, Michael; Holz, Daniel E.; Kesden, MichaelWe compare binary evolution models with different assumptions about black-hole natal kicks to the first gravitational-wave observations performed by the LIGO detectors. Our comparisons attempt to reconcile merger rate, masses, spins, and spin-orbit misalignments of all current observations with state-of-the-art formation scenarios of binary black holes formed in isolation. We estimate that black holes (BHs) should receive natal kicks at birth of the order of sigma similar or equal to 200 (50) km/s if tidal processes do (not) realign stellar spins. Our estimate is driven by two simple factors. The natal kick dispersion sigma is bounded from above because large kicks disrupt too many binaries (reducing the merger rate below the observed value). Conversely, the natal kick distribution is bounded from below because modest kicks are needed to produce a range of spin-orbit misalignments. A distribution of misalignments increases our models' compatibility with LIGO's observations, if all BHs are likely to have natal spins. Unlike related work which adopts a concrete BH natal spin prescription, we explore a range of possible BH natal spin distributions. Within the context of our models, for all of the choices of s used here and within the context of one simple fiducial parameterized spin distribution, observations favor low BH natal spin.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 Precessional Instability in Binary Black Holes with Aligned Spins(2015-10-02) Gerosa, Davide; Kesden, Michael; O'Shaughnessy, Richard; Klein, Antoine; Berti, Emanuele; Sperhake, Ulrich; Trifiro, Daniele; Kesden, MichaelBinary black holes on quasicircular orbits with spins aligned with their orbital angular momentum have been test beds for analytic and numerical relativity for decades, not least because symmetry ensures that such configurations are equilibrium solutions to the spin-precession equations. In this work, we show that these solutions can be unstable when the spin of the higher-mass black hole is aligned with the orbital angular momentum and the spin of the lower-mass black hole is antialigned. Spins in these configurations are unstable to precession to large misalignment when the binary separation r is between the values (r{ud±}= √X̅₁ ± √q̅x̅)⁴ (1-q)⁻² M where M is the total mass, q ≡ m₂/m₁ is the mass ratio, and χ₁ (χ₂) is the dimensionless spin of the more (less) massive black hole. This instability exists for a wide range of spin magnitudes and mass ratios and can occur in the strong-field regime near the merger. We describe the origin and nature of the instability using recently developed analytical techniques to characterize fully generic spin precession. This instability provides a channel to circumvent astrophysical spin alignment at large binary separations, allowing significant spin precession prior to merger affecting both gravitational-wave and electromagnetic signatures of stellar-mass and supermassive binary black holes.Item Multi-Timescale Analysis of Phase Transitions in Precessing Black-Hole Binaries(American Physical Society, 2015-09-14) Gerosa, D.; Kesden, Michael; Sperhake, U.; Berti, E.; O'Shaughnessy, R.; Kesden, MichaelThe dynamics of precessing binary black holes (BBHs) in the post-Newtonian regime has a strong timescale hierarchy: the orbital timescale is very short compared to the spin-precession timescale which, in turn, is much shorter than the radiation-reaction timescale on which the orbit is shrinking due to gravitational-wave emission. We exploit this timescale hierarchy to develop a multiscale analysis of BBH dynamics elaborating on the analysis of Kesden et al. [Phys. Rev. Lett. 114, 081103 (2015)]. We solve the spin-precession equations analytically on the precession time and then implement a quasiadiabatic approach to evolve these solutions on the longer radiation-reaction time. This procedure leads to an innovative "precession-averaged" post-Newtonian approach to studying precessing BBHs. We use our new solutions to classify BBH spin precession into three distinct morphologies, then investigate phase transitions between these morphologies as BBHs inspiral. These precession-averaged post-Newtonian inspirals can be efficiently calculated from arbitrarily large separations, thus making progress towards bridging the gap between astrophysics and numerical relativity.Item Distinguishing Black-Hole Spin-Orbit Resonances by their Gravitational-Wave Signatures(American Physical Society, 2014-06-24) Gerosa, D.; O'Shaughnessy, R.; Kesden, Michael; Berti, E.; Sperhake, U.; 0000 0001 2678 2731 (Kesden, M); Kesden, MichaelIf binary black holes form following the successive core collapses of sufficiently massive binary stars, precessional dynamics may align their spins, S₁ and S₂, and the orbital angular momentum L into a plane in which they jointly precess about the total angular momentum J. These spin orientations are known as spin-orbit resonances since S₁, S₂, and L all precess at the same frequency to maintain their planar configuration. Two families of such spin-orbit resonances exist, differentiated by whether the components of the two spins in the orbital plane are either aligned or antialigned. The fraction of binary black holes in each family is determined by the stellar evolution of their progenitors, so if gravitational-wave detectors could measure this fraction they could provide important insights into astrophysical formation scenarios for binary black holes. In this paper, we show that even under the conservative assumption that binary black holes are observed along the direction of J (where precession-induced modulations to the gravitational waveforms are minimized), the waveforms of many members of each resonant family can be distinguished from all members of the other family in events with signal-to-noise ratios ρ ≃10, typical of those expected for the first detections with Advanced LIGO and Virgo. We hope that our preliminary findings inspire a greater appreciation of the capability of gravitational-wave detectors to constrain stellar astrophysics and stimulate further studies of the distinguishability of spin-orbit resonant families in more expanded regions of binary black-hole parameter space.