Vasileios Paschalidis
 Associate Professor, Astronomy
 Assistant Astronomer, Steward Observatory
 Assistant Professor, Physics
 Member of the Graduate Faculty
Contact
 (520) 6212288
 Steward Observatory, Rm. N208
 Tucson, AZ 85721
 vpaschal@arizona.edu
Biography
Vasileios Paschalidis obtained his Undergraduate degree in Physics from the Aristoteleion University of Thessaloniki, Greece (2003), when he move to the USA to pursuit a Ph.D. in Astrophysics from the University of Chicago which he obtained in 2008. From 20082014 He was a Research Associate and Fortner Fellow in Physics at the University of Illinois at UrbanaChampaign, and from 20142017 a Research Scholar in Physics, at Princeton University. He joined the University of Arizona as a TAP faculty in the Fall of 2017.
Vasileios Paschalidis' research spans a range of topics in gravitational physics, theoretical astrophysics and early Universe cosmology. The ultimate goal of his work is to understand strongfield gravitation and solve longstanding astrophysical/cosmological puzzles such as the nature of the progenitors of short gammaray bursts, the origin of Xshaped radio galaxies, the nature of the equation of state above the nuclear saturation density, the way through which planets may form around isolated pulsars, and how our Universe began, to name a few. He is interested in studying compact objects as multimessenger sources, i.e., as sources of gravitational wave, electromagnetic and neutrino signals. For this reason, compact object binaries, such as black holeblack hole (BHBH), neutron starneutron star (NSNS), black holeneutron star (BHNS) , and white dwarfneutron star (WDNS) systems are a major theme of my research. He develops and uses both (semi)analytic methods and stateoftheart codes at supercomputer centers, to study the equations of relativistic gravitation and simulate systems of interest.
Degrees
 Ph.D. Astronomy and Astrophysics
 The University of Chicago, Chicago, Illinois, United States
 Formulations of General Relativity and Numerical Applications
 M.S. Astronomy and Astrophysics
 The University of Chicago, Chicago, Illinois, United States
Work Experience
 Princeton University, Princeton, New Jersey (2014  2017)
 University of Illinois at UrbanaChampaign (2008  2014)
Awards
 NSF CAREER award
 NSF, Summer 2022
 Lucas Award
 Lucas/San Diego Astronomy Association Junior Faculty Award, Summer 2018
Interests
Teaching
General relativity, compact objects, high energy astrophysics, theoretical astrophysics, gravitational waves astronomy/astrophysics
Research
General relativity, compact objects, high energy astrophysics, theoretical astrophysics, gravitational waves astronomy/astrophysics, early universe cosmology
Courses
202425 Courses

Dissertation
ASTR 920 (Fall 2024) 
Dissertation
PHYS 920 (Fall 2024)
202324 Courses

Dissertation
PHYS 920 (Spring 2024) 
Independent Study
ASTR 599 (Spring 2024) 
Research
ASTR 900 (Spring 2024) 
Dissertation
PHYS 920 (Fall 2023) 
Independent Study
PHYS 599 (Fall 2023) 
Physics of Astrophysics
ASTR 589 (Fall 2023) 
Research
ASTR 900 (Fall 2023)
202223 Courses

Directed Research
PHYS 492 (Spring 2023) 
Dissertation
ASTR 920 (Spring 2023) 
Dissertation
PHYS 920 (Spring 2023) 
HighEnergy Astrophysics
ASTR 582 (Spring 2023) 
HighEnergy Astrophysics
PHYS 582 (Spring 2023) 
Honors Thesis
ASTR 498H (Spring 2023) 
Honors Thesis
PHYS 498H (Spring 2023) 
Independent Study
PHYS 499 (Spring 2023) 
Independent Study
PHYS 599 (Spring 2023) 
Research
ASTR 900 (Spring 2023) 
Dissertation
ASTR 920 (Fall 2022) 
Dissertation
PHYS 920 (Fall 2022) 
Honors Independent Study
PHYS 499H (Fall 2022) 
Honors Thesis
ASTR 498H (Fall 2022) 
Honors Thesis
PHYS 498H (Fall 2022) 
Independent Study
PHYS 599 (Fall 2022) 
Physics of Astrophysics
ASTR 589 (Fall 2022) 
Research
ASTR 900 (Fall 2022)
202122 Courses

Directed Research
ASTR 492 (Spring 2022) 
Directed Research
PHYS 492 (Spring 2022) 
Dissertation
ASTR 920 (Spring 2022) 
Dissertation
PHYS 920 (Spring 2022) 
Honors Independent Study
ASTR 499H (Spring 2022) 
Honors Thesis
ASTR 498H (Spring 2022) 
Independent Study
PHYS 599 (Spring 2022) 
Intro General Relativity
PHYS 569 (Spring 2022) 
Dissertation
ASTR 920 (Fall 2021) 
Dissertation
PHYS 920 (Fall 2021) 
Honors Thesis
ASTR 498H (Fall 2021) 
Independent Study
PHYS 599 (Fall 2021) 
Physics of Astrophysics
ASTR 589 (Fall 2021)
202021 Courses

Dissertation
ASTR 920 (Spring 2021) 
Dissertation
PHYS 920 (Spring 2021) 
Honors Independent Study
ASTR 399H (Spring 2021) 
Independent Study
PHYS 599 (Spring 2021) 
Research
ASTR 900 (Spring 2021) 
Cosmology
ASTR 201 (Fall 2020) 
Dissertation
PHYS 920 (Fall 2020) 
Independent Study
ASTR 599 (Fall 2020) 
Independent Study
PHYS 599 (Fall 2020) 
Research
ASTR 900 (Fall 2020)
201920 Courses

Dissertation
PHYS 920 (Spring 2020) 
Independent Study
ASTR 599 (Spring 2020) 
Independent Study
PHYS 599 (Spring 2020) 
Intro General Relativity
PHYS 569 (Spring 2020) 
Research
ASTR 900 (Spring 2020) 
Computational Physics
PHYS 305 (Fall 2019) 
Directed Research
ASTR 492 (Fall 2019) 
Directed Research
PHYS 492 (Fall 2019) 
Dissertation
PHYS 920 (Fall 2019) 
Independent Study
ASTR 599 (Fall 2019) 
Independent Study
PHYS 599 (Fall 2019) 
Research
ASTR 900 (Fall 2019)
201819 Courses

Directed Research
ASTR 492 (Spring 2019) 
Dissertation
ASTR 920 (Spring 2019) 
Independent Study
ASTR 599 (Spring 2019) 
Research
ASTR 900 (Spring 2019) 
Directed Research
PHYS 492 (Fall 2018) 
Dissertation
ASTR 920 (Fall 2018) 
Independent Study
ASTR 599 (Fall 2018) 
Research
ASTR 900 (Fall 2018)
201718 Courses

Directed Research
ASTR 492 (Spring 2018) 
HighEnergy Astrophysics
ASTR 582 (Spring 2018) 
Independent Study
ASTR 599 (Spring 2018) 
Research
ASTR 900 (Spring 2018)
Scholarly Contributions
Journals/Publications
 Bozzola, G., Chan, C., & Paschalidis, V. (2022). Black Hole Physics and Computer Graphics. Computing in Science Engineering, 11.
 Paschalidis, V. (2021). Multimessenger constraints for ultradense matter. Phys. Rev. X, 12, 011058.More infoRecent rapid progress in multimessenger observations of neutron stars (NSs)offers great potential to constrain the properties of strongly interactingmatter under the most extreme conditions. In order to fully exploit the currentobservational inputs and to study the impact of future observations, we analyzea large ensemble of randomly generated modelindependent equations of state(EoSs) and the corresponding rotating stellar structures without the use ofquasiuniversal relations. We discuss the compatibility and impact of varioushypotheses and measurements on the EoS, including those involving the mergerproduct in GW170817, the binary merger components in GW190814, and radiusmeasurements of PSR J0740+6620. We obtain an upper limit for the dimensionlessspin of a rigidly rotating NS, chi < 0.81, an upper limit for the compactnessof a NS, GM/(Rc^2) < 0.33, and find that the conservative hypothesis that theremnant in GW170817 ultimately collapsed to a black hole strongly constrainsthe EoS and the maximal mass of NSs, implying M_TOV < 2.53M_sol (or M_TOV
 Paschalidis, V. (2021). The fate of twin stars on the unstable branch: implications for the formation of twin stars. Phys. Rev. D, 105, 043014.More infoHybrid hadronquark equations of state that give rise a third family ofstable compact stars have been shown to be compatible with the LIGOVirgo eventGW170817. Stable configurations in the third family are called hybridhadronquark stars. The equilibrium stable hybrid hadronquark star branch isseparated by the stable neutron star branch with a branch of unstable hybridhadronquark stars. The endstate of these unstable configurations has not beenstudied, yet, and it could have implications for the formation and existence oftwin stars  hybrid stars with the same mass as neutron stars but differentradii. We modify existing hybrid hadronquark equations of state with afirstorder phase transition in order to guarantee a wellposed initial valueproblem of the equations of general relativistic hydrodynamics, and study thedynamics of nonrotating or rotating unstable twin stars via 3dimensionalsimulations in full general relativity. We find that unstable twin starsnaturally migrate toward the hadronic branch. Before settling into the hadronicregime, these stars undergo (quasi)radial oscillations on a dynamical timescalewhile the core bounces between the two phases. Our study suggests that it maybe difficult to form stable twin stars if the phase transition is sustainedover a large jump in energy density, and hence it may be more likely thatastrophysical hybrid hadronquark stars have masses above the twin star regime.We also study the minimummass instability for hybrid stars, and find thatthese configurations do not explode, unlike the minimummass instability forneutron stars. Additionally, our results suggest that oscillations between thetwo Quantum Chromodynamic phases could provide gravitational wave signalsassociated with such phase transitions in corecollapse supernovae and whitedwarfneutron star mergers.[Journal_ref: ]
 Paschalidis, V. (2022). Astrophysics with the Laser Interferometer Space Antenna. Living Reviews in Relativity.More infoLaser Interferometer Space Antenna (LISA) will be a transformative experimentfor gravitational wave astronomy as it will offer unique opportunities toaddress many key astrophysical questions in a completely novel way. The synergywith groundbased and other spacebased instruments in the electromagneticdomain, by enabling multimessenger observations, will add further to thediscovery potential of LISA. The next decade is crucial to prepare theastrophysical community for LISA's first observations. This review outlines theextensive landscape of astrophysical theory, numerical simulations, andastronomical observations that are instrumental for modeling and interpretingthe upcoming LISA datastream. To this aim, the current knowledge in three mainsource classes for LISA is reviewed: ultracompact stellarmass binaries,massive black hole binaries, and extreme or intermediate mass ratio inspirals.The relevant astrophysical processes and the established modeling techniquesare summarized. Likewise, open issues and gaps in our understanding of thesesources are highlighted, along with an indication of how LISA could help makeprogress in the different areas. New research avenues that LISA itself, or itsjoint exploitation with studies in the electromagnetic domain, will enable, arealso illustrated. Improvements in modeling and analysis approaches, such as thecombination of numerical simulations and modern data science techniques, arediscussed. This review is intended to be a starting point for using LISA as anew discovery tool for understanding our Universe.[Journal_ref: ]
 Paschalidis, V. (2022). Improving the convergence order of binary neutron star merger simulations in the BSSN formulation. Phys Rev D.More infoHighaccuracy numerical relativity simulations of binary neutron star mergersare a necessary ingredient for constructing gravitational waveform templates toanalyze and interpret observations of compact object mergers. Numericalconvergence in the postmerger phase of such simulations is challenging toachieve with many modern codes. In this paper, we study two ways of improvingthe convergence properties of binary neutron star merger simulations within theBaumgarteShapiroShibataNakamura formulation of Einstein's equations. We showthat discontinuities in a particular constraint damping scheme in thisformulation can destroy the postmerger convergence of the simulation. Acontinuous prescription, in contrast, ensures convergence until late times. Weadditionally study the impact of the equation of state parametrization on thepre and postmerger convergence properties of the simulations. In particular,we compare results for a piecewise polytropic parametrization, which iscommonly used in merger simulations but suffers unphysical discontinuities inthe sound speed, with results using a "generalized" piecewise polytropicparametrization, which was designed to ensure both continuity anddifferentiability of the equation of state. We report on the differences in thegravitational waves depending on which equation of state parametrization isused.[Journal_ref: ]
 Paschalidis, V. (2021). General relativistic simulations of the quasicircular inspiral and merger of charged black holes: GW150914 and fundamental physics implications. Phys. Rev. Lett..More infoWe perform generalrelativistic simulations of charged black holes targetingGW150914. We show that the inspiral is most efficient for detecting black holecharge through gravitational waves and that GW150914 is compatible with havingchargetomass ratio as high as 0.3. Our work applies to electric and magneticcharge, and to theories with black holes endowed with U(1) (hidden or dark)charges. Using our results we place an upper bound on the deviation fromgeneral relativity in the dynamical, strongfield regime of the socalledtheory of MOdified Gravity (MOG).[Journal_ref: Phys. Rev. Lett. 126, 041103 (2021)]
 Paschalidis, V. (2021). Minidisk dynamics in accreting, spinning black hole binaries: Simulations in full general relativity. ApJL.More infoWe perform magnetohydrodynamic simulations of accreting, equalmass binaryblack holes in full general relativity focusing on the impact of black holespin on the dynamical formation and evolution of minidisks. We find that duringthe late inspiral the sizes of minidisks are primarily determined by theinterplay between the tidal field and the effective innermost stable orbitaround each black hole. Our calculations support that a minidisk forms when theHill sphere around each black hole is significantly larger than the blackhole's effective innermost stable orbit. As the binary inspirals, the radius ofthe Hill sphere decreases, and minidisk sconsequently shrink in size. As aresult, electromagnetic signatures associated with minidisks may be expected togradually disappear prior to merger when there are no more stable orbits withinthe Hill sphere. In particular, a gradual disappearance of a hardelectromagnetic component in the spectrum of such systems could provide acharacteristic signature of merging black hole binaries. For a binary of giventotal mass, the timescale to minidisk "evaporation" should therefore depend onthe black hole spins and the mass ratio. We also demonstrate that accretingbinary black holes with spin have a higher efficiency for converting accretionpower to jet luminosity. These results could provide new ways to estimate blackhole spins in the future.[Journal_ref: ApJL 910 L26 (2021)]
 Paschalidis, V. (2021). Numericalrelativity simulations of the quasicircular inspiral and merger of nonspinning, charged black holes: methods and comparison with approximate approaches. Phys. Rev. D.More infoWe present fully general relativistic simulations of the quasicircularinspiral and merger of charged, nonspinning, binary black holes withchargetomass ratio $\lambda \le 0.3$. We discuss the key features thatenabled long term and stable evolutions of these binaries. We also present aformalism for computing the angular momentum carried away by electromagneticwaves, and the electromagnetic contribution to blackhole horizon properties.We implement our formalism and present the results for the first time innumericalrelativity simulations. In addition, we compare our full nonlinearsolutions with existing approximate models for the inspiral and ringdownphases. We show that Newtonian models based on the quadrupole approximationhave errors of 20 %  100 % in key gaugeinvariant quantities. On the otherhand, for the systems considered, we find that estimates of the remnant blackhole spin based on the motion of test particles in KerrNewman spacetimes agreewith our nonlinear calculations to within a few percent. Finally, we discussthe prospects for detecting black hole charge by future gravitationalwavedetectors using either the inspiralmergerringdown signal or the ringdownsignal alone.[Journal_ref: Phys. Rev. D 104, 044004 (2021)]
 Paschalidis, V. (2021). Realistic FiniteTemperature Effects in Neutron Star Merger Simulations. Phys. Rev. D.More infoBinary neutron star mergers provide a unique probe of the densematterequation of state (EoS) across a wide range of parameter space, from thezerotemperature EoS during the inspiral to the hightemperature EoS followingthe merger. In this paper, we implement a new model for calculatingparametrized finitetemperature EoS effects into numerical relativitysimulations. This "M* model" is based on a twoparameter approximation of theparticle effective mass and includes the leadingorder effects of degeneracy inthe thermal pressure and energy. We test our numerical implementation byperforming evolutions of rotating single stars with zero and nonzerotemperature gradients, as well as evolutions of binary neutron star mergers. Wefind that our new finitetemperature EoS implementation can support stablestars over many dynamical timescales. We also perform a first parameter studyto explore the role of the M* parameters in binary neutron star mergersimulations. All simulations start from identical initial data with identicalcold EoSs, and differ only in the thermal part of the EoS. We find that boththe thermal profile of the remnant and the postmerger gravitational wavesignal depend on the choice of M* parameters, but that the total merger ejectadepends only weakly on the finitetemperature part of the EoS across a widerange of parameters. Our simulations provide a first step toward understandinghow the finitetemperature properties of dense matter may affect futureobservations of binary neutron star mergers.[Journal_ref: Phys. Rev. D 104, 063016 (2021)]
 Paschalidis, V. (2021). Searches after Gravitational Waves Using ARizona Observatories (SAGUARO): Observations and Analysis from Advanced LIGO/Virgo's Third Observing Run. ApJ.More infoWith the conclusion of the third observing run for Advanced LIGO/Virgo (O3),we present a detailed analysis of both triggered and serendipitous observationsof 17 gravitational wave (GW) events (7 triggered and 10 purely serendipitous)from the Searches After Gravitationalwaves Using ARizona Observatories(SAGUARO) program. We searched a total of 4935 deg$^2$ down to a median5$\sigma$ transient detection depth of 21.1 AB mag using the Mt Lemmon 1.5 mtelescope, the discovery engine for SAGUARO. In addition to triggered eventswithin 24~hours, our transient search encompassed a time interval following GWevents of $
 Paschalidis, V., Wessel, E., Tsokaros, A., Ruiz, M., & Shapiro, S. L. (2021). Gravitational Waves from Disks Around Spinning Black Holes: Simulations in Full General Relativity. Physical Reviews D, Volume 103(Issue 4), article id.043013. doi:https://doi.org/10.1103/PhysRevD.103.043013More infoWe present fully generalrelativistic numerical evolutions of selfgravitating tori around spinning black holes with dimensionless spin a/M=0.7 parallel or antiparallel to the disk angular momentum. The initial disks are unstable to the hydrodynamic PapaloizouPringle Instability which causes them to grow persistent orbiting matter clumps. The effect of black hole spin on the growth and saturation of the instability is assessed. We find that the instability behaves similarly to prior simulations with nonspinning black holes, with a shift in frequency due to spininduced changes in disk orbital period. Copious gravitational waves are generated by these systems, and we analyze their detectability by current and future gravitational wave observatories for large range of masses. We find that systems of 10M⊙  relevant for black holeneutron star mergers  are detectable by Cosmic Explorer out to ∼300 Mpc, while DECIGO (LISA) will be able to detect systems of 1000M⊙ (105M⊙)  relevant for disks forming in collapsing supermassive stars  out to cosmological redshift of z∼5 (z∼1). Computing the accretion rate of these systems we find that these systems may also be promising sources of coincident electromagnetic signals.
 Paschalidis, V. (2020). Black holeneutron star coalescence: effects of the neutron star spin on jet launching and dynamical ejecta mass. Phys. Rev. D.More infoBlack holeneutron star (BHNS) mergers are thought to be sources ofgravitational waves (GWs) with coincident electromagnetic (EM) counterparts. Tofurther probe whether these systems are viable progenitors of short gammaraybursts (sGRBs) and kilonovae, and how one may use (the lack of) EM counterpartsassociated with LIGO/Virgo candidate BHNS GW events to sharpen parameterestimation, we study the impact of neutron star spin in BHNS mergers. Usingdynamical spacetime magnetohydrodynamic simulations of BHNSs initially on aquasicircular orbit, we survey configurations that differ in the BH spin($a_{\rm BH}/M_{\rm BH}=0$ and $0.75$), the NS spin ($a_{\rm NS}/M_{\rmNS}=0.17,\,0,\,0.23$ and $0.33$), and the binary mass ratio ($q\equiv M_{\rmBH}:M_{\rm NS}=3:1$ and $5:1$). The general trend we find is that increasingthe NS prograde spin increases both the rest mass of the accretion disk ontothe remnant black hole, and the rest mass of dynamically ejected matter. By atime~$\Delta t\sim 35005500M\sim 88138(M_{\rm NS}/1.4M_\odot)\,\rm ms$ afterthe peak gravitational wave amplitude, a magneticallydriven jet is launchedonly for $q=3:1$ regardless of the initial NS spin. The lifetime of the jets[$\Delta t\sim 0.50.8(M_{\rm NS}/1.4 M_\odot)\,\rm s$] and their outgoingPoynting luminosity [$L_{\rm Poyn}\sim 10^{51.5\pm 0.5}\,\rm erg/s$] areconsistent with typical sGRBs luminosities and expectations from theBlandfordZnajek mechanism. By the time we terminate our simulations, we do notobserve either an outflow or a largescale magnetic field collimation for theother systems we considered. The mass range of dynamically ejected matter is$10^{4.5}10^{2}~(M_{\rm NS}/1.4M_\odot)M_\odot$, which can power kilonovaewith peak bolometric luminosities $L_{\rm knova}\sim 10^{40}10^{41.4}$ erg/swith rise times $\lesssim 6.5\,\rm h$ and potentially detectable by the LSST.[Journal_ref: Phys. Rev. D 102, 124077 (2020)]
 Paschalidis, V. (2020). Prospects for Fundamental Physics with LISA. Gen.Rel.Grav..More infoIn this paper, which is of programmatic rather than quantitative nature, weaim to further delineate and sharpen the future potential of the LISA missionin the area of fundamental physics. Given the very broad range of topics thatmight be relevant to LISA, we present here a sample of what we view asparticularly promising directions, based in part on the current researchinterests of the LISA scientific community in the area of fundamental physics.We organize these directions through a "sciencefirst" approach that allows usto classify how LISA data can inform theoretical physics in a variety of areas.For each of these theoretical physics classes, we identify the sources that arecurrently expected to provide the principal contribution to our knowledge, andthe areas that need further development. The classification presented hereshould not be thought of as cast in stone, but rather as a fluid framework thatis amenable to change with the flow of new insights in theoretical physics.[Journal_ref: Gen.Rel.Grav. 52 (2020) 8, 81]
 Bozzola, G., & Paschalidis, V. (2019). Initial data for general relativistic simulations of multiple electrically charged black holes with linear and angular momenta. PRD. doi:10.1103/PhysRevD.99.104044More infoA general relativistic, stationary, and axisymmetric black hole in a fourdimensional asymptotically flat spacetime is fully determined by its mass, angular momentum, and electric charge. The expectation that astrophysically relevant black holes do not posses charge has resulted in a limited number of investigations of moving and charged black holes in the dynamical, strongfield gravitational (and electromagnetic) regime, in which numerical studies are necessary. Apart from having a theoretical interest, the advent of multimessenger astronomy with gravitational waves offers new ways to think about charged black holes. In this work, we initiate an exploration of charged binary black holes by generating valid initial data for general relativistic simulations of black hole systems that have generic electric charge and linear and angular momenta. We develop our initial data formalism within the framework of the conformal transversetraceless (BowenYork) technique using the puncture approach and apply the theory of isolated horizons to attribute physical parameters (mass, charge, and angular momentum) to each hole. We implemented our formalism in the case of a binary system by modifying the publicly available TwoPunctures and QuasiLocalMeasures codes. We demonstrate that our code can recover existing solutions and that it has excellent selfconvergence properties for a generic configuration of two black holes.
 Espino, P., & Paschalidis, V. (2019). Revisiting the maximum mass of differentially rotating neutron stars in general relativity: $backslash$``Ubermassive stars with realistic equations of state. PRD. doi:10.1103/PhysRevD.99.083017
 Paschalidis, V. (2019). Effect of spin on the inspiral of binary neutron stars. Phys. Rev. D. doi:10.1103/PhysRevD.100.024061More infoWe perform longterm simulations of spinning binary neutron stars, with ourhighest dimensionless spin being $\chi \sim 0.32$. To assess the importance ofspin during the inspiral we vary the spin, and also use two equations of state,one that consists of plain nuclear matter and produces compact stars (SLy), anda hybrid one that contains both nuclear and quark matter and leads to largerstars (ALF2). Using high resolution that has grid spacing $\Delta x\sim 98$ mon the finest refinement level, we find that the effects of spin in the phaseevolution of a binary system can be larger than the one that comes from tidalforces. Our calculations demonstrate explicitly that although tidal effects aredominant for small spins ($\lesssim 0.1$), this is no longer true when thespins are larger, but still much smaller than the Keplerian limit.[Journal_ref: Phys. Rev. D 100, 024061 (2019)]
 Paschalidis, V. (2019). Maximum mass and universal relations of rotating relativistic hybrid hadronquark stars. Eur. Phys. J. A. doi:10.1140/epja/i2019128312More infoWe construct equilibrium models of uniformly and differentially rotatinghybrid hadronquark stars using equations of state (EOSs) with a firstorderphase transition that gives rise to a third family of compact objects. We findthat the ratio of the maximum possible mass of uniformly rotatingconfigurations  the supramassive limit  to the TolmanOppenheimerVolkoff(TOV) limit mass is not EOSindependent, and is between 1.15 and 1.31,incontrast with the value of 1.20 previously found for hadronic EOSs. Therefore,some of the constraints placed on the EOS from the observation of thegravitational wave event GW170817 do not apply to hadronquark EOSs. However,the supramassive limit mass for the family of EOSs we treat is consistent withlimits set by GW170817, strengthening the possibility of interpreting GW170817with a hybrid hadronquark EOSs. We also find that along constant angularmomentum sequences of uniformly rotating stars, the third family maximum andminimum mass models satisfy approximate EOSindependent relations, and thesupramassive limit of the third family is approximately 16.5 % larger than thethird family TOV limit. For differentially rotating spheroidal stars, we findthat a lowerlimit on the maximum supportable rest mass is 123 % more than theTOV limit rest mass. Finally, we verify that the recently discovered universalrelations relating angular momentum, rest mass and gravitational mass forturningpoint models hold for hybrid hadronquark EOSs when uniform rotation isconsidered, but have a clear dependence on the degree of differential rotation.[Journal_ref: ]
 Paschalidis, V. (2019). On the dynamical stability of quasitoroidal differentially rotating neutron stars. Phys. Rev. D. doi:10.1103/PhysRevD.100.043014More infoWe investigate the dynamical stability of relativistic, differentiallyrotating, quasitoroidal models of neutron stars through hydrodynamicalsimulations in full general relativity. We find that all quasitoroidalconfigurations studied in this work are dynamically unstable against the growthof nonaxisymmetric modes. Both onearm and bar mode instabilities grow duringtheir evolution. We find that very high rest mass configurations collapse toform black holes. Our calculations suggest that configurations whose rest massis less than the binary neutron star threshold mass for prompt collapse toblack hole transition dynamically to spheroidal, differentially rotating starsthat are dynamically stable, but secularly unstable. Our study shows that theexistence of extreme quasitoroidal neutron star equilibrium solutions does notimply that longlived binary neutron star merger remnants can be much moremassive than previously found. Finally, we find models that are initiallysupraKerr ($J/M^2>1$) and undergo catastrophic collapse on a dynamicaltimescale, in contrast to what was found in earlier works. However, cosmiccensorship is respected in all of our cases. Our work explicitly demonstratesthat exceeding the Kerr bound in rotating neutron star models does not implydynamical stability.[Journal_ref: Phys. Rev. D 100, 043014 (2019)]
 Paschalidis, V. (2019). Searches After Gravitationalwaves Using ARizona Observatories (SAGUARO): System Overview and First Results from Advanced LIGO/Virgo's Third Observing Run. Ap. J. Lett.. doi:10.3847/20418213/ab32f2More infoWe present Searches After Gravitationalwaves Using ARizona Observatories(SAGUARO), a comprehensive effort dedicated to the discovery andcharacterization of optical counterparts to gravitational wave (GW) events.SAGUARO utilizes groundbased facilities ranging from 1.5m to 10m in diameter,located primarily in the Northern Hemisphere. We provide an overview ofSAGUARO's telescopic resources, pipeline for transient detection, and databasefor candidate visualization. We describe SAGUARO's discovery component, whichutilizes the $5$~deg$^2$ fieldofview optical imager on the Mt. Lemmon 1.5mtelescope, reaching limits of $\approx 21.3$~AB mag while rapidly tiling largeareas. We also describe the followup component of SAGUARO, used for rapidvetting and monitoring of optical candidates. With the onset of AdvancedLIGO/Virgo's third observing run, we present results from the first threeSAGUARO searches following the GW events S190408an, S190425z and S190426c,which serve as a valuable proofofconcept of SAGUARO. We triggered andsearched 15, 60 and 60 deg$^{2}$ respectively, 17.6, 1.4 and 41.8 hrs after theinitial GW alerts. We covered 7.8, 3.0 and 5.1\% of the total probabilitywithin the GW event localizations, reaching 3$\sigma$ limits of 19.8, 21.3 and20.8 AB mag, respectively. Although no viable counterparts associated withthese events were found, we recovered 6 known transients and ruled out 5potential candidates. We also present Large Binocular Telescope spectroscopy ofPS19eq/SN2019ebq, a promising kilonova candidate that was later determined tobe a supernova. With the ability to tile large areas and conduct detailedfollowup, SAGUARO represents a significant addition to GW counterpartsearches.[Journal_ref: ]
 Ruiz, M., Tsokaros, A., Paschalidis, V., & Shapiro, S. (2019). Effects of spin on magnetized binary neutron star mergers and jet launching. PRD. doi:10.1103/PhysRevD.99.084032More infoEvents GW170817 and GRB 170817A provide the best confirmation so far that compact binary mergers where at least one of the companions is a neutron star (NS) can be the progenitors of short gammaray bursts (sGRBs). An open question for GW170817 remains the values and impact of the initial NS spins. The initial spins could possibly affect the remnant black hole (BH) mass and spin, the remnant disk and the formation and lifetime of a jet and its luminosity. Here we summarize our general relativistic magnetohydrodynamic simulations of spinning, NS binaries undergoing merger and delayed collapse to a BH. The binaries consist of two identical NSs, modeled as Γ=2 polytropes, in quasicircular orbit, each with spins χNS=−0.053,0,0.24, or 0.36. The stars are endowed initially with a dipolar magnetic field extending from the interior into the exterior, as in a radio pulsar. Following merger, the redistribution of angular momentum by magnetic braking and magnetic turbulent viscosity in the hypermassive neutron star (HMNS) remnant, along with the loss of angular momentum due to gravitational radiation, induce the formation of a massive, nearly uniformly rotating inner core surrounded by a magnetized Keplerian disklike envelope. The HMNS eventually collapses to a BH, with spin a/MBH≃0.78 independent of the initial spin of the NSs, surrounded by a magnetized accretion disk. The larger the initial NS spin the heavier the disk. After Δt∼3000−4000M∼45−60(MNS/1.625M⊙)ms following merger, a mildly relativistic jet is launched. The lifetime of the jet [Δt∼100−140(MNS/1.625M⊙)ms] and its outgoing Poynting luminosity [LEM∼1051.5±1erg/s] are consistent with typical sGRBs, as well as with the BlandfordZnajek mechanism for launching jets and their associated Poynting luminosities.
 Khan, A., Paschalidis, V., Ruiz, M., & Shapiro, S. (2018). Disks around merging binary black holes: From GW150914 to supermassive black holes. prd, 97(4), 044036.
 Nemmen, R., Menezes, R., & Paschalidis, V. (2018). Search for QPOs in Perseus with Fermi LAT. arXiv eprints.
 Paschalidis, V., & Ruiz, M. (2018). Are fast radio bursts the most likely electromagnetic counterpart of neutron star mergers resulting in prompt collapse?. arXiv eprints.
 Paschalidis, V., Yagi, K., AlvarezCastillo, D. .., Blaschke, D., & Sedrakian, A. (2018). Implications from GW170817 and ILoveQ relations for relativistic hybrid stars. prd, 97(8), 084038.
 Yang, H., East, W., Paschalidis, V., Pretorius, F., & Mendes, R. (2018). Evolution of highly eccentric binary neutron stars including tidal effects. prd, 98(4), 044007.
 Yang, H., Paschalidis, V., Yagi, K., Lehner, L., Pretorius, F., & Yunes, N. (2018). Gravitational wave spectroscopy of binary neutron star merger remnants with mode stacking. prd, 97(2), 024049.
 Paschalidis, V. (2017). General relativistic simulations of compact binary mergers as engines for short gammaray bursts. Classical and Quantum Gravity, 34(8), 084002.
 Paschalidis, V., & Stergioulas, N. (2017). Rotating stars in relativity. Living Reviews in Relativity, 20, 7.
 Sun, L., Paschalidis, V., Ruiz, M., & Shapiro, S. (2017). Magnetorotational collapse of supermassive stars: Black hole formation, gravitational waves, and jets. prd, 96(4), 043006.
 Tsokaros, A., Ruiz, M., Paschalidis, V., Shapiro, S., Baiotti, L., & Ury{=, o. K. (2017). Gravitational wave content and stability of uniformly, rotating, triaxial neutron stars in general relativity. prd, 95(12), 124057.
 Yang, H., Yagi, K., Blackman, J., Lehner, L., Paschalidis, V., Pretorius, F., & Yunes, N. (2017). Black Hole Spectroscopy with Coherent Mode Stacking. Physical Review Letters, 118(16), 161101.
Proceedings Publications
 Paschalidis, V., Nemmen, R., & de Menezes, R. (2020, 04/07). Search for QPOs in Perseus with Fermi LAT. In Perseus in Sicily: From Black Hole to Cluster Outskirts, Proceedings of the International Astronomical Union, Volume 342, pp. 167171.
 Yang, H., Paschalidis, V., Yagi, K., Lehner, L., Pretorius, F., & Yunes, N. (2018). Gravitational wave spectroscopy of binary neutron star merger remnants with mode stacking. In APS Meeting Abstracts.
 Paschalidis, V. (2017, may). The status of general relativistic simulations of compact binary mergers as engines of short gammaray bursts. In Journal of Physics Conference Series, 837.