Echoes of ECOs: gravitational-wave signatures of exotic compact objects and of quantum corrections at the horizon scale

Gravitational waves from binary coalescences provide one of the cleanest signatures of the nature of compact objects. It has been recently argued that the post-merger ringdown waveform of exotic ultracompact objects is initially identical to that of a black-hole, and that putative corrections at the horizon scale will appear as secondary pulses after the main burst of radiation. Here we extend this analysis in three important directions: (i) we show that this result applies to a large class of exotic compact objects with a photon sphere for generic orbits in the test-particle limit; (ii) we investigate the late-time ringdown in more detail, showing that it is universally characterized by a modulated and distorted train of "echoes" of the modes of vibration associated with the photon sphere; (iii) we study for the first time equal-mass, head-on collisions of two ultracompact boson stars and compare their gravitational-wave signal to that produced by a pair of black-holes. If the initial objects are compact enough as to mimic a binary black-hole collision up to the merger, the final object exceeds the maximum mass for boson stars and collapses to a black-hole. This suggests that - in some configurations - the coalescence of compact boson stars might be almost indistinguishable from that of black-holes. On the other hand, generic configurations display peculiar signatures that can be searched for in gravitational-wave data as smoking guns of exotic compact objects.Comment: 13 pages, RevTex4. v2: typo in equation 7 corrected, references added, to appear in PR

Large Eddy Simulations of Magnetized Mergers of Neutron Stars with Neutrinos

Neutron star mergers are very violent events involving extreme physical processes: dynamic, strong-field gravity, large magnetic field, very hot, dense matter, and the copious production of neutrinos. Accurate modeling of such a system and its associated multi-messenger signals, such as gravitational waves, short gamma ray burst, and kilonova, requires the inclusion of all these processes, and is increasingly important in light of advancements in multi-messenger astronomy generally, and in gravitational wave astronomy in particular (such as the development of third-generation detectors). Several general relativistic codes have been incorporating some of these elements with different levels of realism. Here, we extend our code MHDuet, which can perform large eddy simulations of magnetohydrodynamics to help capture the magnetic field amplification during the merger, and to allow for realistic equations of state and neutrino cooling via a leakage scheme. We perform several tests involving isolated and binary neutron stars demonstrating the accuracy of the code.Comment: 20 pages, 11 figures (typos corrected

Head-on collisions of boson stars

We study head-on collisions of boson stars in three dimensions. We consider evolutions of two boson stars which may differ in their phase or have opposite frequencies but are otherwise identical. Our studies show that these phase differences result in different late time behavior and gravitational wave output

Robustness of the Blandford-Znajek mechanism

The Blandford-Znajek mechanism has long been regarded as a key ingredient in models attempting to explain powerful jets in AGNs, quasars, blazzars etc. In such mechanism, energy is extracted from a rotating black hole and dissipated at a load at far distances. In the current work we examine the behaviour of the BZ mechanism with respect to different boundary conditions, revealing the mechanism robustness upon variation of these conditions. Consequently, this work closes a gap in our understanding of this important scenario.Comment: 7 pages, accepted in CQ

Simulating binary neutron stars: dynamics and gravitational waves

We model two mergers of orbiting binary neutron stars, the first forming a black hole and the second a differentially rotating neutron star. We extract gravitational waveforms in the wave zone. Comparisons to a post-Newtonian analysis allow us to compute the orbital kinematics, including trajectories and orbital eccentricities. We verify our code by evolving single stars and extracting radial perturbative modes, which compare very well to results from perturbation theory. The Einstein equations are solved in a first order reduction of the generalized harmonic formulation, and the fluid equations are solved using a modified convex essentially non-oscillatory method. All calculations are done in three spatial dimensions without symmetry assumptions. We use the \had computational infrastructure for distributed adaptive mesh refinement.Comment: 14 pages, 16 figures. Added one figure from previous version; corrected typo

Intense Electromagnetic Outbursts from Collapsing Hypermassive Neutron Stars

We study the gravitational collapse of a magnetized neutron star using a novel numerical approach able to capture both the dynamics of the star and the behavior of the surrounding plasma. In this approach, a fully general relativistic magnetohydrodynamics implementation models the collapse of the star and provides appropriate boundary conditions to a force-free model which describes the stellar exterior. We validate this strategy by comparing with known results for the rotating monopole and aligned rotator solutions and then apply it to study both rotating and non-rotating stellar collapse scenarios, and contrast the behavior with what is obtained when employing the electrovacuum approximation outside the star. The non-rotating electrovacuum collapse is shown to agree qualitatively with a Newtonian model of the electromagnetic field outside a collapsing star. We illustrate and discuss a fundamental difference between the force-free and electrovacuum solutions, involving the appearance of large zones of electric-dominated field in the vacuum case. This provides a clear demonstration of how dissipative singularities appear generically in the non-linear time-evolution of force-free fluids. In both the rotating and non-rotating cases, our simulations indicate that the collapse induces a strong electromagnetic transient. In the case of sub-millisecond rotation, the magnetic field experiences strong winding and the transient carries much more energy. This result has important implications for models of gamma-ray bursts.Comment: 28 pages, 20 figures (quality lowered to reduce sizes). Improved initial data and matching condition results in a lower, but still important, energy emission. Added appendix with a discussion on effects of transition laye

Critical Phenomena in Neutron Stars I: Linearly Unstable Nonrotating Models

We consider the evolution in full general relativity of a family of linearly unstable isolated spherical neutron stars under the effects of very small, perturbations as induced by the truncation error. Using a simple ideal-fluid equation of state we find that this system exhibits a type-I critical behaviour, thus confirming the conclusions reached by Liebling et al. [1] for rotating magnetized stars. Exploiting the relative simplicity of our system, we are able carry out a more in-depth study providing solid evidences of the criticality of this phenomenon and also to give a simple interpretation of the putative critical solution as a spherical solution with the unstable mode being the fundamental F-mode. Hence for any choice of the polytropic constant, the critical solution will distinguish the set of subcritical models migrating to the stable branch of the models of equilibrium from the set of subcritical models collapsing to a black hole. Finally, we study how the dynamics changes when the numerically perturbation is replaced by a finite-size, resolution independent velocity perturbation and show that in such cases a nearly-critical solution can be changed into either a sub or supercritical. The work reported here also lays the basis for the analysis carried in a companion paper, where the critical behaviour in the the head-on collision of two neutron stars is instead considered [2].Comment: 15 pages, 9 figure

Method to estimate ISCO and ring-down frequencies in binary systems and consequences for gravitational wave data analysis

Recent advances in the description of compact binary systems have produced gravitational waveforms that include inspiral, merger and ring-down phases. Comparing results from numerical simulations with those of post-Newtonian (PN), and related, expansions has provided motivation for employing PN waveforms in near merger epochs when searching for gravitational waves and has encouraged the development of analytic fits to full numerical waveforms. The models and simulations do not yet cover the full binary coalescence parameter space. For these yet un-simulated regions, data analysts can still conduct separate inspiral, merger and ring-down searches. Improved knowledge about the end of the inspiral phase, the beginning of the merger, and the ring-down frequencies could increase the efficiency of both coherent inspiral-merger-ring-down (IMR) searches and searches over each phase separately. Insight can be gained for all three cases through a recently presented theoretical calculation, which, corroborated by the numerical results, provides an implicit formula for the final spin of the merged black holes, accurate to within 10% over a large parameter space. Knowledge of the final spin allows one to predict the end of the inspiral phase and the quasinormal mode ring-down frequencies, and in turn provides information about the bandwidth and duration of the merger. In this work we will discuss a few of the implications of this calculation for data analysis.Comment: Added references to section 3 14 pages 5 figures. Submitted to Classical and Quantum Gravit