Néstor Ortiz

Testing Black Hole Spacetimes with the S2 Star Orbit: A Bayesian Comparison

We use astrometric data of the S2 star orbit around the radio source Sgr A*, to perform a Bayesian comparison of several non-rotating black hole models.

Phase transition mechanism of spontaneous scalarization

We study the nature of spontaneous scalarization as a phase transition, either second-, first-order, or with a negative scalar susceptibility. Critical exponents are computed, and implications for dynamical scalarization are discussed.

Dynamical transition to spontaneous scalarization in neutron stars: the massive scalar field scenario

We simulate the spontaneous scalarization of neutron stars in a scalar-tensor theory of gravity with a non minimally coupled, massive scalar field. The mass term tends to suppress the range of the scalar field, and thus evade observational constraints on scalarized stars.

Gamma radiation from neutron stars in binary systems

Using numerical simulations, we predict the gamma-ray emission pattern from a black hole - neutron star binary, just a moment before merger. We find that face-on observers receive little emission, equatorial observers see one broad peak, and more generic observers typically see two peaks.

Nonlinear oscillations of neutron stars in scalar-tensor gravity

We analyze the spectrum of neutron star pulsations in scalar-tensor gravity, and we compare results with expectations from linear perturbation theory.

Nonlinear curvature effects in gravitational waves from black hole binaries

For black hole binaries in scalar-Gauss-Bonnet gravity, we compute the leading-order corrections due to curvature nonlinearities in scalar and tensor gravitational waveforms.

Crust meltdown in binary neutron stars

We show that, through resonant tidal excitations in a binary inspiral, neutron star crusts generically undergo elastic-to-plastic transition, which leads to crust heating and eventual meltdown.

New Class of Quasinormal Modes of Neutron Stars in Scalar-Tensor Gravity

We study radial oscillations of neutron stars in massless scalar-tensor gravity. We reveal the existence of new families of modes, with no counterpart in General Relativity, in principle sufficiently well resolved in frequency space to allow for clear detection.

Fixing extensions to General Relativity in the nonlinear regime

Inspired in the approach to 'fixing' viscous relativistic hydrodynamics by Israel and Stewart, we introduce a strategy to evade the ill posedness of certain Cauchy problems. We sketch the implementation of this idea in a couple of effective theories of gravity.

Shadow of a naked singularity

We analyze the redshift suffered by photons originating from an external source, traversing a collapsing dust cloud, and finally received by an asymptotic observer. We then study the shadow that the collapsing cloud casts on the sky of the asymptotic observer.

Linear perturbations of self-gravitating spherically symmetric configurations

We present a new covariant, gauge-invariant formalism describing linear metric perturbation fields on any spherically symmetric background in General Relativity