arXiv subjects

João S. Santos

Publications and source records attributed to João S. Santos.

8 recordsLinked to original sources

arXiv2026

Strong-Field Gravitational-Wave Lensing from Ray-Traced Arrival-Time Surfaces

The diffraction integral formalism has been greatly successful in describing gravitational wave lensing effects in scenarios where the wavelength of the signal is comparable to the characteristic length scale of the lens, known as the wave optics regime. However, the assumptions required to construct its integrand break down as the source approaches the strong gravity region of a black hole lens. In particular, we are interested in the lensed signal from a binary (the source) near a massive black hole (the lens). We present a new formalism for obtaining low frequency corrections to sources being lensed in the strong field regime of gravity by leveraging the fact that null geodesics obey the Hamilton--Jacobi equations. In doing so, we are able to construct a time delay surface without relying on the thin lens approximation, and solve the diffraction integral for the resulting surface directly. For a stationary source at $10M$, deep in the strong field regime, when comparing the results with a relativistic wave scattering model, we find $6.5\%$ and $0.3\%$ agreement in the peak amplification and first diffractive fringe, respectively. Furthermore, we find a $< 1\%$ agreement with the amplitude of the point mass lens with a source at $10^4 M$ in the weak field, but noticeable dephasing between the peaks of the amplification function at higher frequencies. This work allows for clean interpolation between otherwise separate regimes, valid for sources from $\sim 6M \to \infty$. It provides a transparent, physically motivated framework for computing beyond-geometric optics lensing effects in the strong field regime.

gr-qc↗

arXiv2026

Wave optical imaging of an oscillating electric dipole orbiting a black hole

We study the electromagnetic radiation and wave-optical imaging of an oscillating electric dipole orbiting a Kerr black hole. We derive the effective 4-current associated with a pointlike oscillating electric dipole in curved spacetime, and use black hole perturbation theory to compute the resulting radiation field at future null infinity, from first principles. We then develop a wave-optical imaging framework for a moving electromagnetic source in curved spacetime. We obtain images of an orbiting electric dipole, displaying relativistic beaming, gravitational lensing, and Einstein rings. We study polarization-dependent scattering by comparing the images produced by spinning dipoles with opposite helicities, finding a displacement that roughly decreases with the inverse of the radiation frequency, as expected for a beyond-geometric-optics effect. Our results provide a first-principles benchmark for beyond-geometric-optics descriptions of electromagnetic radiation in Kerr spacetime.

gr-qc↗

arXiv2026

Resonances in binary extreme mass ratio inspirals

Stellar-mass binaries evolving in the vicinity of supermassive black holes (SMBHs) may be common in the universe, either in active galactic nuclei or in other astrophysical environments. Here, we study in detail the resonant excitation of SMBH modes driven by a nearby stellar-mass binary. The resulting resonant energy fluxes vary with the orbital location and frequency of the binary, exhibiting a rich and complex structure. In particular, we find that the total energy flux radiated to infinity is maximized at a gravitational-wave frequency that is close to, but not exactly equal to, the real part of the corresponding quasinormal-mode frequency. Moreover, as the binary is moved farther away from the SMBH, this offset from the mode frequency becomes increasingly pronounced. In addition, for suitable orientations, the binary can effectively ``feed'' the light ring of the SMBH, selectively exciting particular oscillation modes. For rotating (Kerr) black holes, the mode spectrum is significantly more intricate; however, individual modes are also less strongly damped, leading to an enhanced -- but more difficult to interpret -- resonant response.

gr-qc↗

arXiv2025

Electromagnetic radiation reaction and energy extraction from black holes: The tail term cannot be ignored

We study electromagnetic radiation reaction in curved space and the dynamics of radiating charged particles. The equation of motion for such particles is the DeWitt-Brehme equation, and it contains a particularly complicated, non-local, tail term. It has been claimed that the tail term can be neglected in certain magnetized black hole spacetimes, and that radiation reaction may then lead to energy extraction ("orbital widening") in the absence of an ergoregion. We show that such claims are incorrect, at least in the Newtonian limit: the tail term can never be neglected consistently in the relevant scenarios, and when it is included the reported energy extraction no longer occurs. Thus, previous results are called into question by our work.

gr-qc↗

arXiv2025

Gravitational waves from b-EMRIs: Doppler shift and beaming, resonant excitation, helicity oscillations and self-lensing

We study gravitational waves from a stellar-mass binary orbiting a spinning supermassive black hole, a system referred to as a binary extreme mass ratio inspiral (b-EMRI). We use Dixon's formalism to describe the stellar-mass binary as a particle with internal structure, and keep terms up to quadrupole order to capture the generation of gravitational waves by the inner motion of the stellar-mass binary. The problem of emission and propagation of waves is treated from first principles using black hole perturbation theory. In the gravitational waveform at future null infinity, we identify for the first time Doppler shifts and beaming due to the motion of the center of mass, as well as helicity breaking gravitational lensing, and resonances with ringdown modes of the supermassive black hole. We establish that previously proposed phenomenological models inadequately capture these effects.

gr-qc↗

arXiv2024

Floating orbits and energy extraction from magnetized Kerr black holes

We study electromagnetic radiation reaction on a charged particle around a weakly magnetized Kerr black hole. We solve numerically the Teukolsky equation to find energy fluxes in electromagnetic radiation at the horizon and at spatial infinity. We also employ analytical methods in the low-frequency limit, finding excellent agreement with the numerical results. For a wide range of parameters, energy fluxes on the horizon are negative for all orbital radii: the modes are amplified through superradiance. More interestingly, the flux on the horizon is larger (in absolute value) than the flux at infinity for orbits with orbital radius $r_0 \gtrsim 9M$ -- floating orbits are a generic outcome of having magnetic fields around black holes. Particles on these orbits extract energy from the BH through their radiation field; the net effect is an increase in the particles' kinetic energy, so that they outspiral to infinity. Although in realistic accretion disks viscous forces are expected to dominate, floating orbits remain an interesting feature of the existence of ergoregions in black hole spacetimes.

gr-qc↗

arXiv2024

Radiation reaction in weakly magnetized black holes: can the tail term be ignored in the strong field regime?

We study radiation from charged particles in circular motion around a Schwarzschild black hole immersed in an asymptotically uniform magnetic field. In curved space, the radiation reaction force is described by the DeWitt-Brehme equation, which includes a complicated, non-local tail term. We show that, contrary to some claims in the literature, this term cannot, in general, be neglected. We account for self-force effects directly by calculating the electromagnetic energy flux at infinity and on the horizon. The radiative field is obtained using black hole perturbation theory. We solve the relevant equations analytically, in the low-frequency and slow-motion approximation, as well as numerically in the general case. Our results show that great care must be taken when neglecting the tail term, which is often fundamental to capture the dynamics of the particle: in fact, it only seems to be negligible when the magnetic force greatly dominates the gravitational force, so that the motion is well described by the Abraham--Lorentz--Dirac equation. We also report a curious "horizon dominance effect" that occurs for a radiating particle in a circular orbit around a black hole (emitting either scalar, electromagnetic or gravitational waves): for fixed orbital radius, the fraction of energy that is absorbed by the black hole can be made arbitrarily large by decreasing the particle velocity.

gr-qc↗

arXiv2021

Hydrodynamic electrons in Graphene: a viscous boundary-layer description

In this paper we dwell over the study of the boundary layer problem in a hydrodynamical description of the electrons in gated graphene. It has been verified experimentally that this fluid can display non-Poiseuille like flow as reproduced in our numerical simulation. In fact, the velocity profile displays a maximum value close to the boundary and then decreases as it approaches the bulk of the graphene layer. This work aims to present a satisfactory theoretical description of the boundary layer problem in graphene. We found that by using the fluid equations and following a method similar to that for deriving Blasius' equation, a non-linear model can be obtained whose solutions display the maximum values of velocity near the edges of the graphene layer. We argue that such a non-monotonic model and behaviour can shed some light on the subject of non-topological edge currents in graphene.

cond-mat.mes-hall↗