Vacuum Birefringence Research Articles

Resonant conversion of gravitational waves in neutron star magnetospheres

High-frequency gravitational waves are the subject of rapidly growing interest in the theoretical and experimental community. In this work we calculate the resonant conversion of gravitational waves into photons in the magnetospheres of neutron stars via the inverse Gertsenshtein mechanism. The resonance occurs in regions where the vacuum birefringence effects cancel the classical plasma contribution to the photon dispersion relation, leading to a massless photon in the medium which becomes kinematically matched to the graviton. We set limits on the amplitude of a possible stochastic background of gravitational waves using X-ray and IR flux measurements of neutron stars. Using Chandra (2–8 keV) and NuSTAR (3–79 keV) observations of RX J1856.6-3754, we set strain limits hclim≃10−26–10−24 in the frequency range 5×1017 Hz≲f≲2×1019 Hz. Our limits are many orders of magnitude stronger than existing constraints from individual neutron stars at the same frequencies. We also use recent JWST observations of the Magnetar 4U 0142+61 in the range 2.7×1013 Hz≲f≲5.9×1013 Hz, setting a limit hclim≃5×10−19. These constraints are in complementary frequency ranges to laboratory searches with CAST, OSQAR and ALPS II. We expect these limits to be improved both in reach and breadth with a more exhaustive use of telescope data across the full spectrum of frequencies and targets. Published by the American Physical Society 2024

IXPE Observations of Magnetar Sources

Among the more than 60 sources observed in the first two years of operations, IXPE addressed four magnetars, neutron stars believed to host ultra-strong magnetic fields. We report here the main implication coming from IXPE measurements for the physics of magnetars. Polarimetric observations confirmed the expectations of high polarization degrees, up to ≈80%, values which have not been detected in any other source so far, providing further proof (independent from the P-P˙ estimate) that magnetars host indeed ultra-magnetized neutron stars. Polarization measurements also indicate that softer X-rays likely come from surface regions where the overlying atmosphere underwent magnetic condensation. The agreement of the phase-dependent polarization angle with a simple rotating vector model strongly supports the presence of vacuum birefringence around the star.

Nonlinear electrodynamics and its possible connection to relativistic superconductivity: an example

Abstract This work presents an illustrative example suggesting a potential connection between the Heisenberg–Euler (HE) model of nonlinear electrodynamics (NLED) and relativistic superconductivity. Within a cylindrical coordinate system, we derive nonlinear Maxwell’s equations from the HE Lagrangian to the fourth order in the electromagnetic (EM) field. By considering static electric and magnetic fields with Gaussian radial profiles, we compute the corresponding HE current density. In parallel, we explore relativistic type-II superconductivity in analogy with the Ginzburg–Landau (GL) theory. We calculate the vortex-supercurrent density of the condensate and propose a link between this current and the HE current. This connection enables the derivation of the relativistic order parameter state. The problem considered in this article may contribute to the understanding of superconductivity in strong EM environments within the broader framework of NLED.

Light-light scattering in Very Special Relativity Quantum Electrodynamics and cosmic anisotropies

We consider scattering of light by light in Very Special Relativity (VSR) Quantum Electrodynamics (QED) with a non-zero photon mass. In order to preserve gauge invariance and Sim(2) symmetry we made use of a recently introduced infrared regularization of VSR theories. Additionally, we show that all one loop diagrams with any number of photon external legs and zero fermion legs reduce to the standard QED result with the effective electron mass, as required by unitarity. It follows that the Euler-Heisenberg Lagrangian is the same as in QED. The total cross section of scattering of light by light exhibits tiny anisotropies that could be detected at cosmological scales. In particular, they should be looked at the Cosmic Microwave Background (CMB) Radiation for low photon frequencies.

X-ray Polarimetry of X-ray Pulsars

Radiation from X-ray pulsars (XRPs) was expected to be strongly linearly polarized owing to a large difference in their ordinary and extraordinary mode opacities. The launch of IXPE allowed us to check this prediction. IXPE observed a dozen X-ray pulsars, discovering pulse-phase dependent variation of the polarization degree (PD) and polarization angle (PA). Although the PD showed rather erratic profiles resembling flux pulse dependence, the PA in most cases showed smooth variations consistent with the rotating vector model (RVM), which can be interpreted as a combined effect of vacuum birefringence and dipole magnetic field structure at a polarization-limiting (adiabatic) radius. Application of the RVM allowed us to determine XRP geometry and to confirm the free precession of the NS in Her X-1. Deviations from RVM in two bright transients led to the discovery of an unpulsed polarized emission likely produced by scattering off the accretion disk wind.

Fermionic signal of vacuum polarization in strong laser fields

Vacuum polarization (VP) is investigated for the interaction of a polarized γ-ray beam of GeV photons with a counterpropagating ultraintense laser pulse. In a conventional setup of a vacuum birefringence measurement, a VP signal is the emerging small circular (linear) polarization of the initially linearly (circularly) polarized probe photons. The pair production via the nonlinear Breit-Wheeler process in such a high-energy environment eliminates part of the γ photons in the outgoing γ beam, increasing the statistical error and decreasing the accuracy of this VP signal. In contrast, we investigate the conversion of the emerging circular polarization of γ photons into longitudinal polarization of the created positrons, considering the latter as the main VP signal. To study the VP effects in the highly nonlinear regime, where the Euler-Heisenberg effective Lagrangian method breaks down, we have developed a Monte Carlo simulation method, incorporating vacuum birefringence and dichroism via the one-loop QED probabilities in the locally constant field approximation. Our Monte Carlo method will enable the study of VP effects in strong fields of arbitrary configuration. With 10 PW laser systems, we demonstrate the feasibility of detecting the fermionic signal of the VP effect at the 5σ confidence level with a few hours of measurement time. Published by the American Physical Society 2024

Lagrangian reverse engineering for regular black holes

Nonlinear extensions of classical Maxwell's electromagnetism are among the prominent candidates for theories admitting regular black hole solutions. A quest for such examples has been fruitful, but mostly unsystematic and littered by the introduction of physically unrealistic Lagrangians. We provide a procedure which admits the reconstruction of a nonlinear electromagnetic Lagrangian, consistent with the Euler–Heisenberg Lagrangian in the weak-field limit, from a given metric representing a regular, magnetically charged black hole.

On the nonlinear electrodynamics in a Lorentz-breaking scenario

In this work, we study a model in nonlinear electrodynamics in the presence of a CPT-even term that violates Lorentz symmetry. The Lorentz-breaking vector, in addition to the usual background magnetic field, produces interesting effects in the dispersion relations. The consequences on the vacuum refractive index and the group velocity are studied. Vacuum birefringence is discussed in the case the nonlinear electrodynamics is a Euler–Heisenberg model.

Gauge field fluctuation corrected QED3 effective action by fermionic particle-vortex duality

We present a non-perturbative framework for incorporating gauge field fluctuations into effective actions of QED3 in the infrared using fermionic particle-vortex duality. This approach is demonstrated through the applications to models containing N species of 2-component Dirac fermions in solvable and interpretable electromagnetic backgrounds, focusing on N = 1 or 2. For the N = 1 model, we establish a correspondence between fermion Casimir energy at finite density and the magnetic Euler-Heisenberg Lagrangian, and further evaluate the corrections to their amplitudes. This predicts amplification of charge susceptibility and reduction of magnetic permeability. We additionally provide physical interpretations for each component of our calculation and offer alternative derivations based on energy density measurements in different characteristic lengths. For N = 2, we show that magnetic catalysis is erased in a U(1) × U(1) QED3, indicating no breakdown of chiral symmetry. Reasoning is offered based on the properties of the lowest Landau level wave functions.

Searching for axion resonances in vacuum birefringence with three-beam collisions

We consider birefringent (i.e., polarization changing) scattering of x-ray photons at the superposition of two optical laser beams of ultrahigh intensity and study the resonant contributions of axions or axionlike particles, which could also be short lived. Applying the specifications of the Helmholtz International Beamline for Extreme Fields (HIBEF), we find that this setup can be more sensitive than previous light-by-light scattering (birefringence) or light-shining-through-wall experiments in a certain domain of parameter space. By changing the pump and probe laser orientations and frequencies, one can even scan different axion masses, i.e., chart the axion propagator. Published by the American Physical Society 2024

Signatures of vacuum birefringence in low-power flying focus pulses

Vacuum birefringence produces a differential phase between orthogonally polarized components of a weak electromagnetic probe in the presence of a strong electromagnetic field. Despite representing a hallmark prediction of quantum electrodynamics, vacuum birefringence remains untested in pure light configurations due to the extremely large electromagnetic fields required for a detectable phase difference. Here, we exploit the programmable focal velocity and extended focal range of a flying focus laser pulse to substantially lower the laser power required for detection of vacuum birefringence. In the proposed scheme, a linearly polarized x-ray probe pulse counterpropagates with respect to a flying focus pulse, whose focus moves at the speed of light in the same direction as the x-ray probe. The peak intensity of the flying focus pulse overlaps the probe over millimeter-scale distances and induces a polarization ellipticity on the order of 10−10, which lies within the detection sensitivity of existing x-ray polarimeters. Published by the American Physical Society 2024

X-ray Polarization from Magnetar Sources

The launch of the IXPE telescope in late 2021 finally made polarization measurements in the 2–8keV band a reality, more than 40 years after the pioneering observations of the OSO-8 satellite. In the first two years of operations, IXPE targeted more than 60 sources, including four magnetars, neutron stars with magnetic fields in the petaGauss range. In this paper we summarize the IXPE main findings and discuss their implications for the physics of ultra-magnetized neutron stars. Polarimetric observations confirmed theoretical predictions, according to which X-ray radiation from magnetar sources is highly polarized, up to ≈80%, the highest value detected so far. This provides an independent confirmation that magnetars are indeed endowed with a super-strong magnetic field and that the twisted magnetosphere scenario is the most likely explanation for their soft X-ray emission. Polarization measurements allowed us to probe the physical conditions of the star’s outermost layers, showing that the cooler surface regions are in a condensed state, with no atmosphere on top. Although no smoking-gun of vacuum QED effects was found, the phase-dependent behavior of the polarization angle strongly hints that vacuum birefringence is indeed at work in magnetar magnetospheres.

On the absence of shock waves and vacuum birefringence in Born–Infeld electrodynamics

We study the interaction of two counter–propagating electromagnetic waves in vacuum in the Born–Infeld electrodynamics. First we investigate the Born case for linearly polarized beams, E · B = 0, i.e., G2=0 (crossed field configuration), which is identical for Born–Infeld and Born electrodynamics; subsequently we study the general Born–Infeld case for beams which are nonlinearly polarized, G2≠0. In both cases, we show that the nonlinear field equations decouple using self-similar solutions and investigate the shock wave formation. We show that the only nonlinear solutions are exceptional traveling wave solutions which propagate with constant speed and which do not turn into shocks for our approximation. We obtain two types of exceptional wave solutions, then we numerically analyze which phase velocities correspond to the counter- or co-propagating beams and subsequently we determine the direction of propagation of the exceptional waves.

Minimal length corrections to magnetic birefringence in vacuum

Minimal length corrections to magnetic birefringence in vacuum

Detection schemes for quantum vacuum diffraction and birefringence

Detection schemes for quantum vacuum diffraction and birefringence

Strong field vacuum birefringence in plane wave pulses

By combining an adiabatic approach based on a ‘locally monochromatic’ approximation with a local Hilbert transform, it is demonstrated how vacuum birefringence in the strong field regime can be calculated using a rate approach suitable for Monte Carlo simulation codes. Results for the flipping of the photon’s polarisation (helicity) are benchmarked with evaluation of exact expressions in a circularly (linearly) polarised plane wave of finite extent. For the circularly polarised case, the Heisenberg–Euler approach predicts a null result; an approximation similar to the ‘locally constant’ form is presented, which recovers the correct low-energy scaling. Example probabilities are given for typical experimental parameters.

A Dirac-material-inspired non-linear electrodynamic model

We propose and study the properties of a non-linear electrodynamics (ED) that emerges inspired on the physics of Dirac materials. This new electrodynamic model is an extension of the one-loop corrected non-linear effective Lagrangian computed in the work of Keser et al (2022 Phys. Rev. Lett. 128 066402). In the particular regime of a strong magnetic and a weak electric field, it reduces to the photonic non-linear model worked out by Keser et al (2022 Phys. Rev. Lett. 128 066402). We pursue our investigation of the proposed model by analyzing properties of the permittivity and permeability tensors, the energy–momentum tensor and wave propagation effects in presence of a uniform magnetic background. It is shown that the ED here presented exhibits the vacuum birefringence phenomenon. Subsequently, we calculate the lowest-order modifications to the interaction energy, considering still the presence of a uniform external magnetic field. Our analysis is carried out within the framework of the gauge-invariant but path-dependent variables formalism. The calculation reveals a screened Coulomb-like potential with an effective electric charge that runs with the external magnetic field but, as expected for Dirac-type materials, the screening disappears whenever the external magnetic field is switched off.

Simulations of spin/polarization-resolved laser–plasma interactions in the nonlinear QED regime

Strong-field quantum electrodynamics (SF-QED) plays a crucial role in ultraintense laser–matter interactions and demands sophisticated techniques to understand the related physics with new degrees of freedom, including spin angular momentum. To investigate the impact of SF-QED processes, we have introduced spin/polarization-resolved nonlinear Compton scattering, nonlinear Breit–Wheeler, and vacuum birefringence processes into our particle-in-cell (PIC) code. In this article, we provide details of the implementation of these SF-QED modules and share known results that demonstrate exact agreement with existing single-particle codes. By coupling normal PIC simulations with spin/polarization-resolved SF-QED processes, we create a new theoretical platform to study strong-field physics in currently running or planned petawatt or multi-petawatt laser facilities.

Remarks on propagating waves in non-linear vacuum electrodynamics

Using the quadratic expansion in the photon fields of Euler–Heisenberg (EH) non-linear electrodynamics (NLED) Lagrangian model we study relevant vacuum properties in a scenario involving the propagation of a photon probe in the presence of a background constant and static magnetic field, mathbf{B_e}. We compute the gauge invariant, symmetric and conserved energy–momentum tensor (EMT) and angular momentum tensor (AMT) for arbitrary magnetic field strength using the Hilbert method under the soft-photon approximation. We discuss how the presence of magneto-electric terms in the EH Lagrangian is a source of anisotropy, induce the non-zero trace in the EMT and leads to differences between EMT calculated by the Hilbert or Noether method. From the Hilbert EMT we analyze some quantities of interest such as the energy density, pressures, Poynting vector, and angular momentum vector, comparing and discussing the differences with respect to the improved Noether method. The magnetized vacuum properties are also studied showing that a photon effective magnetic moment can be defined for different polarization modes. The calculations are done in terms of derivatives of the two scalar invariants of electrodynamics, hence, extension to other NLED Lagrangian is straightforward. We discuss further physical implications and experimental strategies to test magnetization, photon pressure, and effective magnetic moment.

Enhancement of vacuum birefringence with pump laser of flying focus

Enhancement of vacuum birefringence with pump laser of flying focus