Narrow Results

Pulsar Wind Nebulae (PWNe) are bubbles of relativistic plasma that form when the relativistic pulsar wind is confined by the SNR or the ISM. They have a broad band spectrum extending from from Radio to γ-rays, via synchrotron and Inverse Compton emission, and their study can provide important clues on the otherwise unobservable properties of the cold pulsar wind. The richness of emission features, revealed by recent observations, has driven a renewed interest in the theoretical modeling of these objects. In recent years an MHD paradigm has emerged. which has proved successful in explaining the richness of emission features observed in the X-ray band, and that has contributed to settle many old issues. PWNe are perhaps the best systems where relativistic dynamics can be investigated with high accuracy, given their proximity and the fact that they are persistent sources, and understanding their behavior is an essential step for the field of high energy astrophysics in general. I present here the current status of MHD models: what are the key ingredients, their successes, and open questions.

Radio and optical images of the M 87 jet show bright filaments, twisted into an apparent double helix, extending from HST-1 to knot A. Proper motions within the jet suggest a decelerating jet flow passing through a slower, accelerating wave pattern. A mass and energy flux conserving model requires the jet to be an internally hot, but subrelativistic plasma, from HST-1 to knot A. Kelvin-Helmholtz (KH) elliptical mode generated twisted filaments require the cocoon to be cooler than the jet at HST-1 but nearly as hot as the jet at knot A. A pseudo-synchrotron image of our model jet shows both similarities and differences when compared to an image of the real jet.

It is shown that topological changes in space-time are necessary to make General Relativity compatible with the Newtonian limit and to solve the hierarchy of the fundamental interactions. We detail how topology and topological changes appear in General Relativity and how it leaves an observable footprint in space-time. In cosmology we show that such topological observable is the cosmic radiation produced by the acceleration of the universe. The cosmological constant is a very particular case which occurs when the expansion of the universe into the vacuum occurs only in the direction of the cosmic time flow.

Pulsar Wind Nebulae (PWNe) are ideal astrophysical laboratories where high energy relativistic phenomena can be investigated. They are close, well resolved in our observations, and the knowledge derived in their study has a strong impact in many other fields, from AGNs to GRBs. Yet there are still unresolved issues, that prevent us from a full clear understanding of these objects. The lucky combination of high resolution X-ray imaging and numerical codes to handle the outflow and dynamical properties of relativistic MHD, has opened a new avenue of investigation that has lead to interesting progresses in the last years. Despite all of this, we do not understand yet how particles are accelerated, and the functioning of the pulsar wind and pulsar magnetosphere, that power PWNe. I will review what is now commonly known as the MHD paradigm, and in particular I will focus on various approaches that have been and are currently used to model these systems. For each I will highlight its advantages, limitations, and degree of applicability.

Pulsar wind nebulae (PWN) provide a unique test-bed for the study of highly relativistic processes right at our astronomical doorstep. In this contribution we will show results from the first 3D RMHD simulations of PWN. Of key interest to our study is the long standing "sigma-problem" that challenges MHD models of Pulsars and their nebulae now for 3 decades. Earlier 2D MHD models were very successful in reproducing the morphology of the inner Crab nebula showing a jet, torus, concentric wisps and a variable knot. However, these models are limited to a purely toroidal field geometry which leads to an exaggerated compression of the termination shock and polar jet — in contrast to the observations. In three dimensions, the toroidal field structure is susceptible to current driven instabilities; hence kink instability and magnetic dissipation govern the dynamics of the nebula flow. This leads to a resolution of the sigma-problem once also the pulsar's obliqueness (striped wind) is taken into account. In addition, we present polarized synchrotron maps constructed from the 3D simulations, showing the wealth of morphological features reproduced in 2D is preserved in the 3D case.

We study the bulk acceleration in relativistic axisymmetric magnetized outflows, by solving the momentum equation along the flow, the so-called wind equation. The solutions for the bulk Lorentz factor depend on the geometry of the field/streamlines through the "bunching function" S. We investigate the general characteristics of the S function and how its choice affects the acceleration. In our study, various fast rise and slow decay examples are selected for S, with a global maximum near the fast magnetosonic critical point, as required from the regularity condition. For each case we determine the terminal Lorentz factor γ_∞ and the acceleration efficiency γ_∞/μ, where μ is the total energy-to-mass flux ratio (which equals the maximum possible Lorentz factor of the outflow). With proper choices of S we can achieve efficiencies greater than 50%. Last, we examine the shape of the field/streamlines with respect to the choice of the S function. The results of this work, depending on the choices of μ, can be applied to relativistic GRB or AGN jets.

Magnetic fields play a crucial role in many astrophysical scenarios and, in particular, are of paramount importance in the emission mechanism and evolution of Neutron Stars (NSs). To understand the role of the magnetic field in compact objects it is important to obtain, as a first step, accurate equilibrium models for magnetized NSs. Using the conformally flat approximation we solve the Einstein's equations together with the GRMHD equations in the case of a static axisymmetric NS taking into account different types of magnetic configuration. This allows us to investigate the effect of the magnetic field on global properties of NSs such as their deformation.

We present here a kinematic study of dynamo effects in thick accretion disks using the most recent version of the ECHO code. This allowed us to perform high-order simulations of a resistive plasma within the 3 + 1 GRMHD frame with a fully covariant mean-field dynamo closure for the Ohm's law. The magnetic field goes through an exponential growth within few orbital periods of the disk, and we derive butterfly diagrams qualitatively in the same fashion as in the Sun. Finally, a typical time-scale of the field evolution is found, that depends on the dynamo and resistivity parameters and that could be related to the periodicities observed in several accreting systems.

The attempt of this paper is to suggest a new theoretical medium to support the efforts of building particle models as mechanical field structures. The medium model is based on two simple fundamental assumptions. All the properties of the medium, as counterparts in this medium of the well-known physics laws, including energy and momentum conservations, Lorentz transformation and special relativity, electromagnetic interaction, etc., are derived from these two assumptions. The governing equations are established based on the derived properties. Some steady solutions as well as the feasibility of interpreting these solutions as particles are also briefly discussed.

Here, we present a new physical quantity with the aim of unifying the structure of physics. The basic assumption starts with a reduction of all the natural forces into a single force. As a first step, we combine Coulomb’s law and the law of gravity into a single law. Astonishingly, this new law introduces a force never before described, which takes the form of a mass–charge force. This new force may be responsible for holding together the atomic nucleus, counteracting the Coulomb repulsive force. As such, this nuclear force could also be integrated into the new force law. A reduction of the natural forces into a single force would also reduce the number of conservation laws. The discussion herein focuses only on whether gravitational and inertial masses are identical or have different properties: if velocity increases, inertial mass also increases, but gravitational mass appears to remain constant. Furthermore, this may make it possible to measure absolute $\gamma$ factors which are given by the quotient of the inertial and gravitational masses. If true, this would invalidate one of the postulates of the theory of special relativity, leaving the theory of relativity open to new interpretations. Introduction of the new quantity, the complex number consisting of charge and mass $(bq,m_0)$, would not, however, invalidate other well-known formulas of physics, for the following reasons: with respect to all equations relating to gravitational mass, the charges usually sum to zero, yielding $q=0$ and $(bq,m_0)\cong m$, leading to no inconsistencies; with respect to all equations relating to charge, the error would similarly be negligible, since charge changes through m only by adding the summand $m∕b$. Since the gravitational mass of an electron is very small, precise measurements are needed to verify this proposed complex relationship. The inertial mass would remain as has been established.

Accretion disks around stellar-mass black holes (BHs) are observed to emit radiation peaking in the soft X-rays (at about 1 keV) when the source is in the thermal state. This emission is expected to be polarized, because of electron scattering in the disk. However, photons will experience a rotation of the polarization plane due to General Relativity effects. X-ray polarization measurements can then be crucial in assessing strong gravity effects around accreting stellar-mass BHs, providing independent constraints on their spin. In this work, we present the simulated polarization spectra (obtained according to the model by Dovčiak et al.) for the X-ray, thermal radiation emitted by the accretion disk, as expected to be measured by IXPE, an X-ray polarimeter which will be launched in 2021. We also illustrate the code, currently under development, incorporate the contribution of returning radiation and account for absorption effects by the disk material.

The notion of observers’ and their measurements is closely tied to the Lorentzian metric geometry of spacetime, which in turn has its roots in the symmetries of Maxwell’s theory of electrodynamics. Modifying either the one, the other or both ingredients to our modern understanding of physics, requires also a reformulation of the observer model used. In this presentation we will consider a generalized theory of electrodynamics, so called local and linear premetric, or area metric, electrodynamics and its corresponding spacetime structure. On this basis we will describe an observer’s measurement of time and spatial length. A general algorithm how to determine observer measurements will be outlined and explicitly applied to a first order premetric perturbation of Maxwell electrodynamics. The latter contains for example the photon sector of the minimal standard model extension. Having understood an observer’s measurement of time and length we will derive the relativistic observables time dilation and length contraction. In the future a modern relativistic description of the classical test of special relativity shall be performed, including a consistent observer model.

As a matter of fact, modern physics is not taught in the majority of secondary schools; however, recent researches suggest that there are several advantages in introducing modern physics early in school. This is, for instance, the approach of the Einstein-First project. In the context of this approach, we report here some preliminary results of an intervention in the first year of Italian secondary school.

The AstroCamp is an academic excellence program in the field of astronomy and physics for students in the last 3 years of pre-university education, which often includes a course (or a significant part thereof) on Relativity. After an introduction to the principles, goals and structure of the camp, I describe the approach followed by camp lecturers (myself and others) for teaching Special and General Relativity, and some lessons learned and feedback from the students. I also provide some thoughts on the differences between the physics and mathematics secondary school curricula in Portugal and in other countries, and on how these curricula could be modernized.

We show that the soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) can be explained as recently proposed highly magnetized white dwarfs (B-WDs). The radius and magnetic field of B-WDs are perfectly adequate to explain energies in SGRs/AXPs as the rotationally powered energy. While the highly magnetized neutron stars require an extra, observationally not well established yet, source of energy, the magnetized white dwarfs, yet following Chandrasekhar’s theory (C-WDs), exhibit large ultra-violet luminosity which is observationally constrained from a strict upper limit.