Galaxies, Volume 6, Issue 1 (March 2018) – 38 articles

Astronomical X-ray polarimetry was first explored in the end of the 1960s by pioneering rocket instruments. The craze arising from the first discoveries of stellar and supernova remnant X-ray polarization led to the addition of X-ray polarimeters to early satellites. Unfortunately, the inadequacy of the diffraction and scattering technologies required to measure polarization with respect to the constraints driven by X-ray mirrors and detectors, coupled with long integration times, slowed down the field for almost 40 years. Thanks to the development of new, highly sensitive, compact X-ray polarimeters in the beginning of the 2000s, observing astronomical X-ray polarization has become feasible, and scientists are now ready to explore our high-energy sky thanks to modern X-ray polarimeters. In the forthcoming years, several X-ray missions (rockets, balloons, and satellites) will create new observational opportunities. Interest in astronomical X-ray polarimetry field has thus been renewed, and this paper presents for the first time a quantitative assessment, all based on scientific literature, of the growth of this interest. Full article

The study of multi-wavelength flux variability in BL Lacertae objects is very important to discern unstable processes and emission mechanisms underlying their extreme observational features. While the innermost regions of these objects are not accessible from direct observations, we may draw conclusions about their internal structure via the detection of flux variations on various timescales, based on the light-travel argument. In this paper, we review the sub-hour X-ray variability in high-energy peaked BL Lacertae sources (HBLs) that are bright at X-rays and provide us with an effective tool to study the details related to the physics of the emitting particles. The X-ray emission of these sources is widely accepted to be a synchrotron radiation from the highest-energy electrons, and the complex spectral variability observed in this band reflects the injection and radiative evolution of freshly-accelerated particles. The detection of sub-hour X-ray flux variability is very important since it can be related to the small-scale jet turbulent structures or triggered by unstable processes occurring in the vicinity of a central supermassive black hole. We summarize the fastest X-ray variability instances detected in bright HBLs and discuss their physical implications. Full article

Isolated neutron stars are known to be endowed with extreme magnetic fields, whose maximum intensity ranges from ${10}^{12}$ – ${10}^{15}$ G, which permeates their magnetospheres. Their surrounding environment is also strongly magnetized, especially in the compact nebulae powered by the relativistic wind from young neutron stars. The radiation from isolated neutron stars and their surrounding nebulae is, thus, supposed to bring a strong polarization signature. Measuring the neutron star polarization brings important information about the properties of their magnetosphere and of their highly magnetized environment. Being the most numerous class of isolated neutron stars, polarization measurements have been traditionally carried out for radio pulsars, hence in the radio band. In this review, I summarize multi-wavelength linear polarization measurements obtained at wavelengths other than radio both for pulsars and other types of isolated neutron stars and outline future perspectives with the upcoming observing facilities. Full article

Bursts and flares are among the distinctive observational manifestations of magnetars, isolated neutron stars endowed with an ultra-strong magnetic field ( $B\approx {10}^{14}$ – ${10}^{15}$ G). It is believed that these events arise in a hot electron-positron plasma, injected in the magnetosphere, due to a magnetic field instability, which remains trapped within the closed magnetic field lines (the “trapped-fireball” model). We have developed a simple radiative transfer model to simulate magnetar flare emission in the case of a steady trapped fireball. After dividing the fireball surface in a number of plane-parallel slabs, the local spectral and polarization properties are obtained integrating the radiative transfer equations for the two normal modes. We assume that magnetic Thomson scattering is the dominant source of opacity, and neglect contributions from second-order radiative processes, although the presence of double-Compton scattering is accounted for in establishing local thermal equilibrium in the fireball atmospheric layers. The spectra we obtained in the 1–100 keV energy range are in broad agreement with those of available observations. The large degree of polarization (≳80%) predicted by our model should be easily detectable by new-generation X-ray polarimeters, like IXPE, XIPE and eXTP, allowing one to confirm the model predictions. Full article

Multiwavelength blazar variability is produced by noise-like processes with the power-law form of power spectral density (PSD). We present the results of our detailed investigation of multiwavelength ( $\gamma$ -ray and optical) light curves covering decades to minutes timescales, of two BL Lac objects namely, PKS 0735+178 and OJ 287. The PSDs are derived using discrete Fourier transform (DFT) method. Our systematic approach reveals that OJ 287 is, on average, more variable than PKS 0735+178 at both optical and $\gamma$ -ray energies on the corresponding time scales. On timescales shorter than ∼10 days, due to continuous and dense monitoring by the Kepler satellite, a steepening of power spectrum is observed for OJ 287. This indicates the necessity of an intermittent process generating variability on intra-night timescales for OJ 287. Full article

We review the history of astronomical X-ray polarimetry based on the author’s perspective, beginning with early sounding-rocket experiments by Robert Novick at Columbia University and his team, of which the author was a member. After describing various early techniques for measuring X-ray polarization, we discuss the polarimeter aboard the Orbiting Solar Observatory 8 (OSO-8) and its scientific results. Next, we describe the X-ray polarimeter to have flown aboard the ill-fated original Spectrum-X mission, which provided important lessons on polarimeter design, systematic effects, and the programmatics of a shared focal plane. We conclude with a description of the Imaging X-ray Polarimetry Explorer (IXPE) and its prospective scientific return. IXPE, a partnership between NASA and ASI, has been selected as a NASA Astrophysics Small Explorers Mission and is currently scheduled to launch in April of 2021. Full article

Microquasars are Galactic black hole systems in which matter is transferred from a donor star and accretes onto a black hole of, typically, 10–20 solar masses. The presence of an accretion disk and a relativistic jet made them a scaled down analogue of quasars—thence their name. Microquasars feature prominently in the scientific goals of X-ray polarimeters, because a number of open questions, which are discussed in this paper, can potentially be answered: the geometry of the hot corona believed to be responsible for the hard X-ray emission; the role of the jet; the spin of the black hole. Full article

This work summarizes a program intended to unify three burgeoning branches of the high-energy astrophysics of relativistic jets: general relativistic magnetohydrodynamic (GRMHD) simulations of ever-increasing dynamical range, the microphysical theory of particle acceleration under relativistic conditions, and multiwavelength observations resolving ever-decreasing spatiotemporal scales. The process, which involves converting simulation output into time series of images and polarization maps that can be directly compared to observations, is performed by (1) self-consistently prescribing models for emission, absorption, and particle acceleration and (2) performing time-dependent polarized radiative transfer. M87 serves as an exemplary prototype for this investigation due to its prominent and well-studied jet and the imminent prospect of learning much more from Event Horizon Telescope (EHT) observations this year. Synthetic observations can be directly compared with real observations for observational signatures such as jet instabilities, collimation, relativistic beaming, and polarization. The simplest models described adopt the standard equipartition hypothesis; other models calculate emission by relating it to current density or shear. These models are intended for application to the radio jet instead of the higher frequency emission, the disk and the wind, which will be subjects of future investigations. Full article

The PoGO mission, including the PoGOLite Pathfinder and PoGO+, aims to provide polarimetric measurements of the Crab system and Cygnus X-1 in the hard X-ray band. Measurements are conducted from a stabilized balloon-borne platform, launched on a 1 million cubic meter balloon from the Esrange Space Center in Sweden to an altitude of approximately 40 km. Several flights have been conducted, resulting in two independent measurements of the Crab polarization and one of Cygnus X-1. Here, a review of the PoGO mission is presented, including a description of the payload and the flight campaigns, and a discussion of some of the scientific results obtained to date. Full article

The Fermi bubbles, two giant structures above and below the Galactic center (GC), are among the most important discoveries of the Fermi Gamma-ray Space Telescope. Studying their physical origin has been providing valuable insights into cosmic-ray transport, the Galactic magnetic field, and past activity at the GC in the Milky Way galaxy. Despite their importance, the formation mechanism of the bubbles is still elusive. Over the past few years, there have been numerous efforts, both observational and theoretical, to uncover the nature of the bubbles. In this article, we present an overview of the current status of our understanding of the bubbles’ origin, and discuss possible future directions that will help to distinguish different scenarios of bubble formation. Full article

Combined cosmological, astrophysical and numerical tests may shed some light on the viability of theories of gravity beyond Einsteinian relativity. In this letter, we present two different techniques providing complementary ways of testing new physics beyond the $\Lambda$ CDM cosmological paradigm. First, we shall present some of the latest progress and shortcomings in the cosmographic model-independent approach for several modified gravity theories using supernovae catalogues, baryonic acoustic oscillation data and $H\left(z\right)$ differential age compilations. Second, we shall show how once the Einsteinian paradigm is abandoned, the phenomenology of neutron stars changes dramatically since neutron-star masses can be much larger than their General Relativity counterparts. Consequently, the total energy available for radiating gravitational waves could be of the order of several solar masses, and thus a merger of these stars constitutes a privileged wave source. Unfortunately at the present time our persisting lack of understanding in the strong interaction sector does not allow to distinguish the alternative theories from the usual General Relativity predictions. Full article

The Fermi bubbles were possibly created by large injections of energy into the Galactic Center (GC), either by an active galactic nucleus (AGN) or by nuclear starburst more than ~10 Myr ago. However, the origin of the diffuse gamma-ray emission associated with Loop I, a radio continuum loop spanning across 100° on the sky, is still being debated. The northern-most part of Loop I, known as the North Polar Spur (NPS), is the brightest arm and is even clearly visible in the ROSAT X-ray sky map. In this paper, we present a comprehensive review on the X-ray observations of the Fermi bubbles and their possible association with the NPS and Loop I structures. Using uniform analysis of archival Suzaku and Swift data, we show that X-ray plasma with kT~0.3 keV and low metal abundance (Z~0.2 Z_◉) is ubiquitous in both the bubbles and Loop I and is naturally interpreted as weakly shock-heated Galactic halo gas. However, the observed asymmetry of the X-ray-emitting gas above and below the GC has still not been resolved; it cannot be fully explained by the inclination of the axis of the Fermi bubbles to the Galactic disk normal. We argue that the NPS and Loop I may be asymmetric remnants of a large explosion that occurred before the event that created the Fermi bubbles, and that the soft gamma-ray emission from Loop I may be due to either π⁰ decay of accelerated protons or electron bremsstrahlung. Full article

Deep-sea archives all over the world show an enhanced concentration of the radionuclide ⁶⁰Fe, isolated in layers dating from about 2.2 Myr ago. Since this comparatively long-lived isotope is not naturally produced on Earth, such an enhancement can only be attributed to extraterrestrial sources, particularly one or several nearby supernovae in the recent past. It has been speculated that these supernovae might have been involved in the formation of the Local Superbubble, our Galactic habitat. Here, we summarize our efforts in giving a quantitative evidence for this scenario. Besides analytical calculations, we present results from high-resolution hydrodynamical simulations of the Local Superbubble and its presumptive neighbor Loop I in different environments, including a self-consistently evolved supernova-driven interstellar medium. For the superbubble modeling, the time sequence and locations of the generating core-collapse supernova explosions are taken into account, which are derived from the mass spectrum of the perished members of certain, carefully preselected stellar moving groups. The release and turbulent mixing of ⁶⁰Fe is followed via passive scalars, where the yields of the decaying radioisotope were adjusted according to recent stellar evolution calculations. The models are able to reproduce both the timing and the intensity of the ⁶⁰Fe excess observed with rather high precision. We close with a discussion of recent developments and give future perspectives. Full article

The first part of this paper contains a brief description of the beginnings of modern cosmology, which, the author will argue, was most likely born in the year 1912. Some of the pieces of evidence presented here have emerged from recent research in the history of science and are not usually shared with the general audiences in popular science books. In particular, the issue of the correct formulation of the original Big Bang concept, according to the precise words of Fred Hoyle, is discussed. Too often, this point is very deficiently explained (when not just misleadingly) in most of the available generalist literature. Other frequent uses of the same words, Big Bang, as to name the initial singularity of the cosmos, and also whole cosmological models, are then addressed, as evolutions of its original meaning. Quantum and inflationary additions to the celebrated singularity theorems by Penrose, Geroch, Hawking and others led to subsequent results by Borde, Guth and Vilenkin. Additionally, corresponding corrections to the Einstein field equations have originated, in particular, $R^2$ , $f\left(R\right)$ , and scalar-tensor gravities, giving rise to a plethora of new singularities. For completeness, an updated table with a classification of the same is given. Full article

In arXiv:1601.02203 and arXiv:1702.07063, we have proposed a topological model with a simple Lagrangian density and have tried to solve one of the cosmological constant problems. The Lagrangian density is the BRS exact and therefore the model can be regarded as a topological theory. In this model, the divergence of the vacuum energy coming from the quantum corrections from matters can be absorbed into the redefinition of the scalar field. In this paper, we consider the extension of the model in order to apply the mechanism to other kinds of divergences coming from the quantum correction and consider the cosmology in an extended model. Full article

We analyze the unitarity properties of higher derivative quantum field theories which are free of ghosts and ultraviolet singularities. We point out that in spite of the absence of ghosts most of these theories are not unitary. This result confirms the difficulties of finding a consistent quantum field theory of quantum gravity. Full article

We demonstrate that the existence of a Noether symmetry in $f\left(R\right)$ theories of gravity gives rise to an additional gravitational radius, besides the standard Schwarzschild one, determining the dynamics at galactic scales. By this feature, it is possible to explain the baryonic Tully-Fisher relation and the rotation curve of gas-rich galaxies without the dark matter hypothesis. Furthermore, under the same standard, the Fundamental Plane of elliptical galaxies can be addressed. Full article

We address the quantisation of a model that induces the Little Sibling of the Big Rip (LSBR) abrupt event, where the dark energy content is described by means of a phantom-like fluid or a phantom scalar field. The quantisation is done in the framework of the Wheeler–DeWitt (WDW) equation and imposing the DeWitt boundary condition; i.e., the wave function vanishes close to the abrupt event. We analyse the WDW equation within two descriptions: First, when the dark energy content is described with a perfect fluid. This leaves the problem with the scale factor as the single degree of freedom. Second, when the dark energy content is described with a phantom scalar field in such a way that an additional degree of freedom is incorporated. Here, we have applied the Born–Oppenheimer (BO) approximation in order to simplify the WDW equation. In all cases, the obtained wave function vanishes when the LSBR takes place, thus fulfilling the DeWitt boundary condition. Full article

We report on the optical observations and analysis of the high-energy peaked BL Lac object (HBL), Mrk 501, at redshift z = 0.033. We can confirm microvariable behavior over the course of minutes on several occasions per night. As an alternative to the commonly understood dynamical model of random variations in intensity of the AGN, we develop a relativistic beaming model with a minimum of free parameters, which allows us to infer changes in the line of sight angles for the motion of the different relativistic components. We hope our methods can be used in future studies of beamed emission in other active microvariable sources, similar to the one we explored. Full article

Within the framework of the third quantization, we consider the possibility that an initially recollapsing baby universe can enter a stage of near de Sitter inflation by tunnelling through a Euclidean wormhole that connects the recollapsing and inflationary geometries. We present the solutions for the evolution of the scale factor in the Lorentzian and Euclidean regions as well as the probability that the baby universe indeed crosses the wormhole when it reaches its maximum size. Full article

The blazar PKS 1510-089 has shown intense activity at $\gamma$ -rays in the recent years. In this work, we discussed the results of our 7 mm radio continuum and optical polarimetric monitoring between 2009 and 2013. In 2009, we detected a large rotation of the optical polarization angle that we attributed to the ejection of new polarized components. In 2011, after the occurrence of several $\gamma$ -rays flares, the radio emission started to increase, reaching values never observed before. We interpreted this increase as the consequence of the superposition of several new components ejected during the $\gamma$ -rays flares. A delay was measured between the maximum in the radio emission and the $\gamma$ -ray flares, which favors models involving expanding components like the shock-in-jet models. Finally, we tried to understand the polarization angle variability behavior filling the gaps in our observations with published results of other polarimetric campaigns, and using the criterion of minimum variation in the polarization angle between successive observations to solve the 180° multiplicity. Full article

Combined polarization imaging of radio jets from Active Galactic Nuclei (AGN) in circular and linear polarization, also known as full Stokes imaging, has the potential to constrain both the magnetic field structure and particle properties of jets. Although only a small fraction of the emission when detected, typically less than a few tenths of a percent but up to as much as a couple of percent in the strongest resolved sources, circular polarization directly probes the magnetic field and particles within the jet itself and is not expected to be modified by external screens. A key to using full Stokes observations to constrain jet properties is obtaining a better understanding of the emission of circular polarization, including its variability and spectrum. We discuss what we have learned so far from parsec scale monitoring observations in the MOJAVE program and from multi-frequency observations of selected AGN. Full article

The polarization of relativistic jets is of interest for the understanding of their origin, confinement, and propagation. However, even though numerous measurements have been performed, the mechanisms behind jet variability, creation, and composition are still debated. We have performed simultaneous gamma-ray and optical photopolarimetry observations of 45 blazars with the Kanata telescope since July 2008 to investigate the mechanisms of variability and search for a basic relation between the several subclasses of relativistic jets. Consequently, we found that a correlation between the gamma-ray and optical flux might be related to gamma-ray luminosity, and the maximum polarization degree might be related to gamma-ray luminosity or the ratio of gamma-ray to optical flux. These results imply that low gamma-ray luminosity blazars emit from multiple regions. Full article

A key question in the formation of the relativistic jets in active galactic nuclei (AGNs) is the collimation process of their energetic plasma flow launched from the central supermassive black hole (SMBH). Recent observations of nearby low-luminosity radio galaxies exhibit a clear picture of parabolic collimation inside the Bondi accretion radius. On the other hand, little is known of the observational properties of jet collimation in more luminous quasars, where the accretion flow may be significantly different due to much higher accretion rates. In this paper, we present preliminary results of multi-frequency observations of the archetypal quasar 3C 273 with the Very Long Baseline Array (VLBA) at 1.4, 15, and 43 GHz, and Multi-Element Radio Linked Interferometer Network (MERLIN) at 1.6 GHz. The observations provide a detailed view of the transverse structure resolved on a broad range of spatial scales from sub-parsec to kilo parsecs, allowing us to profile the jet width as a function of the distance from the core for the first time in jets of bright quasars. We discovered a transition from a parabolic stream to a conical stream, which has been seen in much lower-luminosity radio galaxies. The similarity in the profile to the much lower-powered radio galaxy M87 suggests the universality of jet collimation among AGNs with different accretion rates. Full article

The binary star AR Scorpii was recently discovered to exhibit high amplitude coherent variability across the electromagnetic spectrum (ultraviolet to radio) at two closely spaced ∼2 min periods, attributed to the spin period of a white dwarf and the beat period. There is strong evidence (low X-ray luminosity, lack of flickering and absense of broad emission lines) that AR Sco is a detached non-accreting system whose luminosity is dominated by the spin-down power of a white dwarf, due to magnetohydrodynamical (MHD) interactions with its M5 companion. Optical polarimetry has revealed highly pulsed linear polarization on the same periods, reaching a maximum of 40%, consistent with a pulsar-like dipole, with the Stokes Q and U variations reminiscent of the Crab pulsar. These observations, coupled with the spectral energy distribution (SED) which is dominated by non-thermal emission, characteristic of synchrotron emission, support the notion that a strongly magnetic (∼200 MG) white dwarf is behaving like a pulsar, whose magnetic field interacts with the secondary star’s photosphere and magnetosphere. Radio synchrotron emission is produced from the pumping action of the white dwarf’s magnetic field on coronal loops from the M-star companion, while emission at high frequencies (UV/optical/X-ray) comes from the particle wind, driven by large electric potential, again reminiscent of processes seen in neutron star pulsars. Full article

There have been different scenarios describing the nature of a dusty source, noted as Dusty S-cluster Object (DSO) or G2, orbiting around the Galactic centre super-massive black hole. Observing the polarized continuum emission of this source provides information on its nature and geometry. We show that this source is intrinsically polarized with polarization degree of 30%, implying that it has a non-spherical geometry, and a varying polarization angle in the ambient medium of the black hole. Its main observable properties can be well described and modeled with a pre-main-sequence star forming a bow shock as it approaches the Sgr A* position. Full article

The time problem is a problem of canonical quantum gravity that has long been known about; it is related to the relativistic invariance and the consequent absence of an explicit time variable in the quantum equations. This fact complicates the interpretation of the wave function of the universe. Following proposals to assign the clock function to a scalar field, we look at the scalar degree of freedom contained in $f\left(R\right)$ theories. For this purpose we consider a quadratic $f\left(R\right)$ theory in an equivalent formulation with a scalar field, with a FRW metric, and consider its Wheeler-DeWitt equation. The wave function is obtained numerically and is consistent with the interpretation of the scalar field as time by means of a conditional probability, from which an effective time-dependent wave function follows. The evolution the scale factor is obtained by its mean value, and the quantum fluctuations are consistent with the Heisenberg relations and a classical universe today. Full article

Supersymmetry arises in certain theories of fermions coupled to gauge fields and gravity in a spacetime of 11 dimensions. The dynamical brane background has mainly been studied for the class of purely bosonic solutions only, but recent developments involving a time-dependent brane solution have made it clear that one can get more information by asking what happens on supersymmetric systems. In this proceeding, we construct an exact supersymmetric solution of a dynamical M-brane background in the 11-dimensional supergravity and investigate supersymmetry breaking, the geometric features near the singularity and the black hole horizon. Full article

The cold dark matter (CDM) paradigm successfully explains the cosmic structure over an enormous span of redshifts. However, it fails when probing the innermost regions of dark matter halos and the properties of the Milky Way’s dwarf galaxy satellites. Moreover, the lack of experimental detection of Weakly Interacting Massive Particle (WIMP) favors alternative candidates such as light axionic dark matter that naturally arise in string theory. Cosmological N-body simulations have shown that axionic dark matter forms a solitonic core of size of ≃150 pc in the innermost region of the galactic halos. The oscillating scalar field associated to the axionic dark matter halo produces an oscillating gravitational potential that induces a time dilation of the pulse arrival time of ≃400 ns/(m ${}_{\mathrm{B}}$ /10 ${}^{-22}$ eV) for pulsar within such a solitonic core. Over the whole galaxy, the averaged predicted signal may be detectable with current and forthcoming pulsar timing array telescopes. Full article

It is theoretically expected that active galactic nucleus (AGN) jets should carry helical magnetic (B) fields, which arise due to the rotation of the central black hole and accretion disk combined with the jet outflow. The direction of the toroidal component of the helical B field ${\mathbf{B}}_{\phi }$ is determined by the direction of the poloidal component ${\mathbf{B}}_p$ of the initial seed field that is “wound up” and the direction of rotation of the central black hole and accretion disk. The presence of the jet’s helical B field can be manifest both through the presence of Faraday rotation gradients across the jet, and the presence of appreciable circular polarization, which comes about when linearly polarized emission from the far side of the jet is partially converted to circularly polarized emission as it passes through the magnetized plasma at the front side of the jet on its way towards the observer. When both of these properties are manifest, they can be used jointly with the jet linear polarization structure to uniquely determine both the direction of ${\mathbf{B}}_p$ and the direction of the central rotation. This technique has been applied to 12 AGNs. The results indicate statistically equal numbers of outward and inward ${\mathbf{B}}_p$ and of clockwise (CW) and counter-clockwise (CCW) rotations of the central black holes on the sky. However, they suggest that the directions of ${\mathbf{B}}_p$ and of the central rotation are coupled: CW/CCW central rotation is preferentially associated with inward/outward poloidal B field. This leads to a preferred orientation for the toroidal B-field component corresponding to inward current along the jet. Full article