On the distribution of fluxes of gamma-ray blazars: hints for a stochastic process?

We examine a model for the observed temporal variability of powerful blazars in the $\gamma$-ray band in which the dynamics is described in terms of a stochastic differential equation, including the contribution of a deterministic drift and a stochastic term. The form of the equation is motivated by the current astrophysical framework, accepting that jets are powered through the extraction of the rotational energy of the central supermassive black hole mediated by magnetic fields supported by a so-called \emph{magnetically arrested} accretion disk. We apply the model to the $\gamma$-ray light curves of several bright blazars and we infer the parameters suitable to describe them. In particular, we examine the differential distribution of fluxes ($dN/dF_{\gamma}$) and we show that the predicted probability density function for the assumed stochastic equation naturally reproduces the observed power law shape at large fluxes $dN/dF_{\gamma} \propto F_{\gamma}^{-\alpha}$ with $\alpha>2$.Comment: 7 pages, 4 figures, accepted for publication in MNRA

Astrophysical hints of axion-like particles

After reviewing three astrophysical hints of the existence of axionlike particles (ALPs), we describe in more detail a new similar hint involving flat spectrum radio quasars (FSRQs). Detection of FSRQs above about 20GeV pose a challenge to very-high-energy (VHE) astrophysics, because at those energies the ultraviolet emission from their broad line region should prevent photons produced by the central engine to leave the source. Although a few astrophysical explanations have been put forward, they are totally ad hoc. We show that a natural explanation instead arises within the conventional models of FSRQs provided that photon-ALP oscillations occur inside the source. Our analysis takes the FSRQ PKR 1222+206 as an example, and it looks tantalizing that basically the same choice of the free model parameters adopted in this case is consistent with those that provide the other three hints of the existence of ALPs

Evidence for an axion-like particle from PKS 1222+216?

The surprising discovery by MAGIC of an intense, rapidly varying emission in the energy range 70 - 400 GeV from the flat spectrum radio quasar PKS 1222+216 represents a challenge for all interpretative scenarios. Indeed, in order to avoid absorption of \gamma rays in the dense ultraviolet radiation field of the broad line region (BLR), one is forced to invoke some unconventional astrophysical picture, like for instance the existence of a very compact (r\sim 10^{14} cm) emitting blob at a large distance (R \sim10^{18} cm) from the jet base. We offer the investigation of a scenario based on the standard blazar model for PKS 1222+216 where \gamma rays are produced close to the central engine, but we add the new assumption that inside the source photons can oscillate into axion-like particles (ALPs), which are a generic prediction of several extensions of the Standard Model of elementary particle interactions. As a result, a considerable fraction of very-high-energy photons can escape absorption from the BLR through the mechanism of photon-ALP oscillations much in the same way as they largely avoid absorption from extragalactic background light when propagating over cosmic distances in the presence of large-scale magnetic fields in the nG range. In addition we show that the above MAGIC observations and the simultaneous Fermi/LAT observations in the energy range 0.3 - 3 GeV can both be explained by a standard spectral energy distribution for experimentally allowed values of the model parameters. In particular, we need a very light ALP just like in the case of photon-ALP oscillations in cosmic space. Moreover, we find it quite tantalizing that the most favorable value of the photon-ALP coupling happens to be the same in both situations. Although our ALPs cannot contribute to the cold dark matter, they are a viable candidate for the quintessential dark energy. [abridged]Comment: 32 pages, 10 figures, accepted for publication in Physical Review

On the Structure of the Magnetic Field in a Kinematic ABC Flow Dynamo

The kinematic induction equation of MHD is solved numerically in the case of the normal ``111'' ABC flow using a general staggered mesh method. Careful 3-D visualizations of the topology of the magnetic field reveal that previous conclusions about the modes of operation of this type of kinematic dynamo must be revised. The two known windows of dynamo action at low and high magnetic Reynolds number, correspond to two distinct modes, both relying crucially on the replenishing of the magnetic field near a discontinuity at the beta-type stagnation points in the flow. One of these modes display double magnetic structures that were previously found only to obscure the physics of the dynamo: They turn out, however, to play an important part in the process of amplifying the magnetic field. Invariant properties of the mode in the second magnetic Reynolds number window support the case for the normal ABC flow as a fast dynamo.Comment: Associated webpage, see http://www.astro.su.se/~dorch/dynamo

Body fat assessment in international elite soccer referees

Soccer referees are a specific group in the sports population that are receiving increasing attention from sports scientists. A lower fat mass percentage (FM%) is a useful parameter to monitor fitness status and aerobic performance, while being able to evaluate it with a simple and quick field-based method can allow a regular assessment. The aim of this study was to provide a specific profile for referees based on morphological and body composition features while comparing the accuracy of different skinfold-based equations in estimating FM% in a cohort of soccer referees. Forty-three elite international soccer referees (age 38.8 ± 3.6 years), who participated in the 2018 Russian World Cup, underwent body composition assessments with skinfold thickness and dual-energy X-ray absorptiometry (DXA). Six equations used to derive FM% from skinfold thickness were compared with DXA measurements. The percentage of body fat estimated using DXA was 18.2 ± 4.1%, whereas skinfold-based FM% assessed from the six formulas ranged between 11.0% ± 1.7% to 15.6% ± 2.4%. Among the six equations considered, the Faulkner's formula showed the highest correlation with FM% estimated by DXA (r = 0.77; R2 = 0.59 p < 0.001). Additionally, a new skinfold-based equation was developed: FM% = 8.386 + (0.478 × iliac crest skinfold) + (0.395 × abdominal skinfold, r = 0.78; R2 = 0.61; standard error of the estimate (SEE) = 2.62 %; p < 0.001). Due to these findings, national and international federations will now be able to perform regular body composition assessments using skinfold measurements. (c) 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/)

Protocell Communication Through the Eyes of Synthetic Organic Chemists

The bottom-up fabrication of synthetic cells (protocells) from molecules and materials, is a major challenge of modern chemistry. A significant breakthrough has been the engineering of protocells capable of chemical communication using bio- derived molecules and ex situ stabilised cell machineries. These, however, suffer from short shelf-lives, high costs, and require mild aqueous conditions. In this Concept Article we analyse the chemistry at the heart of protocell communication to highlight new opportunities for synthetic chemists in protocell engineer- ing. Specifically, we (i) categorise the main bio-derived chemical communication machineries in enzyme cascades, DNA strand displacement, and gene-mediated communication; (ii) review the chemistries of these signal transduction machineries; and (iii) introduce new types of bio-inspired, fully synthetic artificial enzymes to replace their natural counterparts. Developing protocells that incorporate synthetic analogues of bio-derived signal transduction machineries will improve the robustness, stability, and versatility of protocells, and broaden their applications to highly strategic fields such as photocatalysis and fine chemicals production

Lagrangian analysis of alignment dynamics for isentropic compressible magnetohydrodynamics

After a review of the isentropic compressible magnetohydrodynamics (ICMHD) equations, a quaternionic framework for studying the alignment dynamics of a general fluid flow is explained and applied to the ICMHD equations.Comment: 12 pages, 2 figures, submitted to a Focus Issue of New Journal of Physics on "Magnetohydrodynamics and the Dynamo Problem" J-F Pinton, A Pouquet, E Dormy and S Cowley, editor