Internal shocks in relativistic outflows: collisions of magnetized shells

(Abridged): We study the collision of magnetized irregularities (shells) in relativistic outflows in order to explain the origin of the generic phenomenology observed in the non-thermal emission of both blazars and gamma-ray bursts. We focus on the influence of the magnetic field on the collision dynamics, and we further investigate how the properties of the observed radiation depend on the strength of the initial magnetic field and on the initial internal energy density of the flow. The collisions of magnetized shells and the radiation resulting from these collisions are calculated using the 1D relativistic magnetohydrodynamics code MRGENESIS. The interaction of the shells with the external medium prior to their collision is also determined using an exact solver for the corresponding 1D relativistic magnetohydrodynamic Riemann problem. Our simulations show that two magnetization parameters - the ratio of magnetic energy density and thermal energy density, \alpha_B, and the ratio of magnetic energy density and mass-energy density, \sigma - play an important role in the pre-collision phase, while the dynamics of the collision and the properties of the light curves depend mostly on the magnetization parameter \sigma. The interaction of the shells with the external medium changes the flow properties at their edges prior to the collision. For sufficiently dense shells moving at large Lorentz factors (\simgt 25) these properties depend only on the magnetization parameter \sigma. Internal shocks in GRBs may reach maximum efficiencies of conversion of kinetic into thermal energy between 6% and 10%, while in case of blazars, the maximum efficiencies are \sim 2%.Comment: 17 pages, 18 figures. 2 new references have been added. Accepted for publication in Astronomy and Astrophysic

Transformation of the Poynting flux into the kinetic energy in relativistic jets

The acceleration of relativistic jets from the Poynting to the matter dominated stage is considered. The are generally two collimation regimes, which we call equilibrium and non-equilibrium, correspondingly. In the first regime, the jet is efficiently accelerated till the equipartition between the kinetic and electro-magnetic energy. We show that after the equilibrium jet ceases to be Poynting dominated, the ratio of the electro-magnetic to the kinetic energy decreases only logarithmically so that such jets become truly matter dominated only at extremely large distances. Non-equilibrium jets remain generally Poynting dominated till the logarithmically large distances. In the only case when a non-equilibrium jet is accelerated till the equipartition level, we found that the flow is not continued to the infinity but is focused towards the axis at a finite distance from the origin.Comment: Submitted to MNRAS Minor changes in the Conclusion

Fulminantne müokardiit. Haigusjuht ja kirjanduse ülevaade

Müokardiit on heterogeenne haigus nii etioloogia kui ka kliinilise kulu poolest. Haiguse aluseks on põletikuline südamelihase kahjustus, mis kulgeb ägeda, enamasti taaspöörduva sündroomina. Ebasoodsa kulu korral võib tekkida krooniline dilateeriv kardiomüopaatia, mis võib viia progresseeruva südamepuudulikkuseni. Äge müokardiit võib kulgeda ka eluohtliku seisundina ning sel puhul on tegu fulminantse müokardiidiga. Selle harva, kuid raske kuluga haiguse puhul võib olla vaja rakendada kehavälist vereringetoetust. Fulminantse müokardiidi kahtluse korral tuleb patsient suunata keskusesse, kus on võimalik müokardiidi spetsiifiline diagnostika ning ravi, sealhulgas MRTuuringute ning EMB tegemine. Müokardiidi diagnoosimisel on oluline järgida rahvusvahelisi diagnostilisi kriteeriume ning vältida ühelt poolt valediagnoose ning teiselt poolt diagnoosimise ja ravi hilinemisest tingitud püsivat elukvaliteedi halvenemist. Raviprotsessis tuleb lähtuda juhtivast kliinilisest sündroomist – südamepuudulikkust tuleb ravida tõenduspõhiste meetodite ja ravimiannustega, rindkerevalu valuvaigistitega. MSPVAde kasutamine on näidustatud valusündroomi või perikardi haaratuse korral. Dilateeriva kardiomüopaatia põhjuseks võib olla krooniline müokardiit ning võimaluse korral tuleks nendel patsientidel teha EMB ning otsida seerumist patogeenseid autoantikehi. Ägeda müokardiidi põdemise järel on soovitatav patsiendile teha kordusuuringuna ehhokardiograafia ning müokardi düsfunktsiooni korral on soovitatav jälgimine kardioloogi poolt. Müokardiidi diagnostika ning ravi põhineb senini vähesel tõenduspõhisusel ning vajalikud on täpsustavad uuringud.Eesti Arst 2018; 97(1):29–3

Relativistic MHD Simulations of Jets with Toroidal Magnetic Fields

This paper presents an application of the recent relativistic HLLC approximate Riemann solver by Mignone & Bodo to magnetized flows with vanishing normal component of the magnetic field. The numerical scheme is validated in two dimensions by investigating the propagation of axisymmetric jets with toroidal magnetic fields. The selected jet models show that the HLLC solver yields sharper resolution of contact and shear waves and better convergence properties over the traditional HLL approach.Comment: 12 pages, 5 figure

More Than Words can Tell - Using Multimodal Texts to Support Reading Comprehension of Literary Texts in English

This thesis explores the possibilities of multimodality in supporting text comprehension of literary texts in language learning of the L2. While multimodal texts offer multiple ways of meaning making that sometimes go beyond the written text, I have focussed on multimodal expressions that mirror the context of a given text. I conducted an empirical study with 114 students (grade 9; 13-14 years) in two schools in Trondheim, Norway. The material I used consisted of three literary texts (excerpts from Penguin Readers books adapted for the age group) for which the supported by illustrations and audio files was systematically varied in an experimental design. The students carried out a standardized reading test (Hodder reading test) in a first session to establish different levels of reading proficiency, before they took the main reading comprehension test which included the multimodal text expressions in a second session, about 1-2 weeks later. In addition, their personal experience of support by the multimodal expressions was measured in a questionnaire. Differences between the different versions of the texts were analysed by chi-square tests, analyses of variance, and analyses of covariance. The reading proficiency test proved to be a good indicator for successful understanding of the literary texts. The different modalities seem to support reading comprehension, but the effects appear to depend partly on the students’ level of reading proficiency and fatigue. The results are interpreted in the light of cognitive theory of multimedia learning and dual coding theory

More Than Words can Tell - Using Multimodal Texts to Support Reading Comprehension of Literary Texts in English

This thesis explores the possibilities of multimodality in supporting text comprehension of literary texts in language learning of the L2. While multimodal texts offer multiple ways of meaning making that sometimes go beyond the written text, I have focussed on multimodal expressions that mirror the context of a given text. I conducted an empirical study with 114 students (grade 9; 13-14 years) in two schools in Trondheim, Norway. The material I used consisted of three literary texts (excerpts from Penguin Readers books adapted for the age group) for which the supported by illustrations and audio files was systematically varied in an experimental design. The students carried out a standardized reading test (Hodder reading test) in a first session to establish different levels of reading proficiency, before they took the main reading comprehension test which included the multimodal text expressions in a second session, about 1-2 weeks later. In addition, their personal experience of support by the multimodal expressions was measured in a questionnaire. Differences between the different versions of the texts were analysed by chi-square tests, analyses of variance, and analyses of covariance. The reading proficiency test proved to be a good indicator for successful understanding of the literary texts. The different modalities seem to support reading comprehension, but the effects appear to depend partly on the students’ level of reading proficiency and fatigue. The results are interpreted in the light of cognitive theory of multimedia learning and dual coding theory

Equation of State in Relativistic Magnetohydrodynamics: variable versus constant adiabatic index

The role of the equation of state for a perfectly conducting, relativistic magnetized fluid is the main subject of this work. The ideal constant $\Gamma$-law equation of state, commonly adopted in a wide range of astrophysical applications, is compared with a more realistic equation of state that better approximates the single-specie relativistic gas. The paper focus on three different topics. First, the influence of a more realistic equation of state on the propagation of fast magneto-sonic shocks is investigated. This calls into question the validity of the constant $\Gamma$-law equation of state in problems where the temperature of the gas substantially changes across hydromagnetic waves. Second, we present a new inversion scheme to recover primitive variables (such as rest-mass density and pressure) from conservative ones that allows for a general equation of state and avoids catastrophic numerical cancellations in the non-relativistic and ultrarelativistic limits. Finally, selected numerical tests of astrophysical relevance (including magnetized accretion flows around Kerr black holes) are compared using different equations of state. Our main conclusion is that the choice of a realistic equation of state can considerably bear upon the solution when transitions from cold to hot gas (or viceversa) are present. Under these circumstances, a polytropic equation of state can significantly endanger the solution.Comment: 14 pages, 14 figure

An HLLC Solver for Relativistic Flows -- II. Magnetohydrodynamics

An approximate Riemann solver for the equations of relativistic magnetohydrodynamics (RMHD) is derived. The HLLC solver, originally developed by Toro, Spruce and Spears, generalizes the algorithm described in a previous paper (Mignone & Bodo 2004) to the case where magnetic fields are present. The solution to the Riemann problem is approximated by two constant states bounded by two fast shocks and separated by a tangential wave. The scheme is Jacobian-free, in the sense that it avoids the expensive characteristic decomposition of the RMHD equations and it improves over the HLL scheme by restoring the missing contact wave. Multidimensional integration proceeds via the single step, corner transport upwind (CTU) method of Colella, combined with the contrained tranport (CT) algorithm to preserve divergence-free magnetic fields. The resulting numerical scheme is simple to implement, efficient and suitable for a general equation of state. The robustness of the new algorithm is validated against one and two dimensional numerical test problems.Comment: 17 pages, 12 figure

Extragalactic jets with helical magnetic fields: relativistic MHD simulations

Extragalactic jets are inferred to harbor dynamically important, organized magnetic fields which presumably aid in the collimation of the relativistic jet flows. We here explore by means of grid-adaptive, high resolution numerical simulations the morphology of AGN jets pervaded by helical field and flow topologies. We concentrate on morphological features of the bow shock and the jet beam behind the Mach disk, for various jet Lorentz factors and magnetic field helicities. We investigate the influence of helical magnetic fields on jet beam propagation in overdense external medium. We use the AMRVAC code, employing a novel hybrid block-based AMR strategy, to compute ideal plasma dynamics in special relativity. The helicity of the beam magnetic field is effectively transported down the beam, with compression zones in between diagonal internal cross-shocks showing stronger toroidal field regions. In comparison with equivalent low-relativistic jets which get surrounded by cocoons with vortical backflows filled by mainly toroidal field, the high speed jets demonstrate only localized, strong toroidal field zones within the backflow vortical structures. We find evidence for a more poloidal, straight field layer, compressed between jet beam and backflows. This layer decreases the destabilizing influence of the backflow on the jet beam. In all cases, the jet beam contains rich cross-shock patterns, across which part of the kinetic energy gets transferred. For the high speed reference jet considered here, significant jet deceleration only occurs beyond distances exceeding ${\cal O}(100 R_j)$, as the axial flow can reaccelerate downstream to the internal cross-shocks. This reacceleration is magnetically aided, due to field compression across the internal shocks which pinch the flow.Comment: 16 pages, Astronomy and Astrophysics accepted for publicatio