47 research outputs found
Hydrodynamical Simulations of Recollimation Shocks within Relativistic Astrophysical Jets
Abstract: Astrophysical jets launched from active galactic nuclei can remain tightly collimated over large distances due, in part, to recollimation shocks. Formed within the jets due to their supersonic nature, recollimation shocks are predicted to leave signatures in the observed radio emission due to magnetic flux freezing and the geometric relationship between magnetic fields and the polarization of synchrotron radiation. In the course of this work, we will compare how predictions of emission from recollimation shocks change when the flow is modelled using a hydrodynamical code, as opposed to semi-dynamical and magnetohydrodynamical codes. Jets generally exhibit low levels of polarization, which implies a substantially disordered magnetic field. It is difficult to model such fields using magnetohydrodynamics, hence this work uses hydrodynamical code and a statistical treatment of the magnetic field (c.f. Scheuer and Matthews, 1990). It should then be possible to assess whether certain radio jet phenomena, such as knots and radio-cores, may be modelled as singular or multiple recollimation shocks. To date, the hydrodynamical code has been successfully built and executed on UCLan’s supercomputer cluster, and parallelepiped vector triads have been included to monitor the fluid deformation within the simulation, so that the emergent flux and polarization may be calculated. The parallelepiped advection is currently being verified and some results are discussed. Code for radiative transfer throughout the jet is also being implemented, in order to simulate images for comparison with previous works and observations
Magnetic fields of relativistic jets in active galactic nuclei
The motivation of my work is to understand the role played by the magnetic field in the dynamics and emission of relativistic jets. In order to achieve this objective I have been carrying it out in two parallel ways. In the first of them I have performed numerical magnetohydrodynamic and emission (eRMHD) simulations of relativistic jets. The simulations have been performed in collaboration with the Relativistic Astrophysics Group in the University of Valencia, using a numerical code that solves the RMHD equations in conservative form and cylindrical coordinates with axial symmetry (see Leismann et al., 2005, for more details). I have focused the study on the role played by the magnetic field in the dynamics of the jet, analyzing the balance of the main driving forces which determine the jet evolution. By using these relativistic magnetohydrodynamic (RMHD) simulations as input I have computed the non-thermal (synchrotron) emission which allows to obtain synthetic radio maps that can be directly compared with actual observations (Roca-Sogorb et al., 2008a,b, 2009). The synergy between simulations and observations is proven to be a powerful tool in the under- standing of the physical processes taking place in jets. For this, my second line of work has been the comparison of the eRMHD results with actual sources. I have started the study with a very well known source: the radiogalaxy 3C 120. I have carried out new observations, taken in November 2007 making use of all available observing frequencies (from 1 to 86 GHz) with the VLBA and VLA. These observations, taken also in polarimetric mode, allow to study the jet in 3C120 from pc to Kpc scales with great detail. The comparison of these observations with those from 1999 to 2001 (Gomez et al., o 2008) provides information about the source of Faraday rotation in the jet of the radio galaxy 3C 120. The results indicate that the emitting jet and the source of Faraday rotation are not closely connected physically and have different configurations for the magnetic field and/or kinematical properties, favoring a model in which a significant fraction of the RM originates in foreground clouds (Gomez et al., o 2011). The higher frequency 2007 observations reveal a new component located 80 mas from the core (which corresponds to a deprojected distance of 140 pc), with a brightness temperature about 600 times higher than expected at such distances. I have analyzed the different processes that could be responsible for the enhanced brightness temperature observed, its sudden appearance, and apparent 1 stationary (Roca-Sogorb et al., 2010)
A recollimation shock 80 mas from the core in the jet of the radio galaxy 3C120: Observational evidence and modeling
We present Very Long Baseline Array observations of the radio galaxy 3C120 at
5, 8, 12, and 15 GHz designed to study a peculiar stationary jet feature
(hereafter C80) located ~80 mas from the core, which was previously shown to
display a brightness temperature ~600 times lager than expected at such
distances. The high sensitivity of the images -- obtained between December 2009
and June 2010 -- has revealed that C80 corresponds to the eastern flux density
peak of an arc of emission (hereafter A80), downstream of which extends a large
(~20 mas in size) bubble-like structure that resembles an inverted bow shock.
The linearly polarized emission closely follows that of the total intensity in
A80, with the electric vector position angle distributed nearly perpendicular
to the arc-shaped structure. Despite the stationary nature of C80/A80,
superluminal components with speeds up to ~3 c have been detected downstream
from its position, resembling the behavior observed in the HST-1 emission
complex in M87. The total and polarized emission of the C80/A80 structure, its
lack of motion, and brightness temperature excess are best reproduced by a
model based on synchrotron emission from a conical shock with cone opening
angle \eta=10 degrees, jet viewing angle \theta=16 degrees, a completely
tangled upstream magnetic field, and upstream Lorentz factor \gamma=8.4. The
good agreement between our observations and numerical modeling leads us to
conclude that the peculiar feature associated with C80/A80 corresponds to a
conical recollimation shock in the jet of 3C120 located at a de-projected
distance of ~190 pc downstream from the nucleus.Comment: Accepted for publication in Ap
APRICOT: Advanced Platform for Reproducible Infrastructures in the Cloud via Open Tools
[EN] Background Scientific publications are meant to exchange knowledge among researchers but the inability to properly reproduce computational experiments limits the quality of scientific research. Furthermore, bibliography shows that irreproducible preclinical research exceeds 50%, which produces a huge waste of resources on nonprofitable research at Life Sciences field. As a consequence, scientific reproducibility is being fostered to promote Open Science through open databases and software tools that are typically deployed on existing computational resources. However, some computational experiments require complex virtual infrastructures, such as elastic clusters of PCs, that can be dynamically provided from multiple clouds. Obtaining these infrastructures requires not only an infrastructure provider, but also advanced knowledge in the cloud computing field.
Objectives The main aim of this paper is to improve reproducibility in life sciences to produce better and more cost-effective research. For that purpose, our intention is to simplify the infrastructure usage and deployment for researchers.
Methods This paper introduces Advanced Platform for Reproducible Infrastructures in the Cloud via Open Tools (APRICOT), an open source extension for Jupyter to deploy deterministic virtual infrastructures across multiclouds for reproducible scientific computational experiments. To exemplify its utilization and how APRICOT can improve the reproduction of experiments with complex computation requirements, two examples in the field of life sciences are provided. All requirements to reproduce both experiments are disclosed within APRICOT and, therefore, can be reproduced by the users.
Results To show the capabilities of APRICOT, we have processed a real magnetic resonance image to accurately characterize a prostate cancer using a Message Passing Interface cluster deployed automatically with APRICOT. In addition, the second example shows how APRICOT scales the deployed infrastructure, according to the workload, using a batch cluster. This example consists of a multiparametric study of a positron emission tomography image reconstruction.
Conclusion APRICOT's benefits are the integration of specific infrastructure deployment, the management and usage for Open Science, making experiments that involve specific computational infrastructures reproducible. All the experiment steps and details can be documented at the same Jupyter notebook which includes infrastructure specifications, data storage, experimentation execution, results gathering, and infrastructure termination. Thus, distributing the experimentation notebook and needed data should be enough to reproduce the experiment.This study was supported by the program "Ayudas para la contratación de personal investigador en formación de carácter predoctoral, programa VALi+d" under grant number ACIF/2018/148 from the Conselleria d'Educació of the Generalitat Valenciana and the "Fondo Social Europeo" (FSE). The authors would like to thank the Spanish "Ministerio de Economía, Industria y Competitividad" for the project "BigCLOE" with reference number TIN2016-79951-R and the European Commission, Horizon 2020 grant agreement No 826494 (PRIMAGE). The MRI prostate study case used in this article has been retrospectively collected from a project of prostate MRI biomarkers validation.Giménez-Alventosa, V.; Segrelles Quilis, JD.; Moltó, G.; Roca-Sogorb, M. (2020). APRICOT: Advanced Platform for Reproducible Infrastructures in the Cloud via Open Tools. Methods of Information in Medicine. 59(S 02):e33-e45. https://doi.org/10.1055/s-0040-1712460Se33e4559S 02Donoho, D. L., Maleki, A., Rahman, I. U., Shahram, M., & Stodden, V. (2009). Reproducible Research in Computational Harmonic Analysis. Computing in Science & Engineering, 11(1), 8-18. doi:10.1109/mcse.2009.15Freedman, L. P., Cockburn, I. M., & Simcoe, T. S. (2015). The Economics of Reproducibility in Preclinical Research. 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Stratification in polarization and Faraday rotation in the jet of 3C 120
Very long baseline interferometric observations of the radio galaxy 3C 120
show a systematic presence of gradients in Faraday rotation and degree of
polarization across and along the jet. These are revealed by the passage of
multiple superluminal components throughout the jet as they move out from the
core in a sequence of 12 monthly polarimetric observations taken with the VLBA
at 15, 22, and 43 GHz. The degree of polarization has an asymmetric profile in
which the northern side of the jet is more highly polarized. The Faraday
rotation measure is also stratified across the jet width, with larger values
for the southern side. Superposed on this structure we find a localized region
of high Faraday rotation measure (about 6000 rad/m^2) between approximately 3
and 4 mas from the core. This region of enhanced Faraday rotation may result
from the interaction of the jet with the ambient medium, which may also explain
the stratification in degree of polarization. The data are also consistent with
a helical magnetic field in a two-fluid jet model, consisting of an inner
emitting jet and a sheath of nonrelativistic electrons.Comment: To be published by the Memorie della Societa Astronomica Italiana,
Vol. 79, in the Proceedings of the Workshop: "The Central Kiloparsec: Active
Galactic Nuclei and Their Hosts", eds. A.Lobanov, E.Angelakis, M.Perucho, and
A.Zensus. 4 pages (including 4 figures
Faraday rotation and polarization gradients in the jet of 3C~120: Interaction with the external medium and a helical magnetic field?
We present a sequence of 12 monthly polarimetric 15, 22, and 43 GHz VLBA
observations of the radio galaxy 3C 120 revealing a systematic presence of
gradients in Faraday rotation and degree of polarization across and along the
jet. The degree of polarization increases with distance from the core and
toward the jet edges, and has an asymmetric profile in which the northern side
of the jet is more highly polarized. The Faraday rotation measure is also
stratified across the jet width, with larger values for the southern side. We
find a localized region of high Faraday rotation measure superposed on this
structure between approximately 3 and 4 mas from the core, with a peak of about
6000 rad/m^2. Interaction of the jet with the external medium or a cloud would
explain the confined region of enhanced Faraday rotation, as well as the
stratification in degree of polarization and the flaring of superluminal knots
when crossing this region. The data are also consistent with a helical field in
a two-fluid jet model, consisting of an inner, emitting jet and a sheath
containing nonrelativistic electrons. However, this helical magnetic field
model cannot by itself explain the localized region of enhanced Faraday
rotation. The polarization electric vectors, predominantly perpendicular to the
jet axis once corrected for Faraday rotation, require a dominant component
parallel to the jet axis (in the frame of the emitting plasma) for the magnetic
field in the emitting region.Comment: Accepted for publication in ApJ Letters. 4 pages (including 5
figures
On the source of Faraday rotation in the jet of the radio galaxy 3C120
The source of Faraday rotation in the jet of the radio galaxy 3C120 is
analyzed through Very Long Baseline Array observations carried out between 1999
and 2007 at 86, 43, 22, 15, 12, 8, 5, 2, and 1.7 GHz. Comparison of
observations from 1999 to 2001 reveals uncorrelated changes in the linear
polarization of the underlying jet emission and the Faraday rotation screen:
while the rotation measure (RM) remains constant between approximately 2 and 5
mas from the core, the RM-corrected electric vector position angles (EVPAs) of
two superluminal components are rotated by almost 90 degrees when compared to
other components moving through similar jet locations. On the other hand, the
innermost 2 mas experiences a significant change in RM -- including a sign
reversal -- but without variations in the RM-corrected EVPAs. Similarly,
observations in 2007 reveal a double sign reversal in RM along the jet, while
the RM-corrected EVPAs remain perpendicular to the jet axis. Although the
observed coherent structure and gradient of the RM along the jet supports the
idea that the Faraday rotation is produced by a sheath of thermal electrons
that surrounds the emitting jet, the uncorrelated changes in the RM and
RM-corrected EVPAs indicate that the emitting jet and the source of Faraday
rotation are not closely connected physically and have different configurations
for the magnetic field and/or kinematical properties. Furthermore, the
existence of a region of enhanced RM whose properties remain constant over
three years requires a localized source of Faraday rotation, favoring a model
in which a significant fraction of the RM originates in foreground clouds.Comment: 12 pages, 11 figures; Accepted for publication in Ap
Anisotropic scaling features and complexity in magnetospheric-cusp: a case study
Magnetospheric cusps are high-latitude regions characterized by a highly turbulent plasma, playing a special role in the solar wind-magnetosphere interaction. Here, using POLAR satellite magnetic field vector measurements we investigate the anisotropic scaling features of the magnetic field fluctuations in the northern cusp region. Our results seem to support the hypothesis of a 2D-MHD turbulent scenario which is consequence of a strong background magnetic field. The observed turbulent fluctuations reveal a high degree of complexity, which might be due to the interplay of many competing scales. A discussion of our findings in connection with the complex scenario proposed by Chang et al. (2004) is provided