@article{KobzarNiemiecPohletal.2017, author = {Kobzar, Oleh and Niemiec, Jacek and Pohl, Martin and Bohdan, Artem}, title = {Spatio-temporal evolution of the non-resonant instability in shock precursors of young supernova remnants}, series = {Monthly notices of the Royal Astronomical Society}, volume = {469}, journal = {Monthly notices of the Royal Astronomical Society}, publisher = {Oxford Univ. Press}, address = {Oxford}, organization = {ANTARES Collaboration;H E S S Collaboration}, issn = {0035-8711}, doi = {10.1093/mnras/stx1201}, pages = {4985 -- 4998}, year = {2017}, abstract = {A non-resonant cosmic ray (CR) current-driven instability may operate in the shock precursors of young supernova remnants and be responsible for magnetic-field amplification, plasma heating and turbulence. Earlier simulations demonstrated magnetic-field amplification, and in kinetic studies a reduction of the relative drift between CRs and thermal plasma was observed as backreaction. However, all published simulations used periodic boundary conditions, which do not account for mass conservation in decelerating flows and only allow the temporal development to be studied. Here we report results of fully kinetic particle-in-cell simulations with open boundaries that permit inflow of plasma on one side of the simulation box and outflow at the other end, hence allowing an investigation of both the temporal and the spatial development of the instability. Magnetic-field amplification proceeds as in studies with periodic boundaries and, observed here for the first time, the reduction of relative drifts causes the formation of a shock-like compression structure at which a fraction of the plasma ions are reflected. Turbulent electric field generated by the non-resonant instability inelastically scatters CRs, modifying and anisotropizing their energy distribution. Spatial CR scattering is compatible with Bohm diffusion. Electromagnetic turbulence leads to significant non-adiabatic heating of the background plasma maintaining bulk equipartition between ions and electrons. The highest temperatures are reached at sites of large-amplitude electrostatic fields. Ion spectra show supra-thermal tails resulting from stochastic scattering in the turbulent electric field. Together, these modifications in the plasma flow will affect the properties of the shock and particle acceleration there.}, language = {en} } @article{NuzaParisiScannapiecoetal.2014, author = {Nuza, Sebastian E. and Parisi, Florencia and Scannapieco, Cecilia and Richter, Philipp and Gottloeber, Stefan and Steinmetz, Matthias}, title = {The distribution of gas in the Local Group from constrained cosmological simulations: the case for Andromeda and the Milky Way galaxies}, series = {Monthly notices of the Royal Astronomical Society}, volume = {441}, journal = {Monthly notices of the Royal Astronomical Society}, number = {3}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/stu643}, pages = {2593 -- 2612}, year = {2014}, abstract = {We study the gas distribution in the Milky Way and Andromeda using a constrained cosmological simulation of the Local Group (LG) within the context of the CLUES (Constrained Local UniversE Simulations) project. We analyse the properties of gas in the simulated galaxies at z = 0 for three different phases: 'cold', 'hot' and H i, and compare our results with observations. The amount of material in the hot halo (M-hot a parts per thousand 4-5 x 10(10) M-aS (TM)), and the cold (M-cold(r a parts per thousand(2) 10 kpc) a parts per thousand 10(8) M-aS (TM)) and H i components displays reasonable agreement with observations. We also compute the accretion/ejection rates together with the H i (radial and all-sky) covering fractions. The integrated H i accretion rate within r = 50 kpc gives similar to 0.2-0.3 M-aS (TM) yr(-1), i.e. close to that obtained from high-velocity clouds in the Milky Way. We find that the global accretion rate is dominated by hot material, although ionized gas with T a parts per thousand(2) 10(5) K can contribute significantly too. The net accretion rates of all material at the virial radii are 6-8 M-aS (TM) yr(-1). At z = 0, we find a significant gas excess between the two galaxies, as compared to any other direction, resulting from the overlap of their gaseous haloes. In our simulation, the gas excess first occurs at z similar to 1, as a result of the kinematical evolution of the LG.}, language = {en} } @article{MizunoPohlNiemiecetal.2014, author = {Mizuno, Yosuke and Pohl, Martin and Niemiec, Jacek and Zhang, Bing and Nishikawa, Ken-Ichi and Hardee, Philip E.}, title = {Magnetic field amplification and saturation in turbulence behind a relativistic shock}, series = {Monthly notices of the Royal Astronomical Society}, volume = {439}, journal = {Monthly notices of the Royal Astronomical Society}, number = {4}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/stu196}, pages = {3490 -- 3503}, year = {2014}, abstract = {We have investigated via 2D relativistic magnetohydrodynamic simulations the long-term evolution of turbulence created by a relativistic shock propagating through an inhomogeneous medium. In the post-shock region, magnetic field is strongly amplified by turbulent motions triggered by pre-shock density inhomogeneities. Using a long-simulation box we have followed the magnetic field amplification until it is fully developed and saturated. The turbulent velocity is subrelativistic even for a strong shock. Magnetic field amplification is controlled by the turbulent motion and saturation occurs when the magnetic energy is comparable to the turbulent kinetic energy. Magnetic field amplification and saturation depend on the initial strength and direction of the magnetic field in the pre-shock medium, and on the shock strength. If the initial magnetic field is perpendicular to the shock normal, the magnetic field is first compressed at the shock and then can be amplified by turbulent motion in the post-shock region. Saturation occurs when the magnetic energy becomes comparable to the turbulent kinetic energy in the post-shock region. If the initial magnetic field in the pre-shock medium is strong, the post-shock region becomes turbulent but significant field amplification does not occur. If the magnetic energy after shock compression is larger than the turbulent kinetic energy in the post-shock region, significant field amplification does not occur. We discuss possible applications of our results to gamma-ray bursts and active galactic nuclei.}, language = {en} } @article{TepperGarciaRichterSchaye2013, author = {Tepper-Garcia, Thor and Richter, Philipp and Schaye, Joop}, title = {Absorption signatures of warm-hot gas at low redshift - ne viii}, series = {Monthly notices of the Royal Astronomical Society}, volume = {436}, journal = {Monthly notices of the Royal Astronomical Society}, number = {3}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/stt1712}, pages = {2063 -- 2081}, year = {2013}, abstract = {At z < 1 a large fraction of the baryons is thought to reside in diffuse gas that has been shock-heated to high temperatures (10 (5)-10 (6) K). Absorption by the 770.41, 780.32 A doublet of Ne viii in quasar spectra represents a unique tool to study this elusive warm-hot phase. We have developed an analytic model for the properties of Ne viii absorbers that allows for an inhomogeneous metal distribution. Our model agrees with the predictions of a simulation from the OverWhelmingly Large Simulations project indicating that the average line-of-sight metal-filling fraction within the absorbing gas is low (c(L) similar to 0.1). Most of the Ne viii in our model is produced in low-density, collisionally ionized gas (n(H) = 10(-6)-10(-4) cm(-3), T = 10 (5)-10 (6) K). Strong Ne viii absorbers (log(10)(N-NeVIII/cm(-2))14), like those recently detected by Hubble Space Telescope/Cosmic Origins Spectrograph, are found to arise in higher density gas (n(H) greater than or similar to 10(-4) cm(-3), T approximate to 5 x 10 (5) K). Ne viii cloudlets harbour only 1 per cent of the cosmic baryon budget. The baryon content of the surrounding gas (which has similar densities and temperatures as the Ne viii cloudlets) is a factor c(-1)L higher. We conclude that Ne viii absorbers are robust probes of shock-heated diffuse gas, but that spectra with signal-to-noise ratios S/N > 100 would be required to detect the bulk of the baryons in warm-hot gas.}, language = {en} } @article{ValoriDemoulinPariatetal.2013, author = {Valori, Gherarod and Demoulin, Pascal and Pariat, E. and Masson, S.}, title = {Accuracy of magnetic energy computations}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {553}, journal = {Astronomy and astrophysics : an international weekly journal}, number = {2}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/201220982}, pages = {14}, year = {2013}, abstract = {Context. For magnetically driven events, the magnetic energy of the system is the prime energy reservoir that fuels the dynamical evolution. In the solar context, the free energy (i.e., the energy in excess of the potential field energy) is one of the main indicators used in space weather forecasts to predict the eruptivity of active regions. A trustworthy estimation of the magnetic energy is therefore needed in three-dimensional (3D) models of the solar atmosphere, e. g., in coronal fields reconstructions or numerical simulations. Aims. The expression of the energy of a system as the sum of its potential energy and its free energy (Thomson's theorem) is strictly valid when the magnetic field is exactly solenoidal. For numerical realizations on a discrete grid, this property may be only approximately fulfilled. We show that the imperfect solenoidality induces terms in the energy that can lead to misinterpreting the amount of free energy present in a magnetic configuration. Methods. We consider a decomposition of the energy in solenoidal and nonsolenoidal parts which allows the unambiguous estimation of the nonsolenoidal contribution to the energy. We apply this decomposition to six typical cases broadly used in solar physics. We quantify to what extent the Thomson theorem is not satisfied when approximately solenoidal fields are used. Results. The quantified errors on energy vary from negligible to significant errors, depending on the extent of the nonsolenoidal component of the field. We identify the main source of errors and analyze the implications of adding a variable amount of divergence to various solenoidal fields. Finally, we present pathological unphysical situations where the estimated free energy would appear to be negative, as found in some previous works, and we identify the source of this error to be the presence of a finite divergence. Conclusions. We provide a method of quantifying the effect of a finite divergence in numerical fields, together with detailed diagnostics of its sources. We also compare the efficiency of two divergence-cleaning techniques. These results are applicable to a broad range of numerical realizations of magnetic fields.}, language = {en} } @article{HollerSchoeckEgeretal.2012, author = {Holler, M. and Schoeck, F. M. and Eger, P. and Kiessling, D. and Valerius, K. and Stegmann, Christian}, title = {Spatially resolved X-ray spectroscopy and modeling of the nonthermal emission of the pulsar wind nebula in G0.9+0.1}, series = {ASTRONOMY \& ASTROPHYSICS}, volume = {539}, journal = {ASTRONOMY \& ASTROPHYSICS}, number = {2}, publisher = {EDP SCIENCES S A}, address = {LES ULIS CEDEX A}, issn = {0004-6361}, doi = {10.1051/0004-6361/201118121}, pages = {8}, year = {2012}, abstract = {Aims. We performed a spatially resolved spectral X-ray study of the pulsar wind nebula ( PWN) in the supernova remnant G0.9+ 0.1. Furthermore, we modeled its nonthermal emission in the X-ray and very high-energy (VHE, E > 100 GeV) gamma-ray regime. Methods. Using Chandra ACIS-S3 data, we investigated the east-west dependence of the spectral properties of G0.9+ 0.1 by calculating hardness ratios. We analyzed the EPIC-MOS and EPIC-pn data of two on-axis observations of the XMM-Newton telescope and extracted spectra of four annulus-shaped regions, centered on the region of brightest emission of the source. A radially symmetric leptonic model was applied in order to reproduce the observed X-ray emission of the inner part of the PWN. Using the optimized model parameter values obtained from the X-ray analysis, we then compared the modeled inverse Compton (IC) radiation with the published H.E.S.S. gamma-ray data. Results. The spectral index within the four annuli increases with growing distance to the pulsar, whereas the surface brightness drops. With the adopted model we are able to reproduce the characteristics of the X-ray spectra. The model results for the VHE. radiation, however, strongly deviate from the H.E.S.S. data.}, language = {en} } @article{TepperGarciaRichterSchayeetal.2012, author = {Tepper-Garcia, Thorsten and Richter, Philipp and Schaye, Joop and Booth, C. M. and Dalla Vecchia, Claudio and Theuns, Tom}, title = {Absorption signatures of warm-hot gas at low redshift: broad H?i Lya absorbers}, series = {Monthly notices of the Royal Astronomical Society}, volume = {425}, journal = {Monthly notices of the Royal Astronomical Society}, number = {3}, publisher = {Wiley-Blackwell}, address = {Hoboken}, issn = {0035-8711}, doi = {10.1111/j.1365-2966.2012.21545.x}, pages = {1640 -- 1663}, year = {2012}, abstract = {We investigate the physical state of H?i absorbing gas at low redshift (z = 0.25) using a subset of cosmological, hydrodynamic simulations from the OverWhelmingly Large Simulations project, focusing in particular on broad (bHI=40 km s-1) H?i Lya absorbers (BLAs), which are believed to originate in shock-heated gas in the warm-hot intergalactic medium (WHIM). Our fiducial model, which includes radiative cooling by heavy elements and feedback by supernovae and active galactic nuclei, predicts that by z = 0.25 nearly 60?per cent of the gas mass ends up at densities and temperatures characteristic of the WHIM and we find that half of this fraction is due to outflows. The standard H?i observables (distribution of H?i column densities NH?I, distribution of Doppler parameters bHI, bHINH?I correlation) and the BLA line number density predicted by our simulations are in remarkably good agreement with observations. BLAs arise in gas that is hotter, more highly ionized and more enriched than the gas giving rise to typical Lya forest absorbers. The majority of the BLAs arise in warm-hot [log?(T/?K) similar to 5] gas at low (log?? < 1.5) overdensities. On average, thermal broadening accounts for at least 60?per cent of the BLA linewidth, which in turn can be used as a rough indicator of the thermal state of the gas. Detectable BLAs account for only a small fraction of the true baryon content of the WHIM at low redshift. In order to detect the bulk of the mass in this gas phase, a sensitivity at least one order of magnitude better than achieved by current ultraviolet spectrographs is required. We argue that BLAs mostly trace gas that has been shock heated and enriched by outflows and that they therefore provide an important window on a poorly understood feedback process.}, language = {en} } @article{NiemiecPohlBretetal.2012, author = {Niemiec, Jacek and Pohl, Martin and Bret, Antoine and Wieland, Volkmar}, title = {Nonrelativistic parallel shocks in unmagnetized and weakly magnetized plasmas}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {759}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {1}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.1088/0004-637X/759/1/73}, pages = {20}, year = {2012}, abstract = {We present results of 2D3V particle-in-cell simulations of nonrelativistic plasma collisions with absent or parallel large-scale magnetic field for parameters applicable to the conditions at young supernova remnants. We study the collision of plasma slabs of different density, leading to two different shocks and a contact discontinuity. Electron dynamics play an important role in the development of the system. While nonrelativistic shocks in both unmagnetized and magnetized plasmas can be mediated by Weibel-type instabilities, the efficiency of shock-formation processes is higher when a large-scale magnetic field is present. The electron distributions downstream of the forward and reverse shocks are generally isotropic, whereas that is not always the case for the ions. We do not see any significant evidence of pre-acceleration, neither in the electron population nor in the ion distribution.}, language = {en} } @article{TepperGarciaRichterSchayeetal.2011, author = {Tepper-Garcia, Thorsten and Richter, Philipp and Schaye, Joop and Booth, C. M. and Vecchia, Claudio Dalla and Theuns, Tom and Wiersma, Robert P. C.}, title = {Absorption signatures of warm-hot gas at low redshift o vi}, series = {Monthly notices of the Royal Astronomical Society}, volume = {413}, journal = {Monthly notices of the Royal Astronomical Society}, number = {1}, publisher = {Wiley-Blackwell}, address = {Hoboken}, issn = {0035-8711}, doi = {10.1111/j.1365-2966.2010.18123.x}, pages = {190 -- 212}, year = {2011}, abstract = {We investigate the origin and physical properties of O vi absorbers at low redshift (z = 0.25) using a subset of cosmological, hydrodynamical simulations from the OverWhelmingly Large Simulations (OWLS) project. Intervening O vi absorbers are believed to trace shock-heated gas in the warm-hot intergalactic medium (WHIM) and may thus play a key role in the search for the missing baryons in the present-day Universe. When compared to observations, the predicted distributions of the different O vi line parameters (column density, Doppler parameter, rest equivalent width W-r) from our simulations exhibit a lack of strong O vi absorbers, a discrepancy that has also been found by Oppenheimer \& Dave. This suggests that physical processes on subgrid scales (e.g. turbulence) may strongly influence the observed properties of O vi systems. We find that the intervening O vi absorption arises mainly in highly metal enriched (10-1 < Z/Z(circle dot) less than or similar to 1) gas at typical overdensities of 1 < /<<>> less than or similar to 102. One-third of the O vi absorbers in our simulation are found to trace gas at temperatures T < 105 K, while the rest arises in gas at higher temperatures, most of them around T = 105.3 +/- 0.5 K. These temperatures are much higher than inferred by Oppenheimer \& Dave, probably because that work did not take the suppression of metal-line cooling by the photoionizing background radiation into account. While the O vi resides in a similar region of (, T)-space as much of the shock-heated baryonic matter, the vast majority of this gas has a lower metal content and does not give rise to detectable O vi absorption. As a consequence of the patchy metal distribution, O vi absorbers in our simulations trace only a very small fraction of the cosmic baryons (< 2 per cent) and the cosmic metals. Instead, these systems presumably trace previously shock-heated, metal-rich material from galactic winds that is now mixing with the ambient gas and cooling. The common approach of comparing O vi and H i column densities to estimate the physical conditions in intervening absorbers from QSO observations may be misleading, as most of the H i (and most of the gas mass) is not physically connected with the high-metallicity patches that give rise to the O vi absorption.}, language = {en} } @article{MizunoPohlNiemiecetal.2011, author = {Mizuno, Yosuke and Pohl, Martin and Niemiec, Jacek and Zhang, Bing and Nishikawa, Ken-Ichi and Hardee, Philip E.}, title = {Magnetic-field amplification by turbulence in a relativistic shockpropagating through an inhomogeneous medium}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {726}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {2}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.1088/0004-637X/726/2/62}, pages = {11}, year = {2011}, abstract = {We perform two-dimensional relativistic magnetohydrodynamic simulations of a mildly relativistic shock propagating through an inhomogeneous medium. We show that the postshock region becomes turbulent owing to preshock density inhomogeneity, and the magnetic field is strongly amplified due to the stretching and folding of field lines in the turbulent velocity field. The amplified magnetic field evolves into a filamentary structure in two-dimensional simulations. The magnetic energy spectrum is flatter than the Kolmogorov spectrum and indicates that a so-called small-scale dynamo is occurring in the postshock region. We also find that the amount of magnetic-field amplification depends on the direction of the mean preshock magnetic field, and the timescale of magnetic-field growth depends on the shock strength.}, language = {en} }