1,414 research outputs found
Probing dissipation mechanisms in BL Lac jets through X-ray polarimetry
The dissipation of energy flux in blazar jets plays a key role in the
acceleration of relativistic particles. Two possibilities are commonly
considered for the dissipation processes, magnetic reconnection -- possibly
triggered by instabilities in magnetically-dominated jets -- , or shocks -- for
weakly magnetized flows. We consider the polarimetric features expected for the
two scenarios analyzing the results of state-of-the-art simulations. For the
magnetic reconnection scenario we conclude, using results from global
relativistic MHD simulations, that the emission likely occurs in turbulent
regions with unstructured magnetic fields, although the simulations do not
allow us to draw firm conclusions. On the other hand, with local
particle-in-cell simulations we show that, for shocks with a magnetic field
geometry suitable for particle acceleration, the self-generated magnetic field
at the shock front is predominantly orthogonal to the shock normal and becomes
quasi-parallel downstream. Based on this result we develop a simplified model
to calculate the frequency-dependent degree of polarization, assuming that
high-energy particles are injected at the shock and cool downstream. We apply
our results to HBLs, blazars with the maximum of their synchrotron output at
UV-soft X-ray energies. While in the optical band the predicted degree of
polarization is low, in the X-ray emission it can ideally reach 50\%,
especially during active/flaring states. The comparison between measurements in
the optical and in the X-ray band made during active states (feasible with the
planned {\it IXPE} satellite) are expected to provide valuable constraints on
the dissipation and acceleration processes.Comment: 9 pages, 6 figures, accepted for publication by MNRA
Heraclitus in Verse: The Poetic Fragments of Scythinus of Teos
The two fragments of Scythinus\u2019 poetic version of Heraclitus help to clarify several aspects of the philosopher\u2019s thought that are obscure in his own fragments, especially as pertains to time, \u3c7\u3c1\u3cc\u3bd\u3bf\u3c2
Radio light curves during the passage of cloud G2 near Sgr A*
We calculate radio light curves produced by the bow shock that is likely to
form in front of the G2 cloud when it penetrates the accretion disk of Sgr A*.
The shock acceleration of the radio-emitting electrons is captured
self-consistently by means of first-principles particle-in-cell simulations. We
show that the radio luminosity is expected to reach maximum in early 2013,
roughly a month after the bow shock crosses the orbit pericenter. We estimate
the peak radio flux at 1.4 GHz to be 1.4 - 22 Jy depending on the assumed orbit
orientation and parameters. We show that the most promising frequencies for
radio observations are in the 0.1<nu<1 GHz range, for which the bow shock
emission will be much stronger than the intrinsic radio flux for all the models
considered.Comment: 15 pages, 10 figures, accepted for publication in MNRA
Particle-in-cell simulations of shock-driven reconnection in relativistic striped winds
By means of two- and three-dimensional particle-in-cell simulations, we
investigate the process of driven magnetic reconnection at the termination
shock of relativistic striped flows. In pulsar winds and in magnetar-powered
relativistic jets, the flow consists of stripes of alternating magnetic field
polarity, separated by current sheets of hot plasma. At the wind termination
shock, the flow compresses and the alternating fields annihilate by driven
magnetic reconnection. Irrespective of the stripe wavelength "lambda" or the
wind magnetization "sigma" (in the regime sigma>>1 of magnetically-dominated
flows), shock-driven reconnection transfers all the magnetic energy of
alternating fields to the particles, whose average Lorentz factor increases by
a factor of sigma with respect to the pre-shock value. In the limit
lambda/(r_L*sigma)>>1, where r_L is the relativistic Larmor radius in the wind,
the post-shock particle spectrum approaches a flat power-law tail with slope
around -1.5, populated by particles accelerated by the reconnection electric
field. The presence of a current-aligned "guide" magnetic field suppresses the
acceleration of particles only when the guide field is stronger than the
alternating component. Our findings place important constraints on the models
of non-thermal radiation from Pulsar Wind Nebulae and relativistic jets.Comment: 25 pages, 14 figures, movies available at
https://www.cfa.harvard.edu/~lsironi/sironi_movies.tar ; in press, special
issue of Computational Science and Discovery on selected research from the
22nd International Conference on Numerical Simulation of Plasma
Probabilistic evaluation of n traces with no putative source: A likelihood ratio based approach in an investigative framework
Analysis of marks recovered from different crime scenes can be useful to detect a linkage between criminal cases, even though a putative source for the recovered traces is not available. This particular circumstance is often encountered in the early stage of investigations and thus, the evaluation of evidence association may provide useful information for the investigators. This association is evaluated here from a probabilistic point of view: a likelihood ratio based approach is suggested in order to quantify the strength of the evidence of trace association in the light of two mutually exclusive propositions, namely that the n traces come from a common source or from an unspecified number of sources. To deal with this kind of problem, probabilistic graphical models are used, in form of Bayesian networks and object-oriented Bayesian networks, allowing users to intuitively handle with uncertainty related to the inferential problem
A dual output polarimeter devoted to the study of the Cosmic Microwave Background
We have developed a correlation radiometer at 33 GHz devoted to the search
for residual polarization of the Cosmic Microwave Background (CMB). The two
instruments`s outputs are linear combination of two Stokes Parameters (Q and U
or U and V). The instrument is therefore directly sensitive to the polarized
component of the radiation (respectively linear and circular). The radiometer
has a beam-width oif 7 or 14 deg, but it can be coupled to a telescope
increasing the resolution. The expected CMB polarization is at most a part per
milion. The polarimeter has been designed to be sensitive to this faint signal,
and it has been optimized to improve its long term stability, observing from
the ground. In this contribution the performances of the instrument are
presented, together with the preliminary test and observations.Comment: 12 pages, 6 figures, in print on the Proc. SPIE Conf. - August 200
La nutrice di Saffo in P.Oxy. 2289 e i paralleli omerici nel ''Carme dei fratelli''
Secondo l\u2019ipotesi avanzata da Anja Bettenworth, nel cosiddetto "Carme dei fratelli" Saffo si rivolge probabilmente a una nutrice anzich\ue9 alla madre. Alla luce di ci\uf2, il termine omerico \u3bc\u3b1\u1fd6\u3b1 potrebbe essere recuperato nel frammentario adonio di P.Oxy. 2289. Inoltre, i paralleli omerici riscontrabili nel carme di Saffo inducono a pensare che questa traesse ispirazione da alcuni passaggi dell\u2019Odissea, cosa che sembra giustificare tale integrazione
Acceleration in perpendicular relativistic shocks for plasmas consisting of leptons and hadrons
We investigate the acceleration of light particles in perpendicular shocks
for plasmas consisting of a mixture of leptonic and hadronic particles.
Starting from the full set of conservation equations for the mixed plasma
constituents, we generalize the magneto-hydrodynamical jump conditions for a
multi-component plasma, including information about the specific adiabatic
constants for the different species. The impact of deviations from the standard
model of an ideal gas is compared in theory and particle-in-cell simulations,
showing that the standard-MHD model is a good approximation. The simulations of
shocks in electron-positron-ion plasmas are for the first time
multi-dimensional, transverse effects are small in this configuration and 1D
simulations are a good representation if the initial magnetization is chosen
high. 1D runs with a mass ratio of 1836 are performed, which identify the
Larmor frequency \omega_{ci} as the dominant frequency that determines the
shock physics in mixed component plasmas. The maximum energy in the non-thermal
tail of the particle spectra evolves in time according to a power-law
proportional to t^\alpha with \alpha in the range 1/3 < \alpha < 1, depending
on the initial parameters. A connection is made with transport theoretical
models by Drury (1983) and Gargate & Spitkovsky (2011), which predict an
acceleration time proportional to \gamma and the theory for small wavelength
scattering by Kirk & Reville (2010), which predicts a behavior rather as
proportional to \gamma^2. Furthermore, we compare different magnetic field
orientations with B_0 inside and out of the plane, observing qualitatively
different particle spectra than in pure electron-ion shocks
Adaptive General Search Framework for Games and Beyond
The research field of Artificial General Intelligence (AGI) is concerned with the creation of adaptive programs that can autonomously address tasks of a different nature. Search and planning have been identified as core capabilities of AGI, and have been successful in many scenarios that require sequential decision-making. However, many search algorithms are developed for specific problems and exploit domain-specific knowledge, which makes them not applicable to perform different tasks autonomously. Although some domain-independent search algorithms have been proposed, a programmer still has to make decisions on their design, setup and enhancements. Thus, the performance is limited by the programmer's decisions, which are usually biased. This paper proposes to develop a framework that, in line with the goals of AGI, autonomously addresses a wide variety of search tasks, adapting automatically to each new, unknown task. To achieve this, we propose to encode search algorithms in a formal language and combine algorithm portfolios with automatic algorithm generation. In addition, we see games as the ideal test bed for the framework, because they can model a wide variety of complex problems. Finally, we believe that this research will have an impact not only on the AGI research field, but also on the game industry and on real-world problems
La 'Batracomiomachia' di Alessandro Garioni tra greco, italiano e milanese
Nel 1793 il domenicano Alessandro Garioni pubblic\uf2 a Milano un\u2019edizione della Batracomiomachia corredata di traduzione letterale italiana e \u201cparafrasi\u201d poetica in milanese. Nell\u2019introduzione, Garioni attribuisce con convinzione il poemetto ad Omero. In quest\u2019ottica, si tratta dunque del primo tentativo di tradurre Omero in milanese. L\u2019analisi di quest\u2019opera e del suo rapporto con la letteratura precedente, erudita e non solo, si propone di collocarla nella storia della fortuna dei Classici e della letteratura milanese. In particolare, si evidenzia come la parafrasi milanese non possa considerarsi parodia del poemetto greco.In 1793 the Dominican monk Alessandro Garioni published an edition of the Battle of mice and frogs with a literal Italian translation and poetic \u201cparaphrasing\u201d in Milanese. In the introduction, Garioni ascribes the poem to Homer. In his view, this was the first attempt to translate Homer into Milanese. The analysis of the work and its relationship to earlier literature, both scholarly and not, aims to place it within the tradition of Classical and Milanese literature. In particular, it is noted that the paraphrasing in Milanese is not a parody of the Greek poem
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