414 research outputs found
Linear analysis of the Kelvin-Helmholtz instability in relativistic magnetized symmetric flows
We study the linear stability of a planar interface separating two fluids in
relative motion, focusing on the symmetric configuration where the two fluids
have the same properties (density, temperature, magnetic field strength, and
direction). We consider the most general case with arbitrary sound speed
, Alfv\'en speed , and magnetic field orientation. For
the instability associated with the fast mode, we find that the lower bound of
unstable shear velocities is set by the requirement that the projection of the
velocity onto the fluid-frame wavevector is larger than the projection of the
Alfv\'en speed onto the same direction, i.e., shear should overcome the effect
of magnetic tension. In the frame where the two fluids move in opposite
directions with equal speed , the upper bound of unstable velocities
corresponds to an effective relativistic Mach number , where is the fast speed assuming a
magnetic field perpendicular to the wavevector (here, all velocities are in
units of the speed of light), and is the laboratory-frame angle
between the flow velocity and the wavevector projection onto the shear
interface. Our results have implications for shear flows in the magnetospheres
of neutron stars and black holes -- both for single objects and for merging
binaries -- where the Alfv\'en speed may approach the speed of light.Comment: 11 pages, 7 figures, 1 table, Accepted for publication in Monthly
Notices of the Royal Astronomical Societ
The nature of the X-ray halo of the plerion G21.5-0.9 unveiled by XMM-Newton and Chandra
The nature of the radio-quiet X-ray halo around the plerionic SNR G21.5-0.9
is under debate. On the basis of spatial and spectral analysis of a large
Chandra and XMM-Newton dataset of this source, we have developed a
self-consistent scenario which explains all the observational features. We
found that the halo is composed by diffuse extended emission due to dust
scattering of X-rays from the plerion, by a bright limb which traces particle
acceleration in the fast forward shock of the remnant, and by a bright spot
(the ``North Spur'') which may be a knot of ejecta in adiabatic expansion. By
applying a model of interaction between the PWN, the SNR and supernova
environment, we argue that G21.5-0.9 progenitor may be of Type IIP or Ib/Ic,
and that the remnant may be young (200-1000 yr).Comment: 11 pages, 10 figures, accepted by A&A, also avalaible at
http://www.astropa.unipa.it/Library/OAPA_preprints/fb2870.ps.g
A habituation account of change detection in same/different judgments
We investigated the basis of change detection in a short-term priming task. In two experiments, participants were asked to indicate whether or not a target word was the same as a previously presented cue. Data from an experiment measuring magnetoencephalography failed to find different patterns for “same” and “different” responses, consistent with the claim that both arise from a common neural source, with response magnitude defining the difference between immediate novelty versus familiarity. In a behavioral experiment, we tested and confirmed the predictions of a habituation account of these judgments by comparing conditions in which the target, the cue, or neither was primed by its presentation in the previous trial. As predicted, cue-primed trials had faster response times, and target-primed trials had slower response times relative to the neither-primed baseline. These results were obtained irrespective of response repetition and stimulus–response contingencies. The behavioral and brain activity data support the view that detection of change drives performance in these tasks and that the underlying mechanism is neuronal habituation
The Relation Between the Surface Brightness and the Diameter for Galactic Supernova Remnants
In this work, we have constructed a relation between the surface brightness
() and diameter (D) of Galactic C- and S-type supernova remnants
(SNRs). In order to calibrate the -D dependence, we have carefully
examined some intrinsic (e.g. explosion energy) and extrinsic (e.g. density of
the ambient medium) properties of the remnants and, taking into account also
the distance values given in the literature, we have adopted distances for some
of the SNRs which have relatively more reliable distance values. These
calibrator SNRs are all C- and S-type SNRs, i.e. F-type SNRs (and S-type SNR
Cas A which has an exceptionally high surface brightness) are excluded. The
Sigma-D relation has 2 slopes with a turning point at D=36.5 pc: (at 1
GHz)=8.4 D
WmHzster (for
WmHzster and D36.5 pc) and (at 1
GHz)=2.7 10 D
WmHzster (for
WmHzster and D36.5 pc). We discussed the theoretical
basis for the -D dependence and particularly the reasons for the change
in slope of the relation were stated. Added to this, we have shown the
dependence between the radio luminosity and the diameter which seems to have a
slope close to zero up to about D=36.5 pc. We have also adopted distance and
diameter values for all of the observed Galactic SNRs by examining all the
available distance values presented in the literature together with the
distances found from our -D relation.Comment: 45 pages, 2 figures, accepted for publication in Astronomical and
Astrophysical Transaction
The Local Bubble, Local Fluff, and Heliosphere
The properties of the Local Bubble, Local Fluff complex of nearby
interstellar clouds, and the heliosphere are mutually constrained by data and
theory. Observations and models of the diffuse radiation field, interstellar
ionization, pick-up ion and anomalous cosmic-ray populations, and interstellar
dust link the physics of these regions. The differences between the
one-asymmetric-superbubble and two-superbubble views of the Local Bubble are
discussed.Comment: 10 pages, 2 figure
Entanglements of faith: Discourses, practices of care and homeless people in an Italian City of Saints
This is the author accepted manuscript. The final version is available from SAGE via http://dx.doi.org/10.1177/0042098013514620This paper investigates how Catholic-inspired services for homeless people are delivered in Turin, Italy. The purpose is to critically interrogate particular faith-based organisations’ moral discourses on homelessness, and to show how they are enacted through practices of care directed at the homeless subject. The paper contributes to the geographical literature on faith-based organisations addressing its shortcomings – namely the lack of critical and contextual focus on faith-based organisations’ ‘love for the poor’. To address this point, the paper takes a vitalist perspective on the urban and introduces the notion of the ‘entanglements of faith’, which allows an integrated and grounded perspective on faith-based organisations’ interventions. The outcomes of the work suggest that these faith-based organisations propose standardised services that, producing particular assemblages and affective atmospheres, have deep emotional and relational effects on their recipients. Further lines of research are sketched in the conclusions
Recall termination in free recall
Although much is known about the dynamics of memory search in the free recall task, relatively little is known about the factors related to recall termination. Reanalyzing individual trial data from 14 prior studies (1,079 participants in 28,015 trials) and defining termination as occurring when a final response is followed by a long nonresponse interval, we observed that termination probability increased throughout the recall period and that retrieval was more likely to terminate following an error than following a correct response. Among errors, termination probability was higher following prior-list intrusions and repetitions than following extralist intrusions. To verify that this pattern of results can be seen in a single study, we report a new experiment in which 80 participants contributed recall data from a total of 9,122 trials. This experiment replicated the pattern observed in the aggregate analysis of the prior studies
Resolving the inner parsec of the blazar J1924-2914 with the Event Horizon Telescope
The blazar J1924-2914 is a primary Event Horizon Telescope (EHT) calibrator
for the Galactic Center's black hole Sagittarius A*. Here we present the first
total and linearly polarized intensity images of this source obtained with the
unprecedented 20 as resolution of the EHT. J1924-2914 is a very compact
flat-spectrum radio source with strong optical variability and polarization. In
April 2017 the source was observed quasi-simultaneously with the EHT (April
5-11), the Global Millimeter VLBI Array (April 3), and the Very Long Baseline
Array (April 28), giving a novel view of the source at four observing
frequencies, 230, 86, 8.7, and 2.3 GHz. These observations probe jet properties
from the subparsec to 100-parsec scales. We combine the multi-frequency images
of J1924-2914 to study the source morphology. We find that the jet exhibits a
characteristic bending, with a gradual clockwise rotation of the jet projected
position angle of about 90 degrees between 2.3 and 230 GHz. Linearly polarized
intensity images of J1924-2914 with the extremely fine resolution of the EHT
provide evidence for ordered toroidal magnetic fields in the blazar compact
core
Massive Black Hole Binaries as LISA Precursors in the Roman High Latitude Time Domain Survey
With its capacity to observe faint active galactic nuclei
(AGN) out to redshift , Roman is poised to reveal a population of
black holes during an epoch of vigorous galaxy
assembly. By measuring the light curves of a subset of these AGN and looking
for periodicity, Roman can identify several hundred massive black hole binaries
(MBHBs) with 5-12 day orbital periods, which emit copious gravitational
radiation and will inevitably merge on timescales of years. During
the last few months of their merger, such binaries are observable with the
Laser Interferometer Space Antenna (LISA), a joint ESA/NASA gravitational wave
mission set to launch in the mid-2030s. Roman can thus find LISA precursors,
provide uniquely robust constraints on the LISA source population, help
identify the host galaxies of LISA mergers, and unlock the potential of
multi-messenger astrophysics with massive black hole binaries.Comment: White Paper for the Nancy Grace Roman Space Telescope's Core
Community Surveys (https://roman.gsfc.nasa.gov/science/ccs_white_papers.html
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