177 research outputs found
Modelling the Kinked Jet of the Crab Nebula
We investigate the dynamical propagation of the South-East jet from the Crab
pulsar interacting with supernova ejecta by means of three-dimensional
relativistic MHD numerical simulations with the PLUTO code.
The initial jet structure is set up from the inner regions of the Crab
Nebula.
We study the evolution of hot, relativistic hollow outflows initially
carrying a purely azimuthal magnetic field.
Our jet models are characterized by different choices of the outflow
magnetization ( parameter) and the bulk Lorentz factor ().
We show that the jet is heavily affected by the growth of current-driven kink
instabilities causing considerable deflection throughout its propagation
length.
This behavior is partially stabilized by the combined action of larger flow
velocities and/or reduced magnetic field strengths.
We find that our best jet models are characterized by relatively large values
of () and small values of .
Our results are in good agreement with the recent X-ray (\textit{Chandra})
data of the Crab Nebula South-East jet indicating that the jet changes
direction of propagation on a time scale of the order of few years.
The 3D models presented here may have important implications in the
investigation of particle acceleration in relativistic outflows.Comment: 15 pages, 20 figure
The Surprising Crab Nebula
We will present our study of the flux and spectral variability of the Crab
above 100 MeV on different timescales ranging from days to weeks. In addition
to the four main intense and day-long flares detected by AGILE and Fermi-LAT
between Sept. 2007 and Sept. 2012, we find evidence for week-long and less
intense episodes of enhanced gamma-ray emission that we call "waves".
Statistically significant "waves" show timescales of 1-2 weeks, and can occur
by themselves or in association with shorter flares. The Sept. - Oct. 2007
gamma-ray enhancement episode detected by AGILE shows both "wave" and flaring
behavior. We extend our analysis to the publicly available Fermi-LAT dataset
and show that several additional "wave" episodes can be identified. We discuss
the spectral properties of the September 2007 "wave"/flare event and show that
the physical properties of the "waves" are intermediate between steady and
flaring states. Plasma instabilities inducing "waves" appear to involve spatial
distances cm and enhanced magnetic fields }mG. Day-long flares are characterized by smaller distances and larger
local magnetic fields. Typically, the deduced total energy associated with the
"wave" phenomenon (, where is the kinetic
energy of the emitting particles) is comparable with that associated to the
flares, and can reach a few percent of the total available pulsar spindown
energy. Most likely, flares and waves are the product of the same class of
plasma instabilities that we show acting on different timescales and radiation
intensities.Comment: 2012 Fermi Symposium proceedings - eConf C12102
Improving Matrix-vector Multiplication via Lossless Grammar-Compressed Matrices
As nowadays Machine Learning (ML) techniques are generating
huge data collections, the problem of how to efficiently engineer
their storage and operations is becoming of paramount importance.
In this article we propose a new lossless compression scheme for
real-valued matrices which achieves efficient performance in terms
of compression ratio and time for linear-algebra operations. Ex-
periments show that, as a compressor, our tool is clearly superior
to gzip and it is usually within 20% of xz in terms of compression
ratio. In addition, our compressed format supports matrix-vector
multiplications in time and space proportional to the size of the
compressed representation, unlike gzip and xz that require the full
decompression of the compressed matrix. To our knowledge our
lossless compressor is the first one achieving time and space com-
plexities which match the theoretical limit expressed by the k-th
order statistical entropy of the input.
To achieve further time/space reductions, we propose column-
reordering algorithms hinging on a novel column-similarity score.
Our experiments on various data sets of ML matrices show that our
column reordering can yield a further reduction of up to 16% in the
peak memory usage during matrix-vector multiplication.
Finally, we compare our proposal against the state-of-the-art
Compressed Linear Algebra (CLA) approach showing that ours runs
always at least twice faster (in a multi-thread setting), and achieves
better compressed space occupancy and peak memory usage. This
experimentally confirms the provably effective theoretical bounds
we show for our compressed-matrix approach
Optimizing Flame Retardancy and Durability in Melamine-Formaldehyde/Solid-Urban-Waste Composite Panels
In our previous study, an innovative method for sterilization, inertization, and valorization of the organic fraction of municipal solid waste (OFMSW), to be recycled in the production of composite panels, was developed. In this follow-up work, the effects of fire retardants on fire performance, durability, and the mechanical properties of the composite panels based on OFMSW and melamine-formaldehyde resin were investigated. The performance of panels without fire retardants (control panels) was compared to panels containing either mono-ammonium phosphate (PFR) or aluminium trihydrate (ATH) at a mass fraction of 1% and 10% (modified panels). As shown by cone calorimetry, the total heat released was already low (about 31 MJ/m2 at 50 kW/m2) in the control panels, further decreased in the modified panels with the addition of fire retardants, and reached the lowest value (about 1.4 MJ/m2) with 10% mass fraction of PFR. Hence, the addition of fire retardants had a beneficial effect on the response to fire of the panels; however, it also reduced the mechanical properties of the panels as measured by flexural tests. The deterioration of the mechanical properties was particularly obvious in panels containing 10% mass fraction of fire retardants, and they were further degraded by artificial accelerated weathering, carried out by boiling tests. Ultimately, the panels containing PFR at a mass fraction of 1% offered the best balance of fire resistance, durability, and mechanical performance within the formulations investigated in this study
Gamma-ray observations of Cygnus X-1 above 100 MeV in the hard and soft states
We present the results of multi-year gamma-ray observations by the AGILE
satellite of the black hole binary system Cygnus X-1. In a previous
investigation we focused on gamma-ray observations of Cygnus X-1 in the hard
state during the period mid-2007/2009. Here we present the results of the
gamma-ray monitoring of Cygnus X-1 during the period 2010/mid-2012 carried out
for which includes a remarkably prolonged `soft state' phase (June 2010 -- May
2011). Previous 1--10 MeV observations of Cyg X-1 in this state hinted at a
possible existence of a non-thermal particle component with substantial
modifications of the Comptonized emission from the inner accretion disk. Our
AGILE data, averaged over the mid-2010/mid-2011 soft state of Cygnus X-1,
provide a significant upper limit for gamma-ray emission above 100 MeV of
F_soft < 20 x 10^{-8} ph/cm^2/s, excluding the existence of prominent
non-thermal emission above 100 MeV during the soft state of Cygnus X-1. We
discuss theoretical implications of our findings in the context of high-energy
emission models of black hole accretion. We also discuss possible gamma-ray
flares detected by AGILE. In addition to a previously reported episode observed
by AGILE in October 2009 during the hard state, we report a weak but important
candidate for enhanced emission which occurred at the end of June 2010
(2010-06-30 10:00 - 2010-07-02 10:00 UT) exactly in coincidence with a
hard-to-soft state transition and before an anomalous radio flare. An appendix
summarizes all previous high-energy observations and possible detections of
Cygnus X-1 above 1 MeV.Comment: 16 pages, 12 figures, 1 table, accepted for publication in Ap
The extraordinary gamma-ray flare of the blazar 3C 454.3
We present the gamma-ray data of the extraordinary flaring activity above 100
MeV from the flat spectrum radio quasar 3C 454.3 detected by AGILE during the
month of December 2009. 3C 454.3, that has been among the most active blazars
of the FSRQ type since 2007, was detected in the gamma-ray range with a
progressively rising flux since November 10, 2009. The gamma-ray flux reached a
value comparable with that of the Vela pulsar on December 2, 2009. Remarkably,
between December 2 and 3, 2009 the source more than doubled its gamma-ray
emission and became the brightest gamma-ray source in the sky with a peak flux
of F_{\gamma,p} = (2000 \pm 400) x 10^-8 ph cm^-2 s^-1 for a 1-day integration
above 100 MeV. The gamma-ray intensity decreased in the following days with the
source flux remaining at large values near F \simeq (1000 \pm 200) x 10^-8 ph
cm^-2 s^-1 for more than a week. This exceptional gamma-ray flare dissipated
among the largest ever detected intrinsic radiated power in gamma-rays above
100 MeV (L_{\gamma, source, peak} \simeq 3 x 10^46 erg s^-1, for a relativistic
Doppler factor of {\delta} \simeq 30). The total isotropic irradiated energy of
the month-long episode in the range 100 MeV - 3 GeV is E_{\gamma,iso} \simeq
10^56 erg. We report the intensity and spectral evolution of the gamma-ray
emission across the flaring episode. We briefly discuss the important
theoretical implications of our detection.Comment: 17 pages, 3 figures, ApJ accepte
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