502 research outputs found
A new flaring high energy gamma-ray source
We report the detection of a new gamma-ray source in the Fermi-LAT sky using
a source detection tool based on the Minimal Spanning Tree algorithm. The
source, not reported in previous LAT catalogues but very recently observed in
the X-rays and optical bands, is characterized by an increasing gamma-ray
activity in 2012 June-September, reaching a weekly peak flux of
(3.3+-0.6)*10^-7 photons cm^-2 s^-1. A search for a possible counterpart
provides indication that it can be associated with the radio source NVSS
J141828+354250 whose optical SDSS colours are typical of a blazar.Comment: 4 pages, 3 figures. Accepted for publication in Astronomy &
Astrophysic
The Fermi blazars' divide based on the diagnostic of the SEDs peak frequencies
We have studied the quasi-simultaneous Spectral Energy Distributions (SED) of
48 LBAS blazars, detected within the three months of the LAT Bright AGN Sample
(LBAS) data taking period, combining Fermi and Swift data with radio
NIR-Optical and hard-X/gamma-ray data. Using these quasi-simultaneous SEDs,
sampling both the low and the high energy peak of the blazars broad band
emission, we were able to apply a diagnostic tool based on the estimate of the
peak frequencies of the synchrotron (S) and Inverse Compton (IC) components.
Our analysis shows a Fermi blazars' divide based on the peak frequencies of the
SED. The robust result is that the Synchrotron Self Compton (SSC) region
divides in two the plane were we plot the peak frequency of the synchrotron SED
vs the typical Lorentz factor of the electrons most contributing to the
synchrotron emission and to the inverse Compton process. Objects within or
below this region, radiating likely via the SSC process, are
high-frequency-peaked BL Lac object (HBL), or low/intermediate-frequency peaked
BL Lac object (LBL/IBL). All of the IBLs/LBLs within or below the SSC region
are not Compton dominated. The objects lying above the SSC region, radiating
likely via the External radiation Compton (ERC) process, are Flat Spectrum
Radio Quasars and IBLs/LBLs. All of the IBLs/LBLs in the ERC region show a
significant Compton dominance.Comment: Contribution to the Workshop SciNeGHe 2009/Gamma-ray Physics in the
LHC era (Assisi - Italy, Oct. 7-9 2009
Stochastic Acceleration and the Evolution of Spectral Distributions in SSC Sources: A Self Consistent Modeling of Blazars' Flares
The broad-band spectral distributions of non-thermal sources, such as those
of several known blazars, are well described by a log-parabolic fit. The second
degree term in these fits measures the curvature in the spectrum. In this paper
we investigate whether the curvature parameter observed in the spectra of the
synchrotron emission can be used as a fingerprint of stochastic acceleration.
As a first approach we use the multiplicative Central Limit theorem to show how
fluctuations in the energy gain result in the broadening of the spectral shape,
introducing a curvature into the energy distribution. Then, by means of a
Monte-Carlo description, we investigate how the curvature produced in the
electron distribution is linked to the diffusion in momentum space. To get a
more generic description we turn to the diffusion equation in momentum space.
We first study some "standard" scenarios, in order to understand the conditions
that make the curvature in the spectra significant, and the relevance of the
cooling over the acceleration process. We try to quantify the correlation
between the curvature and the diffusive process in the pre-equilibrium stage,
and investigate how the transition between the Klein-Nishina and the Thompson
regime, in Inverse Compton cooling, can determine the curvature in the
distribution at the equilibrium. We apply these results to some observed
trends, such as the anticorrelation between the peak energy and the curvature
term observed in the spectra of Mrk 421, and a sample of BL Lac objects whose
synchrotron emission peaks at X-ray energies.Comment: 16 pages, 13 figures, Accepted for publication in The Astrophysical
Journa
Processor-in-the-loop architecture design and experimental validation for an autonomous racing vehicle
Self-driving vehicles have experienced an increase in research interest in the last decades. Nevertheless, fully autonomous vehicles are still far from being a common means of transport. This paper presents the design and experimental validation of a processor-in-the-loop (PIL) architecture for an autonomous sports car. The considered vehicle is an all-wheel drive full-electric single-seater prototype. The retained PIL architecture includes all the modules required for autonomous driving at system level: environment perception, trajectory planning, and control. Specifically, the perception pipeline exploits obstacle detection algorithms based on Artificial Intelligence (AI), and the trajectory planning is based on a modified Rapidly-exploring Random Tree (RRT) algorithm based on Dubins curves, while the vehicle is controlled via a Model Predictive Control (MPC) strategy. The considered PIL layout is implemented firstly using a low-cost card-sized computer for fast code verification purposes. Furthermore, the proposed PIL architecture is compared in terms of performance to an alternative PIL using high-performance real-time target computing machine. Both PIL architectures exploit User Datagram Protocol (UDP) protocol to properly communicate with a personal computer. The latter PIL architecture is validated in real-time using experimental data. Moreover, they are also validated with respect to the general autonomous pipeline that runs in parallel on the personal computer during numerical simulation
Log-parabolic spectra and particle acceleration in blazars. III: SSC emission in the TeV band from Mkn 501
Curved broad-band spectral distributions of non-thermal sources like blazars
are described well by a log-parabolic (LP) law where the second degree term
measures the curvature. LP energy spectra can be obtained for relativistic
electrons by means of a statistical acceleration mechanism whose probability of
acceleration depends on energy. In this paper we compute the spectra radiated
by an electron population via synchrotron (S) and Synchro-Self Compton(SSC)
processes to derive the relations between the LP parameters. These spectra were
obtained by means of an accurate numerical code. We found that the ratio
between the curvature parameters of the S spectrum to that of the electrons is
equal to about 0.2 instead of 0.25, the value foreseen in the delta
approximation. Inverse Compton spectra are also intrinsically curved and can be
approximated by a log-parabola only in limited ranges. The curvature parameter,
estimated around the SED peak, may vary from a lower value than that of the S
spectrum up to that of emitting electrons depending on whether the scattering
is in the Thomson or in the Klein-Nishina regime. We applied this analysis to
computing the SSC emission from the BL Lac object Mkn 501 during the large
flare of April 1997. We fit simultaneous BeppoSAX and CAT data and reproduced
intensities and spectral curvatures of both components with good accuracy. The
large curvature observed in the TeV range was found to be mainly intrinsic, and
therefore did not require a large pair production absorption against the
extragalactic background. We regard this finding as an indication that the
Universe is more transparent at these energies than previously assumed by
several models found in the literature. This conclusion is supported by recent
detection of two relatively high redshift blazars with H.E.S.S.Comment: Comments: 12 pages, 11 figures. Accepted for publication in the
Astronomy and Astrophysic
Variability of the Spectral Energy Distribution of the Blazar S5 0716+714
The emission from blazars is known to be variable at all wavelengths. The
flux variability is often accompanied by spectral changes. Spectral energy
distribution (SED) changes must be associated with changes in the spectra of
emitting electrons and/or the physical parameters of the jet. Meaningful
modeling of blazar broadband spectra is required to understand the extreme
conditions within the emission region. Not only is the broadband SED crucial,
but also information about its variability is needed to understand how the
highest states of emission occur and how they differ from the low states. This
may help in discriminating between models. Here we present the results of our
SED modeling of the blazar S5 0716+714 during various phases of its activity.
The SEDs are classified into different bins depending on the optical brightness
state of the source.Comment: 4 pages, 3 figures, contributed talk presented at the conference
Multifrequency Variability of Blazars, Guangzhou, China, September 22-24,
2010. To appear in Journal of Astrophysics and Astronomy (JAA
No efficacy of transcranial direct current stimulation on chronic migraine with medication overuse : a double blind, randomised clinical trial
Background: Transcranial direct current stimulation was suggested to provide beneficial effects in chronic migraine, a condition often associated with medication overuse for which no long-term therapy is available.
Methods: We conducted a randomised controlled trial to assess long-term efficacy of transcranial direct current stimulation. Adults diagnosed with chronic migraine and medication overuse were assigned to receive in a 1:1:1 ratio anodal, cathodal, or sham transcranial direct current stimulation daily for five consecutive days, along with standardised drug withdrawal protocol. Primary outcome was 50% reduction of days of headache per month at 12 months. Co-secondary outcomes were 50% reduction of days of headache per month at 6 months, reduction of analgesic intake per month, and change in disability and quality of life, catastrophising, depression, state and trait anxiety, dependence attitude and allodynia intensity. Patients were not allowed to take any migraine prophylaxis drug for the entire study period.
Results: We randomly allocated 135 patients to anodal (44), cathodal (45), and sham (46) transcranial direct current stimulation. At 6 and 12 months, the percentage of reduction of days of headache and number of analgesics per month ranged between 48.5% and 64.7%, without differences between transcranial direct current stimulation (cathodal, anodal, or the results obtained from the two arms of treatment, anodal plus cathodal) and sham. Catastrophising attitude significantly reduced at 12 months in all groups. There was no difference for the other secondary outcomes.
Conclusions: Transcranial direct current stimulation did not influence the short and long-term course of chronic migraine with medication overuse after acute drug withdrawal. Behavioral and educational measures and support for patients' pain management could provide long-term improvement and low relapse rate. Trial registration number NCT04228809
SWIFT observations of TeV BL Lac objects
Context: We present the results of a set of observations of nine TeV detected
BL Lac objects performed by the XRT and UVOT detectors on board the Swift
satellite between March and December 2005. Aims: We are mainly interested in
measuring the spectral parameters, and particularly the intrinsic curvature in
the X-ray band. Methods: We perform X-ray spectral analysis of observed BL Lac
TeV objects using either a log-parabolic or a simple power-law model . Results:
We found that many of the objects in our sample do show significant spectral
curvature, whereas those having the peak of the spectral energies distribution
at energies lower than ~0.1 keV show power law spectra. In these cases,
however, the statistics are generally low thus preventing a good estimate of
the curvature. Simultaneous UVOT observations are important to verify how X-ray
spectra can be extrapolated at lower frequencies and to search for multiple
emission components. Conclusions: The results of our analysis are useful for
the study of possible signatures of statistical acceleration processes
predicting intrinsically curved spectra and for modelling the SED of BL
Lacertae objects up to TeV energies where a corresponding curvature is likely
to be present.Comment: 10 pages, 7 figures. Accepted for publication in the Astronomy and
Astrophysic
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