5,796 research outputs found
Evidence of Particle Acceleration in the Superbubble 30 Doradus C with NuSTAR
We present evidence of diffuse, non-thermal X-ray emission from the
superbubble 30 Doradus C (30 Dor C) using hard X-ray images and spectra from
NuSTAR observations. For this analysis, we utilize data from a 200 ks targeted
observation of 30 Dor C as well as 2.8 Ms of serendipitous off-axis
observations from the monitoring of nearby SN 1987A. The complete shell of 30
Dor C is detected up to 20 keV, and the young supernova remnant MCSNR
J0536-6913 in the southeast of 30 Dor C is not detected above 8 keV.
Additionally, six point sources identified in previous Chandra and XMM-Newton
investigations have hard X-ray emission coincident with their locations. Joint
spectral fits to the NuSTAR and XMM-Newton spectra across the 30 Dor C shell
confirm the non-thermal nature of the diffuse emission. Given the best-fit
rolloff frequencies of the X-ray spectra, we find maximum electron energies of
70-110 TeV (assuming a B-field strength of 4G), suggesting 30 Dor C is
accelerating particles. Particles are either accelerated via diffusive shock
acceleration at locations where the shocks have not stalled behind the
H shell, or cosmic-rays are accelerated through repeated acceleration
of low-energy particles via turbulence and magnetohydrodynamic waves in the
bubble's interior.Comment: 14 pages, 8 figures, ApJ, in pres
A Chandra View Of Nonthermal Emission In The Northwestern Region Of Supernova Remnant RCW 86: Particle Acceleration And Magnetic Fields
The shocks of supernova remnants (SNRs) are believed to accelerate particles
to cosmic ray (CR) energies. The amplification of the magnetic field due to CRs
propagating in the shock region is expected to have an impact on both the
emission from the accelerated particle population, as well as the acceleration
process itself. Using a 95 ks observation with the Advanced CCD Imaging
Spectrometer (ACIS) onboard the Chandra X-ray Observatory, we map and
characterize the synchrotron emitting material in the northwestern region of
RCW 86. We model spectra from several different regions, filamentary and
diffuse alike, where emission appears dominated by synchrotron radiation. The
fine spatial resolution of Chandra allows us to obtain accurate emission
profiles across 3 different non-thermal rims in this region. The narrow width
(l = 10''-30'') of these filaments constrains the minimum magnetic field
strength at the post-shock region to be approximately 80 {\mu}G.Comment: 7 pages, 3 figures, submitted for publication at the Astrophysical
Journa
Strongly correlated fermions with nonlinear energy dispersion and spontaneous generation of anisotropic phases
Using the bosonization approach we study fermionic systems with a nonlinear
dispersion relation in dimension d>2. We explicitly show how the band curvature
gives rise to interaction terms in the bosonic version of the model. Although
these terms are perturbatively irrelevant in relation to the Landau Fermi
liquid fixed point, they become relevant perturbations when instabilities take
place. Using a coherent state path integral technique we built up the effective
action that governs the dynamics of the Fermi surface fluctuations. We consider
the combined effect of fermionic interactions and band curvature on possible
anisotropic phases triggered by negative Landau parameters. In particular we
study in some detail the phase diagram for the isotropic/nematic/hexatic
quantum phase transition.Comment: RevTeX4, 9 pages, 2 eps figures, Final version as appeared in
Phys.Rev.
The Role of Stellar Feedback in the Dynamics of HII Regions
Stellar feedback is often cited as the biggest uncertainty in galaxy
formation models today. This uncertainty stems from a dearth of observational
constraints as well as the great dynamic range between the small scales (<1 pc)
where the feedback occurs and the large scales of galaxies (>1 kpc) that are
shaped by this feedback. To bridge this divide, in this paper we aim to assess
observationally the role of stellar feedback at the intermediate scales of HII
regions. In particular, we employ multiwavelength data to examine several
stellar feedback mechanisms in a sample of 32 HII regions in the Large and
Small Magellanic Clouds (LMC and SMC, respectively). Using optical, infrared,
radio, and X-ray images, we measure the pressures exerted on the shells from
the direct stellar radiation, the dust-processed radiation, the warm ionized
gas, and the hot X-ray emitting gas. We find that the warm ionized gas
dominates over the other terms in all of the sources, although two have
comparable dust-processed radiation pressures to their warm gas pressures. The
hot gas pressures are comparatively weak, while the direct radiation pressures
are 1-2 orders of magnitude below the other terms. We discuss the implications
of these results, particularly highlighting evidence for hot gas leakage from
the HII shells and regarding the momentum deposition from the dust-processed
radiation to the warm gas. Furthermore, we emphasize that similar observational
work should be done on very young HII regions to test whether direct radiation
pressure and hot gas can drive the dynamics at early times.Comment: 19 pages, 8 figures; accepted by Ap
Requirements engineering related usability techniques adopted in agile development processes
Over the last decade there has been a growing
interest in the integration of agile software development process
(ASDP) and user-centred design (UCD). However, there are no
papers that study which usability techniques related to
requirements engineering are being adopted in the ASDP, and
there are no formalized proposals for their adoption. Objective:
Identify which techniques related to requirements engineering
activities are being adopted in the ASDP and determine how they
are being adopted. Method: We have conducted a systematic
mapping study (SMS) to retrieve the literature reporting the
application of usability techniques in the ASDP. We analysed
these techniques using a catalogue of techniques compiled by
software engineering researchers. We then determined the
manner in which the techniques that are being used in the ASDP
were adopted. Results: The agile community is very much
interested in adopting usability techniques. The most used
techniques are Personas, contextual inquiry and prototyping.
Conclusions: This research offers an overview of the adoption of
usability techniques related to requirements engineering in
ASDPs and reports how they are being adopted. We found that
some of the techniques are being adapted for adoption.This research was funded by the Spanish Ministry of
Education, Culture and Sports FLEXOR and “Realizando
Experimentos en la Industria del Software: Comprensión del
Paso de Laboratorio a la Realidad” projects (TIN2014-52129-R
and TIN2014-60490-P, respectively) and the eMadrid-CM
“Investigación y Desarrollo de Tecnologías Educativas en la
Comunidad de Madrid” project (S2013/ICE-2715
A Novel Hybrid Approach to Improve Performance on Frequency Division Duplex Systems with Linear Precoding
The final publication is available http://dx.doi.org/DOI: 10.1007/978-3-642-13803-4_31[Abstrtact] Linear precoding is an attractive technique to combat interference in multiple-input multiple-output systems because it reduces costs and power consumption in the receiver equipment. Most of the frequency division duplex systems with linear precoding acquire the channel state information at the receiver by using supervised algorithms. Such algorithms make use of pilot symbols periodically sent by the transmitter. In a later step, the channel state information is sent to the transmitter side through a limited feedback channel.
In order to reduce the overhead inherent to the periodical transmission of training data, we propose to acquire the channel state information by combining supervised and unsupervised algorithms, leading to a hybrid and more efficient approach. Simulation results show that the performance achieved with the proposed scheme is clearly better than that with standard algorithms.Consellería de Economía e Industria; 09TIC008105PRMinisterio de Educación y Ciencia; TEC2007-68020-C04-0
DASBE: Decision-Aided Semi-Blind Equalization for MIMO Systems with Linear Precoding
[Abstract] Multiple-Input Multiple-Output (MIMO) digital communications standards usually acquire Channel State Information (CSI) by means of supervised algorithms, which implies a loss of performance since pilot symbols do not convey information. We propose to obtain this CSI by using the so-called semi-blind techniques, which combine both supervised and unsupervised (blind) methods. The key idea consists in introducing a decision criterion to determine when the channel has suffered a significant change. In such a case, transmission of pilot symbols is required. The use of this criterion also allows us to determine the time instants in which CSI has to be sent to the transmitter from the receiver through a low-cost feedback channel.Ministerio de Ciencia e Innovación; 09TIC008105PRMinisterio de Ciencia e Innovación; TEC2007-68020-C04-01Ministerio de Ciencia e Innovación; CSD2008-0001
- …