441 research outputs found
Fermi Large Area Telescope observations of the supernova remnant HESS J1731-347
Context: HESS J1731-347 has been identified as one of the few TeV-bright
shell-type supernova remnants (SNRs). These remnants are dominated by
nonthermal emission, and the nature of TeV emission has been continuously
debated for nearly a decade.
Aims: We carry out the detailed modeling of the radio to gamma-ray spectrum
of HESS J1731-347 to constrain the magnetic field and energetic particles
sources, which we compare with those of the other TeV-bright shell-type SNRs
explored before.
Methods: Four years of data from Fermi Large Area Telescope (LAT)
observations for regions around this remnant are analyzed, leading to no
detection correlated with the source discovered in the TeV band. The Markov
Chain Monte Carlo method is used to constrain parameters of one-zone models for
the overall emission spectrum.
Results: Based on the 99.9% upper limits of fluxes in the GeV range, one-zone
hadronic models with an energetic proton spectral slope greater than 1.8 can be
ruled out, which favors a leptonic origin for the gamma-ray emission, making
this remnant a sibling of the brightest TeV SNR RX J1713.7-3946, the Vela
Junior SNR RX J0852.0-4622, and RCW 86. The best-fit leptonic model has an
electron spectral slope of 1.8 and a magnetic field of about 30 muG, which is
at least a factor of 2 higher than those of RX J1713.7-3946 and RX
J0852.0-4622, posing a challenge to the distance estimate and/or the energy
equipartition between energetic electrons and the magnetic field of this
source. A measurement of the shock speed will address this challenge and has
implications on the magnetic field evolution and electron acceleration driven
by shocks of SNRs.Comment: 7 pages, 3 fogures, A&A in pres
Evolution of High-Energy Particle Distribution in Mature Shell-Type Supernova Remnants
Multi-wavelength observations of mature supernova remnants (SNRs), especially
with recent advances in gamma-ray astronomy, make it possible to constrain
energy distribution of energetic particles within these remnants. In
consideration of the SNR origin of Galactic cosmic rays and physics related to
particle acceleration and radiative processes, we use a simple one-zone model
to fit the nonthermal emission spectra of three shell-type SNRs located within
2 degrees on the sky: RX J1713.7-3946, CTB 37B, and CTB 37A. Although radio
images of these three sources all show a shell (or half-shell) structure, their
radio, X-ray, and gamma-ray spectra are quite different, offering an ideal case
to explore evolution of energetic particle distribution in SNRs. Our spectral
fitting shows that 1) the particle distribution becomes harder with aging of
these SNRs, implying a continuous acceleration process, and the particle
distributions of CTB 37A and CTB 37B in the GeV range are harder than the
hardest distribution that can be produced at a shock via the linear diffusive
shock particle acceleration process, so spatial transport may play a role; 2)
the energy loss timescale of electrons at the high-energy cutoff due to
synchrotron radiation appears to be always a bit (within a factor of a few)
shorter than the age of the corresponding remnant, which also requires
continuous particle acceleration; 3) double power-law distributions are needed
to fit the spectra of CTB 37B and CTB 37A, which may be attributed to shock
interaction with molecular clouds.Comment: Accepted for publication in The Astrophysical Journal, 11 pages, 3
figures, 1 tabl
Evaluating Realistic Performance Gains of Massive Multi-User MIMO System in Urban City Deployments
Parameter Sensitivity and Dependency Analysis for the WECC Dynamic Composite Load Model
An accurate dynamic load model plays a crucial role in the analysis of power system transient stability. The WECC dynamic composite load model (CMPLDW) has been developed recently to better represent fault-induced delayed-voltage-recovery (FIDVR) events, which are of increasing concern to electric utilities. To facilitate the understanding of the CMPLDW, it is worth studying the effect of parameters that describe the model structure on its dynamic response. In this paper, we show that 1) some parameters have very minimal sensitivities under certain FIDVR events; 2) sensitivities of certain parameters are strongly dependent on the temporal profile of given fault, such as its minimum fault voltage or recovery time; and 3) some parameters share similar sensitivity patterns and thus the change of their values may complement each other. These observations are essential for further developing enhanced measurement-based dynamic load modeling approaches by tackling the parameter identifiability issues pointed out in the present work
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