2,166 research outputs found
Linking gamma-ray spectra of supernova remnants to the cosmic ray injection properties in the aftermath of supernovae
The acceleration times of the highest-energy particles which emit gamma-rays
in young and middle-age SNRs are comparable with SNR age. If the number of
particles starting acceleration was varying during early times after the
supernova explosion then this variation should be reflected in the shape of the
gamma-ray spectrum. We use the solution of the non-stationary equation for
particle acceleration in order to analyze this effect. As a test case, we apply
our method to describe gamma-rays from IC443. As a proxy of the IC443 parent
supernova we consider SN1987A. First, we infer the time dependence of injection
efficiency from evolution of the radio spectral index in SN1987A. Then, we use
the inferred injection behavior to fit the gamma-ray spectrum of IC443. We show
that the break in the proton spectrum needed to explain the gamma-ray emission
is a natural consequence of the early variation of the cosmic ray injection,
and that the very-high energy gamma-rays originate from particles which began
acceleration during the first months after the supernova explosion. We conclude
that the shape of the gamma-ray spectrum observed today in SNRs critically
depends on the time variation of the cosmic ray injection process in the
immediate post explosion phases. With the same model, we estimate also the
possibility in the future to detect gamma-rays from SN 1987A.Comment: A&A, accepte
Radio polarization maps of shell-type SNRs II. Sedov models with evolution of turbulent magnetic field
Polarized radio emission has been mapped with great detail in several
Galactic supernova remnants (SNRs), but has not yet been exploited to the
extent it deserves. We have developed a method to model maps of the Stokes
parameters for shell-like SNRs during their Sedov evolution phase. At first,
3-dimensional structure of a SNR has been computed, by modeling the
distribution of the magnetohydrodynamic parameters and of the accelerated
particles. The generation and dissipation of the turbulent component of
magnetic field everywhere in SNR are also considered taking into account its
interaction with accelerated particles. Then, in order to model the emission,
we have used a generalization of the classical synchrotron theory, valid for
the case in which the magnetic field has ordered and disordered components.
Finally, 2-dimensional projected maps have been derived, for different
orientations of SNR and of interstellar magnetic field with respect to the
observer. An important effect to consider is the Faraday rotation of the
polarization planes inside the SNR interior. In this paper we present details
of the model, and describe general properties of the images.Comment: accepted in MNRA
A General Mathematical Formulation for the Determination of Differential Leakage Factors in Electrical Machines with Symmetrical and Asymmetrical Full or Dead-Coil Multiphase Windings
This paper presents a simple and general mathematical formulation for the determination of the differential leakage factor for both symmetrical and asymmetrical full and dead-coil windings of electrical machines. The method can be applied to all multiphase windings and considers Görges polygons in conjunction with masses geometry in order to find an easy and affordable way to compute the differential leakage factor, avoiding the adoption of traditional methods that refer to the Ossanna's infinite series, which has to be obviously truncated under the bound of a predetermined accuracy. Moreover, the method described in this paper allows the easy determination of both the minimum and maximum values of the differential leakage factor, as well as its average value and the time trend. The proposed method, which does not require infinite series, is validated by means of several examples in order to practically demonstrate the effectiveness and the easiness of application of this procedure
A Novel, Simple and Flexible Fault-Tolerant Control Algorithm for Multiphase Electrical Machine Operation Under Open Circuit Faults
This paper presents a novel and simple procedure for the determination of a Fault-Tolerant Algorithm (namely FTA) for the adequate working operation of an electric machine equipped with a general m-phase winding under possible open circuits occurring in some of its phases. The FTA is firstly theoretically derived from the analysis of the magnetic field distribution in polyphase systems, then simulated through finite-element analysis and finally implemented via software for real-time validation. The effectiveness of the proposed procedure is confirmed through experimental tests on a laboratory setup. Key contributions include the flexibility of the algorithm, which can be applied to any m-phase electric machine with various winding configurations. Both the obtained FEA and experimental results demonstrate that the proposed control algorithm can be easily used and applied in electric drives supplied by m-phase motors under fault conditions without significantly affecting the magnetic performance of the whole system and ensuring the continuity of operation even in the presence of faults
Rotor bar pre-fault detection in the squirrel cage induction motors
The paper deals with a diagnosis technique to detect and monitor incipient faults in the rotor bars of squirrel gage induction motors. The failure mode analysis is performed monitoring the motor axial vibrations. To accomplish the task, the authors present a mathematical model that allows relating the occurrence and the severity of the faults to the presence and the magnitude of some frequency components of the axial vibration spectrum. To validate the proposed approach, the results obtained by applying the mathematical model are compared with the ones obtained by experimental tests done on both healthy and faulty motors
Probing the Nature of the Vela X Cocoon
Vela X is a pulsar wind nebula (PWN) associated with the active pulsar
B0833-45 and contained within the Vela supernova remnant (SNR). A collimated
X-ray filament ("cocoon") extends south-southwest from the pulsar to the center
of Vela X. VLA observations uncovered radio emission coincident with the
eastern edge of the cocoon and H.E.S.S. has detected TeV -ray emission
from this region as well. Using XMM-\textit{Newton} archival data, covering the
southern portion of this feature, we analyze the X-ray properties of the
cocoon. The X-ray data are best fit by an absorbed nonequilibrium plasma model
with a powerlaw component. Our analysis of the thermal emission shows enhanced
abundances of O, Ne, and Mg within the cocoon, indicating the presence of
ejecta-rich material from the propagation of the SNR reverse shock, consistent
with Vela X being a disrupted PWN. We investigate the physical processes that
excite the electrons in the PWN to emit in the radio, X-ray and -ray
bands. The radio and non-thermal X-ray emission can be explained by synchrotron
emission. We model the -ray emission by Inverse Compton scattering of
electrons off of cosmic microwave background (CMB) photons. We use a
3-component broken power law to model the synchrotron emission, finding an
intrinsic break in the electron spectrum at keV and a
cooling break at 5.5 keV. This cooling break along with
a magnetic field strength of 5 G indicate that the synchrotron
break occurs at 1 keV.Comment: accepted for publication to ApJ
Observational constraints on the modeling of SN1006
Experimental spectra and images of the supernova remnant SN1006 have been
reported for radio, X-ray and TeV gamma-ray bands. Several comparisons between
models and observations have been discussed in the literature, showing that the
broad-band spectrum from the whole remnant as well as a sharpest radial profile
of the X-ray brightness can be both fitted by adopting a model of SN1006 which
strongly depends on the non-linear effects of the accelerated cosmic rays;
these models predict post-shock magnetic field (MF) strengths of the order of
150 micro G. Here we present a new way to compare models and observations, in
order to put constraints on the physical parameters and mechanisms governing
the remnant. In particular, we show that a simple model based on the classic
MHD and cosmic rays acceleration theories allows us to investigate the
spatially distributed characteristics of SN1006 and to put observational
constraints on the kinetics and MF. Our method includes modelling and
comparison of the azimuthal and radial profiles of the surface brightness in
radio, hard X-rays and TeV gamma-rays as well as the azimuthal variations of
the electron maximum energy. In addition, this simple model also provides good
fits to the radio-to-gamma-ray spectrum of SN1006. We find that our best-fit
model predicts an effective MF strength inside SN1006 of 32 micro G, in good
agreement with the `leptonic' model suggested by the HESS Collaboration (2010).
Finally, some difficulties in both the `classic' and the non-linear models are
discussed. A number of evidences about non-uniformity of MF around SN1006 are
noted.Comment: 15 pages, 13 figures, accepted for publication on MNRA
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