27,233 research outputs found
Wind braking of magnetars: to understand magnetar's multiwave radiation properties
Magnetars are proposed to be peculiar neutron stars powered by their super
strong magnetic field. Observationally, anomalous X-ray pulsars and soft
gamma-ray repeaters are believed to be magnetar candidates. While more and more
multiwave observations of magnetars are available, unfortunately, we see
accumulating failed predictions of the traditional magnetar model. These
challenges urge rethinking of magnetar. Wind braking of magnetars is one of the
alternative modelings. The release of magnetic energy may generate a particle
outflow (i.e., particle wind), that results in both an anomalous X-ray
luminosity and significantly high spindown rate. In this wind braking scenario,
only strong multipole field is necessary for a magnetar (a strong dipole field
is no longer needed). Wind braking of magnetars may help us to understand their
multiwave radiation properties, including (1) Non-detection of magnetars in
Fermi-LAT observations, (2) The timing behaviors of low magnetic field
magnetars, (3) The nature of anti-glitches, (4) The criterion for magnetar's
radio emission, etc. In the wind braking model of magentars, timing events of
magnetars should always be accompanied by radiative events. It is worth noting
that the wind engine should be the central point in the research since other
efforts with any reasonable energy mechanism may also reproduce the results.Comment: 6 pages, 1 figure, submitted to conference proceeding of SMFNS2013
(Strong electromagnetic field and neutron stars 2013
The timing behavior of magnetar Swift J1822.3-1606: timing noise or a decreasing period derivative?
The different timing results of the magnetar Swift J1822.3-1606 is analyzed
and understood theoretically. It is pointed that different timing solutions are
caused not only by timing noise, but also that the period derivative is
decreasing after outburst. Both the decreasing period derivative and the large
timing noise may be originated from wind braking of the magnetar. Future timing
of Swift J1822.3-1606 will help us make clear whether its period derivative is
decreasing with time or not.Comment: 5 pages, 1 figure. Accepted by Research in Astronomy and Astrophysic
Phase diagram of the frustrated Hubbard model
The Mott-Hubbard metal-insulator transition in the paramagnetic phase of the
one-band Hubbard model has long been used to describe similar features in real
materials like VO. Here we show that this transition is hidden inside a
rather robust antiferromagnetic insulator even in the presence of comparatively
strong magnetic frustration. This result raises the question of the relevance
of the Mott-Hubbard metal-insulator transition for the generic phase diagram of
the one-band Hubbard model.Comment: 4 pages, 6 figure
The optical/UV excess of isolated neutron stars in the RCS model
The X-ray dim isolated neutron stars (XDINSs) are peculiar pulsar-like
objects, characterized by their very well Planck-like spectrum. In studying
their spectral energy distributions, the optical/UV excess is a long standing
problem. Recently, Kaplan et al. (2011) have measured the optical/UV excess for
all seven sources, which is understandable in the resonant cyclotron scattering
(RCS) model previously addressed. The RCS model calculations show that the RCS
process can account for the observed optical/UV excess for most sources . The
flat spectrum of RX J2143.0+0654 may due to contribution from bremsstrahlung
emission of the electron system in addition to the RCS process.Comment: 6 pages, 2 figures, 1 table, accepted for publication in Research in
Astronomy and Astrophysic
Life assessment of combustor liner using unified constitutive models
Hot section components of gas turbine engines are subject to severe thermomechanical loads during each mission cycle. Inelastic deformation can be induced in localized regions leading to eventual fatigue cracking. Assessment of durability requires reasonably accurate calculation of the structural response at the critical location for crack initiation. In recent years nonlinear finite element computer codes have become available for calculating inelastic structural response under cyclic loading. NASA-Lewis sponsored the development of unified constitutive material models and their implementation in nonlinear finite element computer codes for the structural analysis of hot section components. These unified models were evaluated with regard to their effect on the life prediction of a hot section component. The component considered was a gas turbine engine combustor liner. A typical engine mission cycle was used for the thermal and structural analyses. The analyses were performed on a CRAY computer using the MARC finite element code. The results were compared with laboratory test results, in terms of crack initiation lives
Unified computer codes: Properties data for low cost nozzle materials
The development of the analytic capability to predict the thermal ablation response of promising low cost materials for rocket nozzles is presented
AXPs and SGRs in the outer gap model: confronting Fermi observations
Anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) are
magnetar candidates, i.e., neutron stars powered by strong magnetic field. If
they are indeed magnetars, they will emit high-energy gamma-rays which are
detectable by Fermi-LAT according to the outer gap model. However, no
significant detection is reported in recent Fermi-LAT observations of all known
AXPs and SGRs. Considering the discrepancy between theory and observations, we
calculate the theoretical spectra for all AXPs and SGRs with sufficient
observational parameters. Our results show that most AXPs and SGRs are
high-energy gamma-ray emitters if they are really magnetars. The four AXPs 1E
1547.0-5408, XTE J1810-197, 1E 1048.1-5937, and 4U 0142+61 should have been
detected by Fermi-LAT. Then there is conflict between out gap model in the case
of magnetars and Fermi observations. Possible explanations in the magnetar
model are discussed. On the other hand, if AXPs and SGRs are fallback disk
systems, i.e., accretion-powered for the persistent emissions, most of them are
not high-energy gamma-ray emitters. Future deep Fermi-LAT observations of AXPs
and SGRs will help us make clear whether they are magnetars or fallback disk
systems.Comment: 15 pages, 3 figures, 1 table, accepted for publication in The
Astrophysical Journa
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