163 research outputs found
Axisymmetric Magnetorotational Instability in Viscous Accretion Disks
Axisymmetric magnetorotational instability (MRI) in viscous accretion disks
is investigated by linear analysis and two-dimensional nonlinear simulations.
The linear growth of the viscous MRI is characterized by the Reynolds number
defined as , where is the Alfv{\'e}n
velocity, is the kinematic viscosity, and is the angular
velocity of the disk. Although the linear growth rate is suppressed
considerably as the Reynolds number decreases, the nonlinear behavior is found
to be almost independent of . At the nonlinear evolutionary stage,
a two-channel flow continues growing and the Maxwell stress increases until the
end of calculations even though the Reynolds number is much smaller than unity.
A large portion of the injected energy to the system is converted to the
magnetic energy. The gain rate of the thermal energy, on the other hand, is
found to be much larger than the viscous heating rate. Nonlinear behavior of
the MRI in the viscous regime and its difference from that in the highly
resistive regime can be explained schematically by using the characteristics of
the linear dispersion relation. Applying our results to the case with both the
viscosity and resistivity, it is anticipated that the critical value of the
Lundquist number for active turbulence
depends on the magnetic Prandtl number in
the regime of and remains constant when , where and is the magnetic diffusivity.Comment: Accepted for publication in ApJ -- 18 pages, 9 figures, 1 tabl
Nonaxisymmetric Magnetorotational Instability in Proto-Neutron Stars
We investigate the stability of differentially rotating proto-neutron stars
(PNSs) with a toroidal magnetic field. Stability criteria for nonaxisymmetric
MHD instabilities are derived using a local linear analysis. PNSs are expected
to have much stronger radial shear in the rotation velocity compared to normal
stars. We find that nonaxisymmetric magnetorotational instability (NMRI) with a
large azimuthal wavenumber is dominant over the kink mode () in
differentially rotating PNSs. The growth rate of the NMRI is of the order of
the angular velocity which is faster than that of the kink-type
instability by several orders of magnitude. The stability criteria are
analogous to those of the axisymmetric magnetorotational instability with a
poloidal field, although the effects of leptonic gradients are considered in
our analysis. The NMRI can grow even in convectively stable layers if the
wavevectors of unstable modes are parallel to the restoring force by the
Brunt-V\"ais\"al\"a oscillation. The nonlinear evolution of NMRI could amplify
the magnetic fields and drive MHD turbulence in PNSs, which may lead to
enhancement of the neutrino luminosity.Comment: 24pages, 7figures, Accepted for publication in the Astrophysical
Journal (December 12, 2005
Dead Zone Formation and Nonsteady Hyperaccretion in Collapsar Disks : A Possible Origin of Short-Term Variability in the Prompt Emission of Gamma-Ray Bursts
The central engine of gamma-ray bursts (GRBs) is believed to be a hot and
dense disk with hyperaccretion onto a few solar-mass black hole. We investigate
where the magnetorotational instability (MRI) actively operates in the
hyperaccretion disk, which can cause angular momentum transport in the disk.
The inner region of hyperaccretion disks can be neutrino opaque, and the
energy- and momentum-transport by neutrinos could affect the growth of the MRI
significantly. Assuming reasonable disk models and a weak magnetic field , it is found that the MRI is strongly suppressed by
the neutrino viscosity in the inner region of hyperaccretion disks. On the
other hand, the MRI can drive active MHD turbulence in the outer
neutrino-transparent region regardless of the field strength. This suggests
that the baryonic matter is accumulated into the inner dead zone where the MRI
grows inactively and the angular momentum transport is inefficient. When the
dead zone gains a large amount of mass and becomes gravitationally unstable,
intense mass accretion onto the central black hole would occur episodically
through the gravitational torque. This process can be a physical mechanism of
the short-term variability in the prompt emission of GRBs. Finally, the origin
of flaring activities in the X-ray afterglow is predicted in the context of our
episodic accretion scenario.Comment: 11pages, 4figures. Accepted for publication in the Astrophysical
Journa
A variable absorption feature in the X-ray spectrum of a magnetar
Soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are slowly
rotating, isolated neutron stars that sporadically undergo episodes of
long-term flux enhancement (outbursts) generally accompanied by the emission of
short bursts of hard X-rays. This behaviour can be understood in the magnetar
model, according to which these sources are mainly powered by their own
magnetic energy. This is supported by the fact that the magnetic fields
inferred from several observed properties of AXPs and SGRs are greater than -
or at the high end of the range of - those of radio pulsars. In the peculiar
case of SGR 0418+5729, a weak dipole magnetic moment is derived from its timing
parameters, whereas a strong field has been proposed to reside in the stellar
interior and in multipole components on the surface. Here we show that the
X-ray spectrum of SGR 0418+5729 has an absorption line, the properties of which
depend strongly on the star's rotational phase. This line is interpreted as a
proton cyclotron feature and its energy implies a magnetic field ranging from
2E14 gauss to more than 1E15 gauss.Comment: Nature, 500, 312 (including Supplementary Information
Direct dark matter search by annual modulation in XMASS-I
A search for dark matter was conducted by looking for an annual modulation
signal due to the Earth's rotation around the Sun using XMASS, a single phase
liquid xenon detector. The data used for this analysis was 359.2 live days
times 832 kg of exposure accumulated between November 2013 and March 2015. When
we assume Weakly Interacting Massive Particle (WIMP) dark matter elastically
scattering on the target nuclei, the exclusion upper limit of the WIMP-nucleon
cross section 4.310cm at 8 GeV/c was obtained and we
exclude almost all the DAMA/LIBRA allowed region in the 6 to 16 GeV/c range
at 10cm. The result of a simple modulation analysis, without
assuming any specific dark matter model but including electron/ events,
showed a slight negative amplitude. The -values obtained with two
independent analyses are 0.014 and 0.068 for null hypothesis, respectively. we
obtained 90\% C.L. upper bounds that can be used to test various models. This
is the first extensive annual modulation search probing this region with an
exposure comparable to DAMA/LIBRA.Comment: 5 pages, 4 figure
Magnetic Reconnection in Extreme Astrophysical Environments
Magnetic reconnection is a basic plasma process of dramatic rearrangement of
magnetic topology, often leading to a violent release of magnetic energy. It is
important in magnetic fusion and in space and solar physics --- areas that have
so far provided the context for most of reconnection research. Importantly,
these environments consist just of electrons and ions and the dissipated energy
always stays with the plasma. In contrast, in this paper I introduce a new
direction of research, motivated by several important problems in high-energy
astrophysics --- reconnection in high energy density (HED) radiative plasmas,
where radiation pressure and radiative cooling become dominant factors in the
pressure and energy balance. I identify the key processes distinguishing HED
reconnection: special-relativistic effects; radiative effects (radiative
cooling, radiation pressure, and Compton resistivity); and, at the most extreme
end, QED effects, including pair creation. I then discuss the main
astrophysical applications --- situations with magnetar-strength fields
(exceeding the quantum critical field of about 4 x 10^13 G): giant SGR flares
and magnetically-powered central engines and jets of GRBs. Here, magnetic
energy density is so high that its dissipation heats the plasma to MeV
temperatures. Electron-positron pairs are then copiously produced, making the
reconnection layer highly collisional and dressing it in a thick pair coat that
traps radiation. The pressure is dominated by radiation and pairs. Yet,
radiation diffusion across the layer may be faster than the global Alfv\'en
transit time; then, radiative cooling governs the thermodynamics and
reconnection becomes a radiative transfer problem, greatly affected by the
ultra-strong magnetic field. This overall picture is very different from our
traditional picture of reconnection and thus represents a new frontier in
reconnection research.Comment: Accepted to Space Science Reviews (special issue on magnetic
reconnection). Article is based on an invited review talk at the
Yosemite-2010 Workshop on Magnetic Reconnection (Yosemite NP, CA, USA;
February 8-12, 2010). 30 pages, no figure
Unusual Central Engine Activity in the Double Burst GRB 110709B
The double burst, GRB 110709B, triggered Swift/BAT twice at 21:32:39 UT and
21:43:45 UT, respectively, on 9 July 2011. This is the first time we observed a
GRB with two BAT triggers. In this paper, we present simultaneous Swift and
Konus-WIND observations of this unusual GRB and its afterglow. If the two
events originated from the same physical progenitor, their different
time-dependent spectral evolution suggests they must belong to different
episodes of the central engine, which may be a magnetar-to-BH accretion system.Comment: 10 pages, 14 figures, 2 tables. ApJ accepte
Rhaponticum acaule (L) DC essential oil: chemical composition, in vitro antioxidant and enzyme inhibition properties
Background: α-glucosidase is a therapeutic target for diabetes mellitus (DM) and α-glucosidase inhibitors play a
vital role in the treatments for the disease. Furthermore, xanthine oxidase (XO) is a key enzyme that catalyzes
hypoxanthine and xanthine to uric acid which at high levels can lead to hyperuricemia which is an important cause
of gout. Pancreatic lipase (PL) secreted into the duodenum plays a key role in the digestion and absorption of fats.
For its importance in lipid digestion, PL represents an attractive target for obesity prevention.
Methods: The flowers essential oil of Rhaponticum acaule (L) DC (R. acaule) was characterized using gas
chromatography-mass spectrometry (GC-MS). The antioxidant activities of R. acaule essential oil (RaEO) were also
determined using 2,2’-azinobis-3-ethylbenzothiazoline-6-sulfonic acid (ABTS), reducing power,
phosphomolybdenum, and DNA nicking assays. The inhibitory power of RaEO against α-glucosidase, xanthine
oxidase and pancreatic lipase was evaluated. Enzyme kinetic studies using Michaelis-Menten and the derived
Lineweaver-Burk (LB) plots were performed to understand the possible mechanism of inhibition exercised by the
components of this essential oil.
Results: The result revealed the presence of 26 compounds (97.4%). The main constituents include germacrene D
(49.2%), methyl eugenol (8.3%), (E)-β-ionone (6.2%), β-caryophyllene (5.7%), (E,E)-α-farnesene (4.2%),
bicyclogermacrene (4.1%) and (Z)-α-bisabolene (3.7%). The kinetic inhibition study showed that the essential oil
demonstrated a strong α-glucosidase inhibiton and it was a mixed inhibitor. On the other hand, our results
evidenced that this oil exhibited important xanthine oxidase inhibitory effect, behaving as a non-competitive
inhibitor. The essential oil inhibited the turkey pancreatic lipase, with maximum inhibition of 80% achieved at
2 mg/mL. Furthermore, the inhibition of turkey pancreatic lipase by RaEO was an irreversible one.
Conclusion: The results revealed that the RaEO is a new promising potential source of antioxidant compounds,
endowed with good practical applications for human health.
Keywords: α-glucosidase, Antioxidant activity, Chemical composition, Pancreatic lipase inhibition, Rhaponticum
acaule essential oil, Xanthine oxidase
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