208 research outputs found
The model constraints from the observed trends for the quasi-periodic oscillation in RE J1034+396
We analyze the time variability of the X-ray emission of RE J1034+396 -- an
active galactic nucleus with the first firm detection of a quasi-periodic
oscillations (QPO). Based on the results of a wavelet analysis, we find a drift
in the QPO central frequency. The change in the QPO frequency correlates with
the change in the X-ray flux with a short time delay. The data specifically
suggest a linear dependence between the QPO period and the flux, and this gives
important constraints on the QPO models. In particular, it excludes explanation
in terms of the orbiting hot spot model close to a black hole. Linear
structures such as shocks, spiral waves, or very distant flares are favored.Comment: Astronomy & Astrophysics, in pres
Quasiperiodic oscillations in a strong gravitational field around neutron stars testing braneworld models
The strong gravitational field of neutron stars in the brany universe could
be described by spherically symmetric solutions with a metric in the exterior
to the brany stars being of the Reissner-Nordstrom type containing a brany
tidal charge representing the tidal effect of the bulk spacetime onto the star
structure. We investigate the role of the tidal charge in orbital models of
high-frequency quasiperiodic oscillations (QPOs) observed in neutron star
binary systems. We focus on the relativistic precession model. We give the
radial profiles of frequencies of the Keplerian (vertical) and radial epicyclic
oscillations. We show how the standard relativistic precession model modified
by the tidal charge fits the observational data, giving estimates of the
allowed values of the tidal charge and the brane tension based on the processes
going in the vicinity of neutron stars. We compare the strong field regime
restrictions with those given in the weak-field limit of solar system
experiments.Comment: 26 pages, 6 figure
Investigation of magneto-structural phase transition in FeRh by reflectivity and transmittance measurements in visible and near-infrared spectral region
Magneto-structural phase transition in FeRh epitaxial layers was studied optically. It is shown that the transition between the low-temperature antiferromagnetic phase and the high-temperature ferromagnetic phase is accompanied by a rather large change of the optical response in the visible and near-infrared spectral ranges. This change is consistent with ab initio calculations of reflectivity and transmittance. Phase transition temperatures in a series of FeRh films with thicknesses ranging from 6 to 100 nm is measured thereby demonstrating the utility of the method to quickly characterise samples. Spatially resolved imaging of their magnetic properties with a micrometer resolution shows that the phase transition occurs at different temperatures in different parts of the sample
Co-encapsulation of human serum albumin and superparamagnetic iron oxide in PLGA nanoparticles: Part I. Effect of process variables on the mean size
PLGA (poly d,l-lactic-co-glycolic acid) nanoparticles (NPs) encapsulating magnetite nanoparticles (MNPs) along with a model drug human serum albumin (HSA) were prepared by double emulsion solvent evaporation method. This Part I will focus on size and size distribution of prepared NPs, whereas encapsulation efficiency will be discussed in Part II. It was found that mean hydrodynamic particle size was influenced by five important process variables. To explore their effects, a five-factorial, three-level experimental design and statistical analysis were carried out using STATISTICA® software. Effect of process variables on the mean size of nanoparticles was investigated and finally conditions to minimize size of NPs were proposed. GAMS™/MINOS software was used for optimization. The mean hydrodynamic size of nanoparticles ranged from 115 to 329 nm depending on the process conditions. Smallest possible mean particle size can be achieved by using low polymer concentration and high dispersion energy (enough sonication time) along with small aqueous/organic volume ratio
Anomalous Magnetoresistance by Breaking Ice Rule in Bi2Ir2O7/Dy2Ti2O7 Heterostructure
While geometrically frustrated quantum magnets are known for a variety of
exotic spin states that are of great interests of understanding emergent
phenomena as well as enabling revolutionary quantum technologies, most of them
are necessarily good insulators which are difficult to be integrated with
modern electrical circuit that relies on moving charge carriers. The grand
challenge of converting fluctuations and excitations of frustrated moments into
electronic responses is finding ways to introduce charge carriers that interact
with the localized spins without destroying the spin states. Here, we show
that, by designing a Bi2Ir2O7/Dy2Ti2O7 heterostructure, the breaking of the
spin ice rule in insulating Dy2Ti2O7 can lead to a charge response in the
Bi2Ir2O7 conducting layer that can be detected as anomalous magnetoresistance.
These results demonstrate a novel and feasible interfacial approach for
electronically probing exotic spin states in insulating magnets, laying out a
blueprint for the metallization of frustrated quantum magnets
Adaptive Honeypot Engagement through Reinforcement Learning of Semi-Markov Decision Processes
A honeynet is a promising active cyber defense mechanism. It reveals the
fundamental Indicators of Compromise (IoCs) by luring attackers to conduct
adversarial behaviors in a controlled and monitored environment. The active
interaction at the honeynet brings a high reward but also introduces high
implementation costs and risks of adversarial honeynet exploitation. In this
work, we apply infinite-horizon Semi-Markov Decision Process (SMDP) to
characterize a stochastic transition and sojourn time of attackers in the
honeynet and quantify the reward-risk trade-off. In particular, we design
adaptive long-term engagement policies shown to be risk-averse, cost-effective,
and time-efficient. Numerical results have demonstrated that our adaptive
engagement policies can quickly attract attackers to the target honeypot and
engage them for a sufficiently long period to obtain worthy threat information.
Meanwhile, the penetration probability is kept at a low level. The results show
that the expected utility is robust against attackers of a large range of
persistence and intelligence. Finally, we apply reinforcement learning to the
SMDP to solve the curse of modeling. Under a prudent choice of the learning
rate and exploration policy, we achieve a quick and robust convergence of the
optimal policy and value.Comment: The presentation can be found at https://youtu.be/GPKT3uJtXqk. arXiv
admin note: text overlap with arXiv:1907.0139
Foundations of Black Hole Accretion Disk Theory
This review covers the main aspects of black hole accretion disk theory. We
begin with the view that one of the main goals of the theory is to better
understand the nature of black holes themselves. In this light we discuss how
accretion disks might reveal some of the unique signatures of strong gravity:
the event horizon, the innermost stable circular orbit, and the ergosphere. We
then review, from a first-principles perspective, the physical processes at
play in accretion disks. This leads us to the four primary accretion disk
models that we review: Polish doughnuts (thick disks), Shakura-Sunyaev (thin)
disks, slim disks, and advection-dominated accretion flows (ADAFs). After
presenting the models we discuss issues of stability, oscillations, and jets.
Following our review of the analytic work, we take a parallel approach in
reviewing numerical studies of black hole accretion disks. We finish with a few
select applications that highlight particular astrophysical applications:
measurements of black hole mass and spin, black hole vs. neutron star accretion
disks, black hole accretion disk spectral states, and quasi-periodic
oscillations (QPOs).Comment: 91 pages, 23 figures, final published version available at
http://www.livingreviews.org/lrr-2013-
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