239 research outputs found
Identification of two new HMXBs in the LMC: a 2013 s pulsar and a probable SFXT
We report on the X-ray and optical properties of two high-mass X-ray binary
systems located in the Large Magellanic Cloud (LMC). Based on the obtained
optical spectra, we classify the massive companion as a supergiant star in both
systems. Timing analysis of the X-ray events collected by XMM-Newton revealed
the presence of coherent pulsations (spin period 2013 s) for XMMU
J053108.3-690923 and fast flaring behaviour for XMMU J053320.8-684122. The
X-ray spectra of both systems can be modelled sufficiently well by an absorbed
power-law, yielding hard spectra and high intrinsic absorption from the
environment of the systems. Due to their combined X-ray and optical properties
we classify both systems as SgXRBs: the 19 confirmed X-ray pulsar
and a probable supergiant fast X-ray transient in the LMC, the second such
candidate outside our Galaxy.Comment: 12 pages, 10 figures, accepted for publication in MNRA
Deciphering the properties of the central engine in GRB collapsars
The central engine in long gamma-ray bursts (GRBs) is thought to be a compact object produced by the core collapse of massive stars, but its exact nature (black hole or millisecond magnetar) is still debatable. Although the central engine of GRB collapsars is hidden to direct observation, its properties may be imprinted on the accompanying electromagnetic signals. We aim to decipher the generic properties of central engines that are consistent with prompt observations of long GRBs detected by the Burst Alert Telescope (BAT) on board the Neil Gehrels Swift Observatory. Adopting a generic model for the central engine, in which the engine power and activity time-scale are independent of each other, we perform Monte Carlo simulations of long GRBs produced by jets that successfully breakout from the star. Our simulations consider the dependence of the jet breakout time-scale on the engine luminosity and the effects of the detector’s flux threshold. The two-dimensional (2D) distribution of simulated detectable bursts in the gamma-ray luminosity versus gamma-ray duration plane is consistent with the observed one for a range of parameter values describing the central engine. The intrinsic 2D distribution of simulated collapsar GRBs peaks at lower gamma-ray luminosities and longer durations than the observed one, a prediction that can be tested in the future with more sensitive detectors. Black hole accretors, whose power and activity time are set by the large-scale magnetic flux through the progenitor star and stellar structure, respectively, are compatible with the properties of the central engine inferred by our model
Application of neural networks to synchro-Compton blazar emission models
Jets from supermassive black holes in the centers of active galaxies are the
most powerful persistent sources of electromagnetic radiation in the Universe.
To infer the physical conditions in the otherwise out-of-reach regions of
extragalactic jets we usually rely on fitting of their spectral energy
distribution (SED). The calculation of radiative models for the jet non-thermal
emission usually relies on numerical solvers of coupled partial differential
equations. In this work machine learning is used to tackle the problem of high
computational complexity in order to significantly reduce the SED model
evaluation time, which is needed for SED fitting with Bayesian inference
methods. We compute SEDs based on the synchrotron self-Compton model for blazar
emission using the radiation code ATHEA, and use them to train Neural
Networks exploring whether these can replace the original computational
expensive code. We find that a Neural Network with Gated Recurrent Unit neurons
can effectively replace the ATHEA leptonic code for this application,
while it can be efficiently coupled with MCMC and nested sampling algorithms
for fitting purposes. We demonstrate this through an application to simulated
data sets and with an application to observational data. We offer this tool in
the community through a public repository. We present a proof-of-concept
application of neural networks to blazar science. This is the first step in a
list of future applications involving hadronic processes and even larger
parameter spaces.Comment: 12 pages, submitted, comments are welcome, code will be soon
available at https://github.com/tzavellas/blazar_m
TeV neutrinos and hard X-rays from relativistic reconnection in the corona of NGC 1068
The recent discovery of astrophysical neutrinos from the Seyfert galaxy NGC
1068 suggests the presence of non-thermal protons within a compact "coronal"
region close to the central black hole. The acceleration mechanism of these
non-thermal protons remains elusive. We show that a large-scale magnetic
reconnection layer, of the order of a few gravitational radii, may provide such
a mechanism. In such a scenario, rough energy equipartition between magnetic
fields, X-ray photons, and non-thermal protons is established in the
reconnection region. Motivated by recent three-dimensional particle-in-cell
simulations of relativistic reconnection, we assume that the spectrum of
accelerated protons is a broken power law, with the break energy being
constrained by energy conservation (i.e., the energy density of accelerated
protons is at most comparable to the magnetic energy density). The proton
spectrum is below the break, and above the break, with IceCube neutrino observations suggesting . Protons above the break lose most of their energy within the
reconnection layer via photohadronic collisions with the coronal X-rays,
producing a neutrino signal in good agreement with the recent observations.
Gamma-rays injected in photohadronic collisions are cascaded to lower energies,
sustaining the population of electron-positron pairs that makes the corona
moderately Compton thick.Comment: 8 pages, 3 figures, with Appendice
Engineering of composite metallic microfibers towards development of plasmonic devices for sensing applications
The paper discusses the analysis of tapered hybrid composite microfibers based on a metal-core and dielectric-cladding composite material system. Its advantages over the pure metal tips conventionally used, are the inherent enhanced environmental robustness due to inert borosilicate cladding and the capability of multiple excitation of the tapered nanowire through the length of the fiber due to the enabled total internal reflection at the borosilicate/air interface. Simulations through finite element method (FEM) have demonstrated an improved field enhancement at the tapered region of such microfibers. Furthermore, experimental results on tapering in copper based microfibers together with light coupling and propagation studies will be demonstrated revealing the potential for the development of plasmonic devices for sensing applications
Tridiagonal Operators and Zeros of Polynomials in Two Variables
The aim of this paper is to connect the zeros of polynomials in two variables with the eigenvalues of a self-adjoint operator. This is done by use of a functional-analytic method. The polynomials in two variables are assumed to satisfy a five-term recurrence relation, similar to the three-term recurrence relation that the classical orthogonal polynomials satisfy
A review of methods for addressing components of interventions in meta-analysis
Many healthcare interventions are complex, consisting of multiple, possibly interacting, components. Several methodological articles addressing complex interventions in the metaanalytical context have been published. We hereby provide an overview of methods used to evaluate the effects of complex interventions with meta-analytical models. We summarized the methodology, highlighted new developments, and described the benefits, drawbacks, and potential challenges of each identified method. We expect meta-analytical methods focusing on components of several multicomponent interventions to become increasingly popular due to recently developed, easy-to-use, software tools that can be used to conduct the relevant analyses. The different meta-analytical methods are illustrated through two examples comparing psychotherapies for panic disorder. Copyright
110 Minima timings of ultra-short orbital period eclipsing binaries
We present 110 times of minima of 7 ultra-short orbital period eclipsing binaries
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