596 research outputs found
Pulsar Prospects for the Cherenkov Telescope Array
In the last few years, the Fermi-LAT telescope has discovered over a 100
pulsars at energies above 100 MeV, increasing the number of known gamma-ray
pulsars by an order of magnitude. In parallel, imaging Cherenkov telescopes,
such as MAGIC and VERITAS, have detected for the first time VHE pulsed
gamma-rays from the Crab pulsar. Such detections have revealed that the Crab
VHE spectrum follows a power-law up to at least 400 GeV, challenging most
theoretical models, and opening wide possibilities of detecting more pulsars
from the ground with the future Cherenkov Telescope Array (CTA). In this
contribution, we study the capabilities of CTA for detecting Fermi pulsars. For
this, we extrapolate their spectra with "Crab-like" power-law tails in the VHE
range, as suggested by the latest MAGIC and VERITAS results.Comment: 4 pages, 3 figures. In Proceedings of the 2012 Heidelberg Symposium
on High Energy Gamma-Ray Astronomy. All CTA contributions at arXiv:1211.184
Very high energy gamma-ray emission from X-ray transients during major outbursts
Context: Some high mass X-ray binaries (HMXB) have been recently confirmed as
gamma-ray sources by ground based Cherenkov telescopes. In this work, we
discuss the gamma-ray emission from X-ray transient sources formed by a Be star
and a highly magnetized neutron star. This kind of systems can produce variable
hadronic gamma-ray emission through the mechanism proposed by Cheng and
Ruderman, where a proton beam accelerated in the pulsar magnetosphere impacts
the transient accretion disk. We choose as case of study the best known system
of this class: A0535+26. Aims: We aim at making quantitative predictions about
the very high-energy radiation generated in Be-X ray binary systems with
strongly magnetized neutron stars. Methods: We study the gamma-ray emission
generated during a major X-ray outburst of a HMXB adopting for the model the
parameters of A0535+26. The emerging photon signal from the disk is determined
by the grammage of the disk that modulates the optical depth. The
electromagnetic cascades initiated by photons absorbed in the disk are
explored, making use of the so-called "Approximation A" to solve the cascade
equations. Very high energy photons induce Inverse Compton cascades in the
photon field of the massive star. We implemented Monte Carlo simulations of
these cascades, in order to estimate the characteristics of the resulting
spectrum. Results: TeV emission should be detectable by Cherenkov telescopes
during a major X-ray outburst of a binary formed by a Be star and a highly
magnetized neutron star. The gamma-ray light curve is found to evolve in
anti-correlation with the X-ray signal.Comment: 8 pages, 7 figures, accepted for publication in Astronomy and
Astrophysical journa
An Eccentric Circumbinary Accretion Disk and the Detection of Binary Massive Black Holes
We present a two-dimensional grid-based hydrodynamic simulation of a thin,
viscous, locally-isothermal corotating disk orbiting an equal-mass Newtonian
binary point mass on a fixed circular orbit. We study the structure of the disk
after multiple viscous times. The binary maintains a central hole in the
viscously-relaxed disk with radius equal to about twice the binary semimajor
axis. Disk surface density within the hole is reduced by orders of magnitude
relative to the density in the disk bulk. The inner truncation of the disk
resembles the clearing of a gap in a protoplanetary disk. An initially circular
disk becomes elliptical and then eccentric. Disturbances in the disk contain a
component that is stationary in the rotating frame in which the binary is at
rest; this component is a two-armed spiral density wave. We measure the
distribution of the binary torque in the disk and find that the strongest
positive torque is exerted inside the central low-density hole. We make
connection with the linear theory of disk forcing at outer Lindblad resonances
(OLRs) and find that the measured torque density distribution is consistent
with forcing at the 3:2 (m=2) OLR, well within the central hole. We also
measure the time dependence of the rate at which gas accretes across the hole
and find quasi-periodic structure. We discuss implications for variability and
detection of active galactic nuclei containing a binary massive black hole.Comment: 10 pages; replaced to match ApJ version; includes new physical
interpretation of torque density (Sec. 4.1); large mpeg animation is
available at http://www.tapir.caltech.edu/~milos/circBinaryEccDisk.mp
Prospects for Observations of Pulsars and Pulsar Wind Nebulae with CTA
The last few years have seen a revolution in very-high gamma-ray astronomy
(VHE; E>100 GeV) driven largely by a new generation of Cherenkov telescopes
(namely the H.E.S.S. telescope array, the MAGIC and MAGIC-II large telescopes
and the VERITAS telescope array). The Cherenkov Telescope Array (CTA) project
foresees a factor of 5 to 10 improvement in sensitivity above 0.1 TeV,
extending the accessible energy range to higher energies up to 100 TeV, in the
Galactic cut-off regime, and down to a few tens GeV, covering the VHE photon
spectrum with good energy and angular resolution. As a result of the fast
development of the VHE field, the number of pulsar wind nebulae (PWNe) detected
has increased from one PWN in the early '90s to more than two dozen firm
candidates today. Also, the low energy threshold achieved and good sensitivity
at TeV energies has resulted in the detection of pulsed emission from the Crab
Pulsar (or its close environment) opening new and exiting expectations about
the pulsed spectra of the high energy pulsars powering PWNe. Here we discuss
the physics goals we aim to achieve with CTA on pulsar and PWNe physics
evaluating the response of the instrument for different configurations.Comment: accepted for publication in Astroparticle Physic
Protective Nanocomposite Vacuum Coatings Deposited by Separated Plasma Flows
The way of generation of protective nanostructure vacuum coatings by separated plasma flows was investigated. In order to limit the crystallite growth in the coatings the compound of the growing condensate was doped by Al. Nanocrystalline (Ti,Al)N coatings with grain size 10-20 nm and microhardness 35-40 GPa by means of vacuum-arc deposition technique from the separated plasma flows were obtained. It was determined that purposeful alloying improved operational coatings properties, and allowed using them as protective layers, deposited on the working surfaces of the cutting tools during timber-based material processing
Pulsar shadow as the origin of double notches in radio pulse profiles
We present the model of eclipsing a rotating, spatially extended source of
directional emission by a central absorber, and apply it to the pulsar
magnetosphere. The model assumes the radially extended inward radio emission
along the local direction of the magnetic field, and the pulsar as the
absorber. The geometry of the magnetic field lines of the rotating dipole is
favourable for the double eclipse events, which we identify with the double
notches observed in pulse profiles of nearby pulsars. For pulsars with large
dipole inclinations 70 <~ alpha <~ 110 deg the double notches are predicted to
occur within a narrow phase range of 20 to 30 deg before the main radio peak.
Application of the model to PSR B0950+08 establishes it as a nearly orthogonal
rotator (alpha =~ 75 deg, beta =~ -10 deg) with many pulse components naturally
interpreted in terms of the inward radio emission from a large range of
altitudes. The inward components include the intermittently strong, leading
component of the main pulse, which would traditionally have been interpeted as
a conal emission in the outward direction. The model also identifies the
magnetic field lines along which the radially extended inward radio emission
occurs in B0950+08. These have a narrow range of the footprint parameter s
close to 1.1 (closed field line region, near the last open field lines). We
describe directional characteristics of inward emission from the radially
extended region and compare them with characteristics of extended outward
emission. Our work shows that pulse profiles of at least some pulsars may be a
superposition of both inward and outward emission.Comment: 28 pages, 13 figures, accepted by ApJ, high-quality figures are
available from http://www.ncac.torun.pl/~michalf/inward1_figs
Учет влияния останова реактора ВВЭР-1200 для перегрузки при расчете активностей продуктов деления
Current work is aimed at the analysis of the fission products decay influence during fuel reloading, when calculating the accumulated fission products activity for the VVER-1200 reactor fuel campaign. The Bateman problem solution based technique was used for calculations, within the framework of the two fissile nuclides approximation. The fission products producing process for the VVER-1200 reactor stationary campaign is considered, taking into account the reactor shutdown periods for refueling and without taking them into account (instant reload approximation). It was shown, that the instant reload approximation for fission products activity calculations gives the similar accurate result, as calculations with taking into account the shutdown periods. The results can be used to significantly simplify the calculations of fission product activity accumulation in nuclear power reactors.Целью исследования являлся анализ влияния распада продуктов деления во время перегрузки топлива, при определении наработанной активности продуктов деления в течение топливной кампании реактора ВВЭР-1200. Для расчетов применялась методика на основе аналитического решения задачи Бейтмана в рамках приближения двух делящихся нуклидов. Рассмотрен процесс наработки продуктов деления для стационарной кампании реактора ВВЭР-1200 с учетом периодов останова реактора для перегрузки топлива и без их учета (приближение мгновенной перегрузки). Было установлено, что расчет удельных активностей продуктов деления в моменты останова реактора для перегрузки в приближении мгновенной перегрузки по точности практически не уступает расчетам, учитывающим распад продуктов деления за период перегрузки. Полученные результаты могут быть использованы для существенного упрощения расчетов наработки активностей продуктов деления в ядерных энергетических реакторах
Study of Gamma Ray Burst Binary Progenitors
Recently much work in studying Gamma-Ray Burst has been devoted to revealing
the nature of outburst mechanism and studies of GRB afterglows. These issues
have also been closely followed by the quest for identifying GRB progenitors.
In this paper we consider the proposed binary star progenitors of GRBs: white
dwarf neutron star binaries, white dwarf black hole binaries, helium core
neutron star mergers, helium core black hole mergers, double neutron stars and
neutron star black hole binaries. Using population synthesis methods we
calculate merger rates of these binary progenitors and we compare them to the
observed BATSE GRB rate. We also calculate the distribution of merger sites
around host galaxies and compare them to the observed locations of GRB
afterglows with respect to their hosts. We find that the rates of binary GRB
progenitors in our standard model are lower than the observed GRB rates if GRBs
are highly collimated. However, the uncertainty in the population synthesis
results is too large to make this a firm conclusion. Although some
observational signatures seem to point to collapsars as progenitors of long
GRBs, we find that mergers of WD-NS, He-NS, He-BH, and NS-NS systems also trace
the star formation regions of their host galaxies, as it is observed for long
GRBs. We also speculate about possible progenitors of short-duration GRBs. For
these, the most likely candidates are still mergers of compact objects. We find
that the locations NS-NS and NS-BH mergers with respect to their hosts are
significantly different. This may allow to distinguish between these two
progenitor models, once current and near future missions, such as HETE-II or
SWIFT, measure the locations of short GRBs.Comment: 26 pages, 12 figures, submitted to Ap
Discovery of VHE gamma-rays from the high-frequency-peaked BL Lac object RGB J0152+017
Aims: The BL Lac object RGB J0152+017 (z=0.080) was predicted to be a very
high-energy (VHE; > 100 GeV) gamma-ray source, due to its high X-ray and radio
fluxes. Our aim is to understand the radiative processes by investigating the
observed emission and its production mechanism using the High Energy
Stereoscopic System (H.E.S.S.) experiment. Methods: We report recent
observations of the BL Lac source RGB J0152+017 made in late October and
November 2007 with the H.E.S.S. array consisting of four imaging atmospheric
Cherenkov telescopes. Contemporaneous observations were made in X-rays by the
Swift and RXTE satellites, in the optical band with the ATOM telescope, and in
the radio band with the Nancay Radio Telescope. Results: A signal of 173
gamma-ray photons corresponding to a statistical significance of 6.6 sigma was
found in the data. The energy spectrum of the source can be described by a
powerlaw with a spectral index of 2.95+/-0.36stat+/-0.20syst. The integral flux
above 300 GeV corresponds to ~2% of the flux of the Crab nebula. The source
spectral energy distribution (SED) can be described using a two-component
non-thermal synchrotron self-Compton (SSC) leptonic model, except in the
optical band, which is dominated by a thermal host galaxy component. The
parameters that are found are very close to those found in similar SSC studies
in TeV blazars. Conclusions: RGB J0152+017 is discovered as a source of VHE
gamma-rays by H.E.S.S. The location of its synchrotron peak, as derived from
the SED in Swift data, allows clearly classification it as a
high-frequency-peaked BL Lac (HBL).Comment: Accepted for publication in A&A Letters (5 pages, 4 figures
H.E.S.S. observations of gamma-ray bursts in 2003-2007
Very-high-energy (VHE; >~100 GeV) gamma-rays are expected from gamma-ray
bursts (GRBs) in some scenarios. Exploring this photon energy regime is
necessary for understanding the energetics and properties of GRBs. GRBs have
been one of the prime targets for the H.E.S.S. experiment, which makes use of
four Imaging Atmospheric Cherenkov Telescopes (IACTs) to detect VHE gamma-rays.
Dedicated observations of 32 GRB positions were made in the years 2003-2007 and
a search for VHE gamma-ray counterparts of these GRBs was made. Depending on
the visibility and observing conditions, the observations mostly start minutes
to hours after the burst and typically last two hours. Results from
observations of 22 GRB positions are presented and evidence of a VHE signal was
found neither in observations of any individual GRBs, nor from stacking data
from subsets of GRBs with higher expected VHE flux according to a
model-independent ranking scheme. Upper limits for the VHE gamma-ray flux from
the GRB positions were derived. For those GRBs with measured redshifts,
differential upper limits at the energy threshold after correcting for
absorption due to extra-galactic background light are also presented.Comment: 9 pages, 4 tables, 3 figure
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