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