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Friday, 18.5.2012, 11:00, 31 Fitzwilliam Place

Dr. Denis Malishev

Bogolyubov Institute of Theoretical Physics, Kiev

Systematic study of the variability of Fermi/LAT sources

Abstract: Gamma-ray emission from the Galactic Plane is composed of three main contributions: diffuse emission from cosmic ray interactions in the interstellar medium, emission from extended sources, like supernova remnants and pulsar wind nebulae and emission from isolated compact source populations. The diffuse emission and emission from the extended sources provide the dominant contribution to the flux almost everywhere in the inner Galaxy, preventing the detection of isolated compact sources. In spite of this difficulty, compact sources in the Galactic Plane could be singled out based on the variability properties of gamma-ray emission. We perform a systematic study of variability of all-sky gamma-ray emission, via construction of variability maps. We find that emission from several directions along the Galactic Plane is significantly variable on month time scale. These directions include, apart from known variable Galactic sources and background blazars, the Galactic Ridge region at positive Galactic longitudes and several regions containing young pulsars. We argue that month-time scale variability associated to the pulsars originates from the inner parts of pulsar wind nebulae, similarly to what is observed in the Crab pulsar. A strong positive correlation between flux and the variability measure was observed for FSRQ type blazars, while the specified parameters are only weakly correlated for BL Lac type blazars, non-blazar AGNs and AGN of uncertain types.

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Wednesday, 16.5.2012, 16:00, 31 Fitzwilliam Place

Dr. Michael Sterzik

European Southern Observatory

Spectropolarimetry of Planet Earth

Abstract: I will discuss linear polarization spectra of the Earthshine that we recently obtained with FORS2. The spectra contain atmospheric oxygen and water vapor signatures and allow to discern the fractional contribution of clouds and ocean surfaces. They also reveal small amounts of vegetated areas on Earth, depending on the viewing geometry of the Earth as seen from the Moon. The measurements demonstrate that polarimetric spectra of the Earth contain robust information on biosignatures. The interpretation of these spectra is largely independent of the knowledge of the planet-hosting spectrum and insensitive to local observing conditions. (Spectro)polarimetry may become key for the imminent search for life in the universe with giant, ground-based, telescopes.

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Friday, 13.4.2012, 15:00, 31 Fitzwilliam Place

Dr. Jane Greaves

University of St. Andrews

Planetary Impacts and Habitability

Abstract: We are close to the discovery of an extrasolar planet of Earth-like mass and suitably far from its star for temperatures allowing liquid water. How inevitable is it that life would arise there? Other hazards exist, notably the presence around many Sun-like stars of large populations of planetesimals. High impact rates could render planetary surfaces uninhabitable, or potentially stimulate the rise of thermophilic life. I will present new astronomical observations and modelling results to put the history of life on Earth in this extrasolar context.

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Thursday, 12.4.2012, 15:00, 31 Fitzwilliam Place

Rachael Ainsworth

DIAS

Radio continuum observations of low mass young stars driving outflows

Abstract: We present 16 GHz deep radio continuum observations of a sample of classic low-mass young stars driving jets. We compile and examine spectral energy distributions for each source and calculate both radio and sub-mm spectral indices. We use these derived spectral indices to place constraints on the physical mechanisms responsible for the radio emission and find that 80% of the objects in this sample have spectral indices consistent with free-free emission from a partially ionized outflow. We examine correlations between the radio luminosity and bolometric luminosity, envelope mass, and outflow force and investigate the error contributions of different spectral parameters to constraining the radio luminosity of these objects.

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Monday, 2.4.2012, 15:00, 31 Fitzwilliam Place

Dr. Marco Padovani

CSIC-IEEC Barcelona

Cosmic-ray ionisation of molecular clouds

Abstract: Cosmic-rays constitute the main ionising and heating agent in dense, starless, molecular cloud cores. The main goal of this work is the calculation of the ionisation rate in a cloud resulting from the collisions between cosmic-ray particles (protons and electrons) and the matter constituting the cloud, mainly molecular hydrogen. We reexamine the physical quantities necessary to determine the cosmic-ray ionisation rate (especially the cosmic ray spectrum at E < GeV and the ionisation cross sections), and calculate the ionisation rate as a function of the column density of molecular hydrogen, through careful numerical integrations. Finally, we compute the attenuation of the cosmic-ray flux rate in a cloud core taking into account magnetic focusing, magnetic mirroring, and all relevant energy loss processes.

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Friday, 23.3.2012, 15:00, 31 Fitzwilliam Place

Nakisa Nooraee

Dublin Institute for Advanced Studies

Study of X-ray light curves of bright soft X-ray transients in M31

Abstract: Studies of Galactic low mass X-ray binaries, and more specifically soft X-ray transients (SXTs), show that members of this category of binary systems might offer a promising place to search for stellar-mass black holes. The launch of high resolution X-ray telescopes in the last decade, has given us the ability to detect and study extragalactic SXTs. As a nearby spiral galaxy with known distance, M31, has been assigned as an observation target for Chandra, XMM and SWIFT numerous times. In this talk, results of studying the X-ray light curve of the bright transients in M31 will be presented.

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Thursday, 22.3.2012, 16:00, 31 Fitzwilliam Place

Rosaria Bonito

Observatory of Palermo

Protostellar jets: a comparison between numerical models and X-ray and optical observations

Abstract: In the last decade X-ray emission from about ten protostellar jets has been discovered and now it appears as a common feature of the most energetic jets. I present the X-ray and optical observations of the protostellar jet HH 154 showing the presence of a stationary shock detected in X-rays at the base of the optical jet. This result can be interpreted as the emission due to shocks formed in a pulsed jet scenario and originating from a nozzle through which the jet is launched into the ambient medium. The origin of this nozzle could be related to the protostellar magnetic field which can be constrained by our model. I discuss the numerical model and compare the emission synthesized from our model with the observations.

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Wednesday, 14.3.2012, 11:00, 31 Fitzwilliam Place

Dr. Chris Conselice

University of Nottingham, UK

The GOODS NICMOS Survey: Watching Galaxy Formation

Abstract: I will present a full analysis of the modes of galaxy formation for massive galaxies with log M > 11 at z < 3 based on data from the Hubble GOODS NICMOS Survey. I will discuss the role of major mergers, star formation, AGN feedback, and for the first time, minor mergers, in the formation of massive galaxies down to z = 0. This is possible due to using new near-infrared Hubble Space Telescope imaging from NICMOS focused on massive galaxies in the distant universe. I will further show that modes besides the above are needed to form galaxies and will argue that gas accretion from the intergalactic medium is an important method for adding mass to the most massive galaxies, and potentially a major new part of the galaxy formation process.

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Friday, 2.3.2012, 15:00, 31 Fitzwilliam Place

Dr. Naoki Itoh

Sophia University, Tokyo, Japan

Seeing galaxy clusters through cosmic microwave background: Sunyaev-Zeldovich Effect

Abstract: Sunyaev and Zeldovich predicted in 1972 that microwave background photons coming from the directions of galaxy clusters will show spectral distortion due to the Compton scattering by high-temperature electrons that exist in the galaxy cluster plasma. This phenomenon was observed by Gull and Northover in 1976. Recent Sunyaev-Zeldovich effect observations with the use of interferometric telescope arrays by University of Chicago group led by John Carlstrom have revolutionized the field. By this method detailed Sunyaev-Zeldovich effect maps of about 60 galaxy clusters have been obtained by University of Chicago group. Many ongoing Sunyaev-Zeldovich survey observation projects of galaxy clusters will reveal thousands of galaxy clusters and will shed light on the evolution of the universe, providing us with a method to measure the Hubble constant, the dark matter content in our universe, and the dark energy content in our universe. On the theoretical side, accurate relativistic corrections to the Sunyaev-Zeldovich effect have been calculated by our group as well as some other groups, thus enabling the precision theoretical treatment. These relativistic corrections are essential for the ongoing high-frequency observations of galaxy clusters.

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Friday, 24.2.2012, 16:00, 31 Fitzwilliam Place

Prof. Luke Drury

Dublin Institute for Advanced Studies

First order Fermi acceleration driven by magnetic reconnection

Abstract: A box model is used to study first order Fermi acceleration driven by magnetic reconnection. It is shown, at least in this simple model, that the spectral index of the accelerated particles is related to the total compression in the same way as in diffusive shock acceleration and is not, as has been suggested, a universal $E^{-5/2}$ spectrum. The acceleration time-scale is estimated and some comments made about the applicability of the process.

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Friday, 3.2.2012, 15:00, 31 Fitzwilliam Place

Dr. Sean Matt

CEA Saclay

Stellar Winds During Star Formation and Evolution

Abstract: During star formation, stellar winds may be an important component of observed large-scale outflows from accreting systems, and theoretical considerations suggest that powerful stellar outflows may exist and may carry substantial angular momentum from young stars. During the main sequence, it is well-established that magnetized winds are responsible for explaining the observed spin-down of sun-like stars on long timescales. However, until a few years ago, our ability to predict the torque from stellar winds was based upon simple scaling arguments. With modern magnetohydrodynamic simulations, it is now possible to simulate magnetized stellar winds in multiple dimensions and to precisely quantify the torque and its dependence on stellar parameters. In my talk, I will describe our recent efforts to do this and to explore the consequences for the angular momentum evolution of stars. These calculations work together with (e.g.) observations of outflows, detailed magnetic field measurements of stars across the HR diagram, continuing advancements in stellar convection and dynamo models, and measurements and modeling of the evolution of stellar spin rates and activity, in order to paint a coherent picture of the evolution of stars, their internal dynamics, and the interaction with their environments.

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Thursday, 2.2.2012, 15:00, 31 Fitzwilliam Place

Dr. Emma Whelan

Dublin Institute for Advanced Studies

High Angular Resolution Observations of Outflow and Accretion Phenomena in Star Forming Regions

Abstract: While great strides have been made in our overall understanding of activity in star forming regions questions remain unanswered. High angular resolution observations are needed to provide constraints to leading models. In this talk I will describe how the techniques of Spectro-astrometry and Integral Field Spectroscopy can be used to address important open questions. I will discuss my current projects which include outflow and accretion activity in young brown dwarfs, jet launching mechanisms in classical T Tauri stars and planet formation at its earliest stages.

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Friday, 20.01.2012, 16:00, TCD-DIAS seminar, SNIAM Conference Room (Trinity College)

Keiichi Ohnaka

Max-Planck-Institut fuer Radioastronomie

Title: Spatially resolving the gas dynamics over the surface of the red supergiants Betelgeuse and Antares with the Very Large Telescope Interferometer

Abstract: The mass-loss mechanism in red supergiants is a long-standing problem. The milliarcsecond angular resolution achieved by infrared long-baselineinterferometry provides us with the only way to spatially resolve the region where the material is accelerated. We present spatially resolved high-spectral resolution observations of the 2.3 micron CO lines in the red supergiants Betelgeus and Antares using the Very Large Telescope Interferometer (VLTI) with a spatial resolution of 9.8 mas---the highest spatial resolution achieved for these well-studied stars. This has enabled us to spatially resolve the gas dynamics over the stellar surface for the first time other than the Sun. We have detected the vigorous motions of large CO gas clumps with velocities of up to 20--30 km/s. Comparison of the CO line data taken 1 year apart shows a significant change in the dynamics of the atmosphere. In contrast to the CO line data, the continuum data reveal no or only marginal time variations, much smaller than the maximum variation predicted by the current 3-D convection simulations. Our VLTI observations suggest the following new picture: the material within ~1.5 stellar radii is strongly stirred possibly by magnetohydrodynamical processes and/or pulsation and may be violently flung out rather than spilling out in an ordered, spherical fashion as often assumed.

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Thursday, 15.12.2011, 16:00, 31 Fitzwilliam Place

Sam Falle

University of Leeds

Smooth particle hydrodynamics (SPH) versus adaptive mesh refinement (AMR)

Abstract: The most widely used computational methods for astrophysical fluid dynamics are SPH (Smoothed Particle Hydrodynamics) and grid codes using AMR (Adaptive Mesh Refinement). The relative merits of these methods have been hotly disputed for a number of years, but in the end the only way to settle this is to compare them for a range of problems for which one has a good idea of the correct solution. I will discuss comparisons between the SPH code, SEREN, and the Godunov-type AMR code, MG for a number of test problems, such as Kelvin-Helmholtz, Rayleigh-Taylor and a simple model of the Santa Barbara cluster formation problem. The main conclusion is that the standard form of SPH is unreliable for a wide range of astrophysical problems and that some of the freely available AMR codes cannot be trusted either.

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Friday, 9.12.2011, 16:00, 31 Fitzwilliam Place

Denis Malyshev

Bogolyubov Institute for Theoretical Physics

Galactic Center at GeV energy range

Abstract: Employing data collected during the first 25 months' observations by the Fermi-LAT, we describe and subsequently seek to model the very high energy (>300 MeV) emission from the central few parsecs of our Galaxy. We analyze the morphological, spectral and temporal characteristics of the central source, 1FGL J1745.6-2900. Remarkably, the data show a clear, statistically significant signal at energies above 10 GeV, where the Fermi-LAT has an excellent angular resolution comparable to the angular resolution of HESS at TeV energies, which makes meaningful the joint analysis of the Fermi and HESS data. Our analysis does not show statistically significant variability of 1FGL J1745.6-2900. Using the combination of Fermi data on 1FGL J1745.6-2900 and HESS data on the coincident, TeV source HESS J1745-290, we show that the spectrum of the central gamma-ray source is inflected with a relatively steep spectral region matching between the flatter spectrum found at both low and high energies. We seek to model the gamma-ray production in the inner 10 pc of the Galaxy and examine, in particular, cosmic ray (CR) proton propagation scenarios that reproduce the observed spectrum of the central source. We show that a model that instantiates a transition from diffusive propagation of the CR protons at low energy to almost rectilinear propagation at high energies (given a reasonable energy-dependence of the assumed diffusion coefficient) can well explain the spectral phenomenology. In general, however, we find considerable degeneracy between different parameter choices which will only be broken with the addition of morphological information that gamma-ray telescopes cannot deliver given current angular resolution limits.We argue that a future analysis done in combination with higher-resolution radio continuum data holds out the promise of breaking this degeneracy.

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Friday, 18.11.2011, 16:00, 31 Fitzwilliam Place

Masha Chernyakova

Dublin City University

Title: Multiwavelength properties of gamma-ray loud binary systems

Abstract: Gamma-ray-loud binary systems are a newly identified class of X-ray binaries in which either accretion onto the compact object (a neutron star, or a black hole), or interaction of an outflow from the compact object with the wind and radiation emitted by the massive companion star leads to the production of very-high energy gamma-ray emission. Three such systems PSR B1259-63, LSI +61 303 and LS 5039, have been firmly detected as persistent or regularly variable TeV gamma-ray emitters. In my talk I will review multiwavelegth properties of these systems, try to explain the observed spectral variability and describe our attempt to find new members of this class.

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Friday, 28.10.2011, 15:00, 31 Fitzwilliam Place

Dr. Aleks Scholz

Dublin Institute for Advanced Studies

What do protostars look like?

Abstract: The process of star formation involves many orders of magnitude in physical scale and temperature. Hence, only multi-wavelength and high-resolution observations can provide a comprehensive picture of the early evolution of protostars and their environment. Here we demonstrate the power of such a synoptic analysis based on a case study for the binary low-mass protostar IRAS04325+2502. We combine new observations from Gemini, SMA, IRAM and ESO with Spitzer data to study the central sources, their disks, envelopes, and outflows. The observations show a complex threedimensional structure on scales ranging from a few AU to several parsec. We highlight the strong misalignment between the disks and the binary orbital plane, which favours turbulent fragmentation as a formation scenario. The study also illustrates the difficulties in assigning ages and masses to protostars. While our analysis confirms the paradigm for the structure of YSOs, it clearly demonstates the need to develop models for complex 3D configurations, including a detailed treatment of the dust chemistry. (based on Scholz et al., MNRAS, 2010, 409, 1557; Scholz et al., ApJL, 2008, 681, 29)

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Friday, 14.10.2011, 16:00, 31 Fitzwilliam Place

Dr. Valenti Bosch-Ramon

Dublin Institute for Advanced Studies

Particle Acceleration in Gamma-Ray Binaries

Abstract: Binary systems hosting compact objects and/or massive stars are powerful sources of non-thermal emission from radio to gamma rays. When the latter are detected, then these objects are known as gamma-ray binaries. In this work, we explore, in the context of gamma-ray binaries, different acceleration processes to estimate their efficiency: Fermi~I, Fermi~II, shear acceleration, the converter mechanism, and magnetic reconnection. We find that Fermi~I acceleration in a mildly relativistic shock can provide, although marginally, the particle energies required to explain observations. Shear acceleration may be a complementary mechanism, allowing particles to reach multi~10 TeV energies. Fermi~II acceleration may be too slow to account for the observed very high energy photons, but may be suitable to explain extended low-energy emission. The converter mechanism seems to require rather high Lorentz factors but cannot be discarded a priori. Efficient relativistic shock acceleration requires a highly turbulent, weakly magnetized downstream medium; magnetic reconnection, by itself possibly insufficient to reach very high energies, could perhaps facilitate such a conditions. Further theoretical developments, and better characterizations of the sources, are needed to pinpoint the dominant acceleration mechanism, which need not be one and the same in all sources.

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Friday, 7.10.2011, 16:00, 31 Fitzwilliam Place

Paul Dawson

Dublin Institute for Advanced Studies

Brown dwarfs in Upper Scorpius: new results from 3 surveys

Abstract: Results from a combination of 3 infra-red surveys in Upper Scorpius have increased the numbers of known brown dwarfs to a point where statistical comparison with other clusters is becoming possible. The same surveys are also yielding information on the lifetimes of circumsubstellar disks. The proposed talk will focus on the meaning of these results for the low mass end of the IMF in Upper Scorpius and the evolution of brown dwarf disks. The Upper Scorpius star forming region is the nearest OB association and represents our best chance of constraining the impact of massive stars on the formation and early evolution of very low mass objects. The Galactic Cluster Survey component of UKIDSS is enabling a census of brown dwarfs in ten large open star clusters and star forming regions. Data from the all sky surveys 2MASS and (most recently) WISE can now be combined with GCS data from Upper Scorpius. The proposed talk will show how this combination is extending the census of brown dwarfs in Upper Scorpius allowing a reliable star to brown dwarf ratio (3.8) be calculated. The talk will also discuss how WISE data is used to derive a new disk fraction (0.2) for brown dwarfs in Upper Scorpius.

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Monday, 26.09.2011, 16:00, 31 Fitzwilliam Place

Dr. Pavel Ivanov

Space Research Institute, Moscow

Dynamic tides in exoplanetary systems and the problem of formation of 'Hot Jupiters'

Abstract: In this talk I would like to discuss the problem of excitation of normal modes in rotating stars and giant planets on highly eccentric orbits as a result of tidal interactions and an associated transfer of energy and angular momentum. I am going to specify conditions on orbital parameters of such systems determining their secular evolution due to tides, in particular, leading to a persistent decrease of orbital eccentricity - the so called orbital circularisation, and corresponding circularisation time scales. The developed theory will be applied to the problem of formation of 'Hot Jupiters' - giant planets having quite small orbital periods of the order of a few days and orbital planes, which can be misaligned with equatorial plane of the host star.

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Thursday, 22.09.2011, 16:00, 31 Fitzwilliam Place

Dr. Pavel Ivanov

Space Research Institute, Moscow

A fully relativistic twisted disc around a slowly rotating Kerr black hole: derivation of dynamical equations and the shape of stationary configurations

Abstract: In this talk I am going to discuss equations describing dynamics and stationary configurations of a twisted fully relativistic thin accretion disc around a slowly rotating black hole and analyze analytically and numerically the shapes of stationary twisted configurations of accretion discs having non-zero inclinations with respect to the black hole equatorial plane at large distances from the black hole. The results reported in the talk may have implications for determining structure and variability of accretion discs close to Rms as well as for modelling of emission spectra coming from different sources, which are supposed to contain black holes. (abstract shortened)

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Thursday, 08.09.2011, 16:00, 31 Fitzwilliam Place

Dr. Dirk Froebrich

University of Kent (UK)

Observational studies of star formation: 1) cloud structure vs. star formation mode, 2) jets and outflows from YSOs

Abstract: In this talk I will present the results from two of our ongoing observational projects to understand the formation of stars in our Galaxy. At first I will concentrate on our efforts to understand how turbulence shapes the structure of nearby giant molecular clouds and how this, in turn, influences the mode of star formation (isolated or clustered) within them. The second half of the talk will be dedicated to the first results from the UKIRT Widefield Infrared Survey for H2 (UWISH2), a deep unbiased survey for molecular hydrogen emission line objects along the northern Galactic Plane.

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Thursday, 25.08.2011, 16:00, 31 Fitzwilliam Place

Dr. Ginny McSwain, Lehigh university

Lehigh university (USA)

Multiwavelength Observations of Gamma-ray Binaries

Abstract: The recent launch of the Fermi Gamma-ray Space Telescope has renewed interest in the rare class of high mass X-ray binaries that also have MeV-TeV gamma-ray emission. These systems consist of a hot, mass donor star and an unknown compact companion (either a neutron star or black hole). Two of the best known gamma-ray binaries are LS I +61 303 and LS 5039. I will present recent observations of LS I +61 303 that reveal a tidal mass stream that interacts with the compact companion for a fraction of the binary orbital period. A correlated burst in radio, X-ray, and gamma-ray emission is observed when the stream reaches the compact companion. Radio and optical observations of LS 5039 do not show evidence for a similar stream in that system, so I will discuss alternate ways to produce the very high energy emission.

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Thursday, 14.07.2011, 16:00, 31 Fitzwilliam Place

Dr. Pat Hartigan

Rice University

Observations and Laboratory Experiments of Fluid Dynamics in Stellar Jets

Abstract: New Hubble Space Telescope images have made it possible to observe how collimated supersonic jets from young stars evolve in real time. The resulting movies reveal shear and entrainment along the jet beams, as well as merging and fragmentation of clumps embedded within the jets. The systems show clear evidence for bow shocks and Mach disks, and also hot spots that may arise from Mach stems at the intersection points of individual shock waves. Other physical phenomena related to radiative precursors and photoablation processes are evident. This talk will summarize these discoveries, and present the results from several scaled laboratory experiments motivated by the observations.

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Thursday, 19.05.2011, 16:00, 31 Fitzwilliam Place

Sean Delaney

DIAS, Ireland

The Particle Spectrum from Relativistic Shock Acceleration, how hard can it be?!

Abstract: We present a newish code to calculate the particle distribution function about relativistic shocks including synchrotron losses using an explicit finite difference scheme. A continuous one dimensional plasma flow is considered, leading to a calculation in three dimensions plus time, the former three being momentum, pitch angle and position. The code should reproduce the expected power law behaviour in momentum at the shock for upstream flow speeds ranging from 0.1c to 0.995c (Gamma ~ 1...10). It also reproduces approximate analytical results for the synchrotron cut off shape for a non-relativistic shock. The code has been implemented as a hybrid OpenMP--MPI parallel code to make efficient use of SMP cluster architectures. It scales well to a few hundred cores for typical run sizes.

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Thursday, 12.05.2011, 16:00, 31 Fitzwilliam Place

Dr. Anna Scaife

DIAS, Ireland

Revealing the thermodynamic state of the intra-cluster medium with high resolution SZ imaging.

Abstract: The first all sky blind Sunyaev-Zel'dovich (SZ) effect cluster catalogue has now been produced by the Planck satellite and has revealed a new population of clusters undetected by X-ray surveys. This sample consists of hot, massive, morphologically disturbed clusters that are under luminous in the X-ray. This exciting new population has implications not only for understanding the thermodynamic state of the intra-cluster medium, but also for cluster cosmology where this population represents an important area of phase space in the cluster mass function. I will explain how the emerging science of high resolution SZ imaging is now allowing us to probe the hot gas in such clusters, and to assess the significance of hot gas in the disturbed ICM biasing both the X-ray selection function and causing discrepancies in the X-ray luminosity - mass scaling relations.

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Thursday, 24.03.2011, 16:00, 31 Fitzwilliam Place

Dr. Anabella Araudo

Instituto Argentino de Radioastronomia (CONICET) and University of La Plata, Argentina

Interactions between clumps from winds of massive stars with jets in high-mass microquasars

Abstract: High-mass microquasars (HMMQs) are binary systems formed by a massive star and an accreting compact object from which relativistic jets are launched. Considerable observational evidence supports the idea that the winds of massive stars are formed by clumps, which should interact with the jets in HMMQs. We will present here a study of the interaction between a microquasar jet and clumps. This interaction produce a weak radiative shock in the clump, and a strong adiabatic shock in the jet. In the former case, we study the main dynamical processes that take place in the clump as a consequence of the passing shock. In the later case, particles can be accelerated up to relativistic energies and produce non-thermal emission. We calculate the spectral energy distribution resulting from the dominant non-thermal processes taking place in the interaction region: synchrotron and inverse Compton radiation from relativistic electrons, and gamma-rays from inelastic proton-proton collisions. Significant levels of X- and gamma-rays are predicted, with luminosities up to 10^34-10^36 erg/s, and associated timescales of about 1 h. These phenomena may explain the fast variability found in some high-mass X-ray binaries such as Cygnus X-1 and Cygnus X-3. Its investigation can help to obtain information on the properties of jets and clumpy winds.

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Wednesday, 9.03.2011, 12:00, 31 Fitzwilliam Place

Dr. Aya Bamba

DIAS, Ireland

X-ray evolution of SNRs and PWNe

Abstract: SNRs and PWNe are one of the main candidates of Galactic cosmic-ray accelerators. In X-ray observations, we can detect synchrotron X-rays from accelerated electrons, and now we know several young SNRs and PWNe emit them, as well as VHE gamma-rays. On the other hand, it is not so clear observationally how these systems evolve with age. This is partly because the older objects expand and their surface brightness in X-rays become fainter and fainter, and as a result, the previous observations failed to detect older systems. Suzaku has X-ray CCDs with large effective area and low and stable background, and good at finding such faint and extended sources. We have made deep observations of older SNRs and PWNe, and found the following results: 1. Synchrotron X-rays from SNRs decays suddenly when SNRs becomes ~10 pc (~1000 years old). 2. PWNe keeps expanding up to more than 105 years. In this talk, we discuss on the origin of their evolution.

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Thurday, 24.02.2011, 16:00, 31 Fitzwilliam Place

Dr. Koraljka Muzic

University of Toronto, Canada

Substellar Objects in Nearby young Clusters

Abstract: Star forming regions harbor a large population of free-floating objects below the stellar-mass boundary. These objects include brown dwarfs, but also a population of objects with masses comparable to those of massive planets (planemos). SONYC, "Substellar Objects in Nearby Young Clusters", is an ongoing project to provide a complete census of the substellar population in nearby young clusters. The survey relies on extremely deep wide-field optical and near-infrared imaging, with follow-up spectroscopy. With the use of facilities such as Subaru, VLT, Gemini, CTIO, NTT and Spitzer, we aim to probe the bottom end of the IMF to unprecedented levels. In this talk, I will discuss the current state of knowledge and the main issues in the studies of the IMF at the low-mass end. I will report on the current status of the SONYC survey and present our results on NGC 1333, Rho Ophiuchus, and Chamaeleon-I. I will also present our future plans and discuss the currently used observing strategies. Finally, I will address the completeness of the current brown dwarf census in young clusters and the implications of our findings for star formation theory.

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Friday, 10.02.2011, 16:00, 31 Fitzwilliam Place

Eva Lefa

Max-Planck-Institut für Kernphysik, Germany

The formation of hard TeV gamma-ray spectra in blazars

Abstract: The TeV gamma-ray spectra of some BL Lacs, after being corrected for intergalactic absorption in the extragalactic background light, appears unusually hard. This finding poses challenges to conventional acceleration and emission models. We analyze the constraints to produce such hard TeV spectra within a time-dependent, leptonic synchrotron-Compton approach, based on the solutions of the relevant kinetic equations introducing adiabatic losses. We examine two types of electron distributions, power-law with high value of "low energy cut-off" and relativistic Maxwellian distribution.

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Thursday, 17.01.2011, 16:00, 31 Fitzwilliam Place

Dr. Manel Perucho

Universitat de València , Spain

Relativistic Outflows in the computer lab: numerical simulations of microquasar and extragalactic jets.

Abstract: Relativistic outflows are commonly associated to accreting compact objects. These systems generate highly collimated jets able to extract large amounts of energy from the surroundings of the compact object to large distances, having an effect on their environments and generating radiation in the whole spectrum. Numerical simulations are our best laboratory to study these objects. In my talk I will review our last results in two different scenarios where jets have an important role: Microquasars and Active Galactic Nuclei. In the field of microquasars, we have performed different 3D simulations to study the interaction of jets with stellar winds in High Mass X-ray Binaries. We show that such interaction may produce candidate locations for particle acceleration and high-energy emission. In addition, disruption of the jet can be produced if the jet power is not large enough, frustrating the growth of these jets. I will also present results on simulations of longer-term evolution of microquasar jets in two dimensions. In particular, we were interested on the interaction with different media such as the Supernova Remnant or shocked Interstellar Medium and its radiative signature. In the second part of my talk, I will present 2D simulations, with a realistic equation of state, of long-term evolution of FRII jets in a typical galaxy and cluster environment, from 1 kpc to almost 1 Mpc, and I will discuss some interesting conclusions regarding cooling flows.