List of Past CTC Theory Lunches : 01-Jan-2013 to 01-Jun-2013

Date:   Wednesday 30-Jan-2013
Speaker:   Anatoly Spitkovsky (Princeton)
Title:  The emerging role of reconnection in high energy phenomena

In this informal talk, I will discuss kinetic simulations of astrophysical objects where, somewhat unexpectedly, reconnection appears to be important in determining the dynamics and the observables. The examples will be drawn from simulations of pulsar magnetospheres, deceleration of poynting-dominated outflows, and magneto-rotational instability in accretion disks. I will also comment on the expected and observed nonthermal acceleration processes during reconnection, and the universality of particle spectra produced in reconnection.

Date:   Monday 04-Feb-2013
Speaker:   Rodrigo Nemmen (GSFC)
Title:  Unifying Black Hole Jets Across the Mass Scale

Relativistic jets are generated in black hole systems with a wide variety of masses: think of gamma-ray bursts, microquasars, and radio galaxies. How is this fascinating black hole activity connected across the vast mass scale? I will give a general overview of previous evidence for such connections involving black hole binaries and AGNs. Then, I will present new results based on observations made with the Fermi and Swift observatories. I will show that, despite tremendous differences in luminosity and black hole mass, the relativistic jets produced in gamma-ray bursts and AGNs follow a universal scaling for their energetics. I will discuss the exciting implications of this finding.

Date:   Monday 11-Feb-2013
Speaker:   Rahul Shetty (Heidelberg)
Title:  Understanding the ISM using Bayesian analysis: Fitting the Kennicutt-Schmidt relationship and Dust SEDs

Fitting data is a central component of many observational investigations, including the comparison of theoretical models with observed systems. Measurement uncertainties, noise, model degeneracies, and hierarchical data structure must be rigorously treated in order to obtain accurate fitting results. I will discuss how Bayesian methods can handle each of these issues when fitting data. After presenting the general background of Bayesian inference, I will demonstrate the accuracy of Bayesian fitting using models with known parameters. I will also contrast the Bayesian parameter estimates with those obtained from traditional methods, such as least-squares fitting. I will then present recent results of the application of Bayesian fitting of observed data to quantify the Kennicutt-Schmidt (KS) relationship and dust spectral energy distributions (SEDs). In applying a hierarchical Bayesian fit on a sample of spiral galaxies compiled by Bigiel et al. (2008), I will show that there is significant variation in the KS parameters, indicating that no single KS relationship holds for all galaxies. This suggests that the relationship between molecular gas and star formation differs from galaxy to galaxy, possibly due to the influence of other physical properties within a given galaxy, such as metallicity, molecular gas fraction, stellar mass, and/or magnetic fields. The mean index of the KS relationship for the population is 0.84, with 2sigma range [0.63, 1.0]. The sub-linear KS relationship estimated for a number of the individual galaxies suggests that CO emission is tracing some molecular gas that is not directly associated with star formation. When fitting dust SEDs, simple least-squares methods produce an artificial anti-correlation between the dust temperature (T) and spectral index (beta), due to noise and the intrinsic degeneracy between T and beta. I will demonstrate that a Bayesian fit accounts for this degeneracy, so that the final parameter estimates are not artificially anti-correlated. When fitting Herschel data of a star forming core, the Bayesian results indicate no anti-correlation between beta and T, whereas a least-squares fit produces the anti-correlation. This suggests that any beta-T anti-correlation may be fully driven by noise in the observed fluxes. The fitting results portray a decreasing spectral index with increasing density and decreasing temperature, possibly suggestive of grain growth in the dense, cold ISM.

Date:   Monday 18-Feb-2013
Speaker:   Margaret Pan (GSFC)
Title:  Stochastic flights of Saturn's propellers

In a solar system analogue to exoplanet-disk interactions, kilometer-sized moonlets embedded in Saturn's A ring create S-shaped wakes called 'propellers' in surrounding material. The Cassini spacecraft has tracked the motions of propellers for several years and finds that they deviate from Keplerian orbits with constant semimajor axes. The inferred orbital migration is known to switch sign. We show using a statistical test that the time series of orbital longitudes of the propeller Bleriot is consistent with that of a time-integrated Gaussian random walk. That is, Bleriot's observed migration pattern is consistent with being stochastic. We further show, using a combination of analytic estimates and collisional N-body simulations, that stochastic migration of the right magnitude to explain the Cassini observations can be driven by encounters with ring particles 10-20 m in radius. That the local ring mass is concentrated in decametre-sized particles is supported on independent grounds by occultation analyses.

Date:   Monday 25-Feb-2013
Speaker:   Sam Leitner (UMD)
Title:  On the formation of disk galaxies

Disk galaxies are sensitive probes of the processes governing the growth of stellar mass in the universe. However, cosmological hydrodynamic simulations still struggle to match the main observed properties of this galaxy population, or to converge on predictions. I will discuss an analysis of observations, which suggests that simulations tend to form stars too early in these galaxies. This finding dovetails with simulation results I will present, showing that improved treatment of stellar feedback in molecular clouds is key to getting disk structures right. I will briefly discuss improved treatment of star formation and feedback processes in simulations. Finally, I will introduce a major code comparison effort that will bring galaxy simulations into better agreement and help identify robust predictions.

Date:   Monday 11-Mar-2013
Speaker:   Sean McWilliams (Princeton)
Title:  Binary Systems as Resonance Detectors for Gravitational Waves

Gravitational waves at suitable frequencies can resonantly interact with a binary system, inducing changes to its orbit. A stochastic gravitational-wave background causes the orbital elements of the binary to execute a classic random walk, with the variance of the orbital elements growing with time. The lack of such a random walk in binaries that have been monitored with high precision over long time-scales can thus be used to place an upper bound on the gravitational-wave background. Using periastron time data from the Hulse-Taylor binary pulsar spanning ~30 years, we obtain a bound of h_c < 7.9 x 10^{-14} at ~10^{-4} Hz, where h_c is the strain amplitude per logarithmic frequency interval. Our constraint complements those from pulsar timing arrays, which probe much lower frequencies, and ground-based gravitational-wave observations, which probe much higher frequencies. Interesting sources in our frequency band, which overlaps the lower sensitive frequencies of proposed space-based observatories, include white-dwarf/supermassive black-hole binaries in the early/late stages of inspiral, and TeV scale preheating or phase transitions. The bound improves as (time span)^{-2} and (sampling rate)^{-1/2}. The Hulse-Taylor constraint can be improved to ~3.8 x 10^{-15} with a suitable observational campaign over the next decade. Our approach can also be applied to other binaries, including (with suitable care) the Earth-Moon system, to obtain constraints at different frequencies. The observation of additional binary pulsars with the SKA could reach a sensitivity of h_c ~ 3 x 10^{-17}.

Date:   Monday 25-Mar-2013
Speaker:   Roman Shcherbakov (UMD)
Title:  A New Channel for Low-Luminosity GRBs: Tidal Disruptions of White Dwarfs by Intermediate Mass Black Holes

Low-luminosity GRBs or X-ray flashes (XRFs), which often accompany supernovae, are typically ascribed to either the supernova shock breakout or weak GRBs powered by the central engine of stellar mass. We propose the tidal disruption of a white dwarf (WD) by an intermediate-mass black hole (IMBH) as another channel for XRFs. Such disruptions last for 100-5000 seconds. The release of gravitational energy over short time generates a powerful flare. The magnetic field is quickly amplified in the fallback material, and then the BH launches a slow uncollimated jet. The emission from jet photosphere dominates X-rays with Comptonized thermal spectrum, while the expanding jet shell produces most of IR/optical. The prompt flare may be followed by an underluminous fast supernova, resulting from a tidal compression and thermonuclear ignition of a WD. High event rate in dwarf galaxies warrants searches among the known and future transients observed with Swift satellite. We perform detailed dynamical and spectral modeling of a candidate disruption source GRB060218/SN2006aj. The BH mass is independently estimated to be 20,000 solar masses based on (1) the event duration, (2) the jet base radius from the thermal X-ray component, and (3) the properties of a host galaxy. The supernova position is consistent with a center of a dwarf host galaxy. Other potential candidates are the flashes with very weak/absent supernovae such as XRF040701.

Date:   Monday 01-Apr-2013
Speaker:   Gianfranco Brunetti (Inst. for Radioastronomy, Bologna)
Title:  Acceleration of cosmic rays and non-thermal emission in galaxy clusters

Galaxy clusters are the most massive structures in the Universe, they grow hierarchically via accretion of matter and mergers between sub-structures. Mergers between clusters are the most energetic events in the Universe and convert a fraction of their energy into the acceleration of relativistic matter in these massive systems. This is probed by radio observations that indeed detect large scale radio (synchrotron) emission in a fraction of merging galaxy clusters. After reviewing the most important observational constraints, from radio to gamma-ray observations, I will outline the theoretical scenario for the origin and evolution of relativistic particles in galaxy clusters. Turbulence plays an important role in this scenario and I will be focussing on the physics of turbulence and turbulent acceleration in these environments. I will discuss the most important implications and expectations based on this scenario and discuss the science case of incoming new generation radio facilities (LOFAR, ASKAP,..) and of future high-energy observatories (Astro-H, CTA, ..).

Date:   Monday 08-Apr-2013
Speaker:   Krista Smith (UMD)
Title:  XBONGs and Optically Elusive AGN: Infrared AGN Identifications and Reasons for Optical Dullness

We have obtained near-infrared (NIR) spectra of four optically-elusive AGN and four XBONGs that were selected by Swift-BAT in the ultra-hard X-ray (14 - 195 keV). We combine this new data with archival observations from SDSS, WISE, 2MASS, and Spitzer to undertake an overview analysis of possible explanations for these objects in the optical and NIR bands. We identify a number of AGN indicators in the NIR and FIR that successfully identify these elusive objects as AGN without the expense and difficulty of X-ray selection; namely, NIR emission line diagnostic ratios, the presence of high ionization lines in the NIR and FIR, the presence of a hidden BLR in the NIR spectrum, and infrared photometry. We also discount the possibility that these eight ob jects are optically elusive or normal due to excess star-formation in the host, the presence of radiatively-inefficient accretion flows (RIAFs) resulting in lack of an ionizing accretion disk, or the presence of significant starlight dilution from the host galaxy.

Date:   Monday 15-Apr-2013
Speaker:   Maithili Kalamkar (Univ. of Amsterdam)
Title:  Probing the Accretion-flow Dynamics using Energy Dependent variability Analyses in Black Hole X-ray Binaries

X-ray binaries have been studied for decades, yet, some major and important aspects of the accretion flow remain poorly understood. Understanding the origin of X-ray variability will play a major role in advancing our current knowledge of the two component accretion flow: the accretion disk and the corona/hot flow. The detailed variability studies in the past 15 years were conducted mostly using the Rossi X-ray Timing Explorer (RXTE) in the energy range > 2 keV, which is dominated by the hot flow emission. With the Swift X-Ray Telescope (XRT), we can explore energies < 2 keV which are dominated by the disk emission. I will present the first detailed energy dependent variability studies performed with Swift XRT of a black hole binary SWIFTJ1753.5-012. This work strongly indicates that the hot flow is more variable in the peak of the outburst and the disk is more variable at low intensities. I will discuss how these results support the scenario with different driving mechanisms for different variability components.

Date:   Monday 22-Apr-2013
Speaker:   Anton Dorodnitsyn (GSFC)
Title:  Hiding a supermassive black hole behind dusty obscuring winds in AGN

Observational properties of active galactic nucleus are essentially influenced by their winds. In the first part of my talk. I will review the current status of the theoretical modeling of AGN unification including radiation-driven and MHD winds. In the second part I will present the results from the radiation hydrodynamics (RHD) simulations of the dusty obscuring flows in AGN.

The illumination of a parsec-scale, geometrically thin, dusty accretion disk by X-rays, and UV radiation leads to the conversion of such radiation into IR. Our flux-limited diffusion RHD simulations show that in result the disk becomes geometrically thick due to pressure of the IR radiation on dust, and eventually develops a rigorous IR-driven accretion disk wind. When AGN luminosity, exceeds 0.1Ledd, the outflowing dusty wind provides the obscuration with IR pressure on dust playing a major role. The outer dense and cold part of the flow provides obscuration while the inner hot, and photoionized has properties similar to those of a warm absorber flow. At luminosities <=0.1Ledd episodes of the outflow are followed by extended periods when the wind switches to slow accretion.

Date:   Monday 29-Apr-2013
Speaker:   Rachel Bean (Cornell)
Title:  Opportunities to test gravity with upcoming large scale structure surveys (Note special time!)

In the search to understand cosmic acceleration a variety of alternatives to Einstein's cosmological constant, including novel matter and modifications to General Relativity, are currently under consideration. We discuss the observational implications of matter and gravity-based dark energy theories and how upcoming cosmological surveys can provide insights into dark energy's nature. In particular we discuss the pivotal roles of upcoming large scale structure surveys measuring weak lensing and redshift space distortions.

Date:   Monday 06-May-2013
Speaker:   Francesca Civano (Dartmouth, CfA)
Title:  Studying Rare Sources in Extragalactic X-ray surveys

The supermassive black holes (SMBHs), found almost ubiquitously at the center of large galaxies, grew by just two mechanisms: accretion and mergers. Extragalactic multiwavelength medium-wide area surveys, as COSMOS, have been proven to be a uniquely effective tool to study the SMBH growth in the Universe, in particular for rare sources, where the best trade between area and depth is needed. In my talk I will present accretion and merger processes, and their signatures, in two specific ways: (1) SMBHs ejected from the galaxy nucleus, during the merger of a tight SMBH binary, are the loudest source of Gravitational Waves (GW) and should sometimes be seen as displaced quasars. (2) SMBH accretion at early times (when the Universe was <2 Gyr old). These SMBHs should all lie in the nascent large scale structures. I will focus on the X-ray surveys of the COSMOS field, showing predictions for the recently approved COSMOS-Legacy, a 2.8 Ms Chandra Visionary Project as well as the harder X-ray survey of the COSMOS field with the NuSTAR telescope.

Date:   Monday 13-May-2013
Speaker:   Or Graur (Tel-Aviv/AMNH)
Title:  The Oldest Explosions: Type-Ia Supernova Rates from Low- and High-Redshift Surveys

For the past decade, Type-Ia supernovae have been instrumental in measuring the cosmological parameters and in revealing the accelerating nature of the Universe's expansion. And yet, we still do not know what kind of stellar system is the progenitor of this type of supernova. In my talk, I will present three supernova surveys. Two are imaging-based surveys for high-redshift supernovae, the first conducted with the Subaru 8.2-m telescope in the Subaru Deep Field, and the second with the Hubble Space Telescope as part of the 3-year Multi-Cycle Treasury program CLASH. The third is a spectroscopic survey that searches for supernovae embedded in SDSS galaxy spectra. Using these supernovae, we measure the rates of Type-Ia supernovae out to redshift z=2. This type of measurement allows us to place constraints on the nature of the progenitor system.

Date:   Monday 20-May-2013
Speaker:   Mark Avara (UMD)
Title:  The role of magnetic field strength and numerical resolution in simulations of the heat-flux driven buoyancy instability

The role played by magnetic fields in the intracluster medium (ICM) of galaxy clusters is complex. Thermal conduction is channelled along field lines and profoundly changes the stability of the ICM atmosphere, with convective instabilities being driven by temperature gradients of either sign. We investigate the local non-linear behavior of the heat-flux driven buoyancy instability (HBI), relevant in the cores of cooling-core clusters. We study a grid of 2-d simulations that span a large range of initial magnetic field strengths and numerical resolutions and explain the formation of magnetic filaments seen in such cluster simulations in terms of the linear stability of the HBI and total energetics of the plasma. While astrophysical conclusions regarding the role of conduction in cooling cores require advanced global models, our local study proves that systems dominated by HBI do not always quench the conductive heat flux.

This page was automatically generated on: 03-Feb-2020.