Tuesday, September 26th, 10:30 AM
ISB 310

Effects of random density fluctuations on Supernovae and solar neutrinos

Timur Rashba (Max-Planck-Institut fuer Physik)

We discuss an influence of matter density fluctuations on Supernovae and solar neutrinos. It is shown that in the case of the Sun the effect is too small for present experimental sensitivity and it is used to put bounds on solar matter density fluctuations. This bound is complementary to heliosemic bounds. In the case of Supernovae the expected spectra of neutrinos depend significantly on the matter density fluctuations.

MONDAY, October 2nd, 10:30 AM
ISB 310
NOTE: UNUSUAL TIME

Title: Modelling the X-ray emission of massive black hole binaries

Steinn Sigurdsson (Penn State)

Abstract: We modeled the electromagnetic signatures of massive black hole binaries (MBHBs) with an associated gas component. We investigate the sub-parsec binaries which have not yet entered the gravitational radiation phase. The results from the first set of calculations, carried out for a coplanar binary and gas disk, suggest that the outbursts in the X-ray light curve are pronounced during pericentric passages and can serve as a fingerprint for this type of binaries if periodic outbursts are a long lived signature of a binary. The modeled Ha emission line profiles may be used as a criterion for selection of MBHB candidates from the existing archival data.

The Energetic Gamma Ray Experiment Telescope (EGRET) aboard the Compton Gamma Ray Observatory has detected pulsed signals from at least six rotation-powered pulsars. Since interpreting gamma-rays should be less ambiguous compared with reprocessed, non-thermal X-rays, the gamma-ray pulsations observed from these objects are particularly important as a direct signature of basic non-thermal processes in pulsar magnetospheres, and potentially should help to discriminate among different emission models.

I investigate the electrodynamic structure of a particle accelerator in the magnetospheres of three gamma-ray pulsars, Crab, Vela, and PSR B1951+32, on the two-dimensional poloidal plane, solving from the first principles. It is demonstrated that a stationary potential gap - a particle accelerator -is maintained for high-energy pulsars and that the gap extends from the neutron star surface to the outer magnetosphere, extracting ions from the stellar surface as a space-charge-limited flow. This solution is a mixture of traditional polar-cap and outer-gap models of pulsar emission mechanisms, which have been considered separately so far.

TUESDAY, October 17th, 10:30 AM
ISB 310

Inverse Compton scattering on stellar photons (,heliospheric modulation, and neutrino astrophysics)

Troy Porter (SCIPP)

I will talk about the inverse Compton scattering of solar photons by Galactic cosmic-ray electrons. Within the heliosphere, we have recently shown that the gamma-ray emission from this process is substantial, with maximum intensity in the direction of the Sun; the angular distribution of the emission is broad. Observations by GLAST can therefore be used to monitor the heliosphere and determine the electron spectrum as a function of position from distances as large as Saturn's orbit to close proximity of the Sun, thus enabling unique studies of solar modulation. This paves the way for the determination of other Galactic cosmic-ray species, primarily protons, near the solar surface which will lead to accurate predictions of gamma rays from pp-interactions in the solar atmosphere. These albedo gamma rays will be observable by GLAST, allowing the study of deep atmospheric layers, magnetic field(s), and cosmic-ray cascade development. The latter is necessary to calculate the neutrino flux from pp-interactions at higher energies (> 1 TeV). Although this flux is small, it is a guaranteed flux in contrast to other astrophysical sources of neutrinos, and may be detectable by km-cubed neutrino telescopes of the near future, such as IceCube. Since the solar core is opaque for very high-energy neutrinos, directly studying the mass distribution of the solar core may thus be possible. Turning further afield, our Sun is not unique, and I will discuss the prospects for detecting similar emission from other stars in the Galaxy.

TUESDAY, October 24th, 10:30 AM
ISB 310

Title: This week's seminar consists of two "mini-talks"

Igor Moskalenko (SLAC)

* GALPROP model for Galactic cosmic ray propagation and diffuse gamma-ray emission

I will describe basic processes of cosmic ray propagation in the Galaxy and production of the diffuse gamma-ray emission. I will discuss an origin of the "GeV excess" and some possible connection with spectra of secondary particles in cosmic rays such as antiprotons and positrons. A new determination of the extragalactic diffuse gamma-ray background is also discussed.

* Dark Matter Burners

We show that a star orbiting close enough to an adiabatically grown supermassive black hole can capture a large number of weakly interacting massive particles (WIMPs) during its lifetime. WIMP annihilation energy release in low- to medium-mass stars is comparable with or even exceeds the luminosity of such stars due to thermonuclear burning. The excessive energy release in the stellar core may result in an evolution scenario different from what is expected for a regular star. The model thus predicts the existence of unusual stars within the central parsec of galactic nuclei. If found, such stars would provide evidence for the existence of particle dark matter. White dwarfs seem to be the most promising candidates to look for. The signature of a white dwarf burning WIMPs would be a very hot star with mass and radius characteristic for a white dwarf, but with luminosity exceeding the typical luminosity of a white dwarf by orders of magnitude <50L_sun. A white dwarf with a highly eccentric orbit around the central black hole may exhibit variations in brightness correlated with the orbital phase.

TUESDAY, November 7th, 10:30AM

ISB 310

Title: Type Ia supernova explosions modeled in supercomputers

Speaker: Friedrich Roepke (UCSC and MPA Garching)

Abstract:

The application of Type Ia supernovae as distance indicators led to
the spectacular discovery of the currently accelerated expansion of
the universe. This points to a dominating Dark Energy component
and Type Ia supernovae are one of the most promising tools to
constrain the nature of this mysterious energy form.

But what are these objects? Although there is consensus about the
general astrophysical scenario -- a white dwarf star undergoing a
thermonuclear explosion -- details of the explosion process remain
unclear. Uncertainties in the ignition and the flame
propagation mechanisms lead to different options in constructing
models. Recent progress in numerical techniques and the constant
advancement of supercomputers, however, facilitate three-dimensional
simulations with high predictive power. The results can be compared
with observations and a validation of the models is possible. This
way, the construction of a self-consistent model has come into reach.

Looking at Type Ia supernovae from a modeler's perspective, I will
show how basic properties of the observed events lead to an explosion
model. The numerical implementation is challenging due to turbulence
effects playing an important role, but can be tackled with a "Large
Eddy Simulation" approach. The different possibilities in the
explosion mechanism will be discussed on the basis of simulations and
comparison of the results with observations. In an illustrative
example I will show how a valid Type Ia supernova explosion model can
eventually be used to address questions arising in their cosmological
application making these measurements more reliable.

TUESDAY, November 14th, 10:30AM

ISB 310

Title: Ionization Imaging A better way to search for 0-v BB decay?

Speaker: David Nygren (Lawrence Berkeley National Laboratory)

Abstract:

The discovery of neutrino oscillations implies a non-zero neutrino mass, one of the conditions for neutrino-less double beta decay. A convincing observation of this decay, at any level, would establish that the neutrino is its own anti-particle, and a Majorana nature for the neutrino. The non-conservation of lepton number would be a direct implication. Consequently, interest in the development of more robust experimental methodologies has greatly increased. High-pressure gaseous xenon appears to offer an opportunity for simultaneous realization of 3-D event localization, detailed event reconstruction, fully active and variable fiducial volume surfaces, and 1% FWHM energy resolution at the 136Xe Q-value of 2.5 MeV. The concept is based on direct detection of ionization in a TPC geometry, and scales naturally to 1000+ kg. A new concept for the identification of the daughter nucleus is also described.

TUESDAY, November 28th, 2:30PM

Note: Unusual Time!!

ISB 310

Title: High Energy Education and Public Outreach

Speaker: Lynn Cominsky and Sarah Silva (Sonoma State University)

Abstract:

The Education and Public Outreach (E/PO) group at Sonoma State University
supports three NASA high-energy astrophysics missions: GLAST, Swift and
XMM-Newton. We train teachers, and develop both classroom and interactive
computer educational activities. Recently, through the GLAST project, we
provided seed money and scientific oversight to both a planetarium show,
"Black Holes: the Other Side of Infinity" and the PBS NOVA show "Monster of
the Milky Way" which premiered on 10/31/06.

In this talk, we will provide an overview of our programs, and will also
demonstrate how we use real data to encourage students to think
scientifically, provide hands-on activities to explain abstract concepts,
and connect familiar and nearby astronomical objects such as the Earth to
explain distant and exotic objects such as pulsars.

TUESDAY, December 12th, 10:30AM

ISB 310

Title: "Search for Electroweak Top Quark Production at D0."

Speaker: Jovan Mitrevski (Columbia University)

Abstract:

Electroweak production of the top quark has many attractive features.
One is that the cross section is proportional to |Vtb|^2 and thus
provides a test of the CKM matrix’s 3-generation unitarity. The
constraints on |Vtb| not assuming unitarity are weak. Another is that
it provides a way to probe the V - A structure of the weak
interaction. Because the top quark decays before it has time to
hadronize, it retains its polarization information, making it
available for study. Furthermore, because the top quark has such a
large mass, close to the scale of electroweak symmetry breaking, one
wonders if it has some special role in beyond the Standard Model
physics. Studying the electroweak interaction in single top
production is a good way to look for such beyond the Standard Model
effects. In this seminar I will discuss the search for electroweak
production of the top quark being undertaken by the D0 collaboration
at Fermilab. I will discuss our selection criteria and background
modeling, and I will focus in particular on the matrix element method
analysis currently under way using 1 fb-1 of data.