Rice Logo

Astronomy & Astrophysics Seminar

ASTR 400/500

+ Graduate and Undergraduate Research Seminar

Fall 2008

Titles and Abstracts

THE SUPERMASSIVE BLACK HOLE AT THE GALACTIC CENTER

Siming Liu, Los Alamos National Laboratory
The supermassive black hole at the Galactic center has been observed extensively during the past decade. Observations in near-infared bands with VLT and KECK show stars at a distance of 7.6(+-0.2) kpc from us orbiting a mass concentration of 3.6(+-0.2) million solar masses within 45 AU. This mass is centered at the location of the compact radio source, Sgr A*, whose intrinsic size has been measured to be 14 r_S in radius at 3.5 millimeter, where r_S =1.1e12 centimeter is the Schwarszchild radius of the black hole. Flares are routinely observed in the millimeter, NIR and X-ray bands from the direction of Sgr A*. Their variability, polarization, and correlation suggest that they are triggered near the event horizon and propagate outward, and the synchrotron and self-Comptonization dominate the emission processes. The longer wavelength emissions may also be associated with outflows from the black hole. Chandra observations also reveal a concentration of hot plasmas with a temperature of 1.9 keV in the direction of Sgr A*. These observations suggest that Sgr A* is powered by accretion of the supermassive black hole in stellar winds and warrantee a detailed study of the accretion process. I will give a review of these observations and related theoretical work and discuss ongoing observational and theoretical investigations to further our understanding of this source.

THE HUNT FOR PARTICLE DARK MATTER

Uwe Oberlack, Rice University
Redundant astrophysical observations imply that baryonic matter makes up less than 5% of the universe. The dominant components, "particle-like" Dark Matter (~25%) and "like-nothing-else" Dark Energy (~70%), are unexplained in the framework of standard physics. A natural explanation for the bulk of matter in the universe is provided by non-relativistic, massive, relic particles. These Weakly Interacting Massive Particles (WIMPs) are predicted in particular by theories invoking supersymmetry, and should at some small rate produce detectable signals from nuclear recoils in sensitive low background experiments. I will provide a brief overview of direct Dark Matter searches, and elaborate on the XENON suite of experiments. The XENON Dark Matter program reached a major milestone with the operation of its first Dark Matter detector, XENON10, at the Gran Sasso underground laboratory in Italy, providing world-best limits on WIMP-nucleon cross-sections last year. Meanwhile we are building the next generation detector XENON100 at the same location, with ten-fold greater fiducial mass and 100 times reduced background. I will report on the status of XENON100 and its projected sensitivity, and conclude with an outlook on future prospects.

SIMULATIONS OF DIFFUSIVE SHOCK ACCELERATION AT OBLIQUE RELATIVISTIC SHOCKS

Errol Summerlin, Rice University
Diffusive shock acceleration (DSA) at relativistic shocks is expected to be an important accel- eration mechanism in a variety of astrophysical objects including extragalactic jets in active galactic nuclei and gamma ray bursts. In this paper, we present results from a Monte Carlo sim- ulation of such diffusive acceleration in test-particle, relativistic, oblique, MHD shocks. Simula- tion output is presented for both large angle and small angle scattering scenarios, and a variety of shock obliquities including superluminal regimes when the de Hoffman-Teller frame does not exist. The distribution functions and power-law indices compare favorably with semi-analytic results in the non-relativistic and ultra-relativistic regimes, as well as published DSA simulation results in the trans-relativistic regime. Angular distributions are also discussed; these provide powerful checks on the accuracy of the simulation. New insights into the character of oblique shocks are addressed, specifically the downstream evolution of the test particle energy spectra and angular distributions that mark the interplay between diffusive and gyrational properties in the shock layer.

MONTE CARLO CODE AND BLAZER VARIABILITY

Xuhui Chen, Rice University
I am working on adopting an existing simulation code to the problem of Blazar Variability. This program uses the Monte Carlo method and the Fokker Planck equation to deal with time dependent Compton Scattering problems. In this talk, I will introduce Blazar variability, and then discuss the fundamentals of the simulation code, what we can do using this code, and what we still need to do in the future.

PROPERTIES OF BLAZAR POPULATIONS FROM MODELING AND PHENOMENOLOGICAL CENSUS

Eileen Meyer, Rice University
I will discuss the progress of my current project, which is the development of a versatile, multi-layered program which simulates underlying blazar populations for the purposes of statistical analysis, including monte carlo. I will put the work in context, citing recent success with similar methods in diffuse x-ray background and stellar population synthesis.

POPULATION STATISTICS STUDY OF RADIO AND GAMMA-RAY PULSARS FROM THE GALACTIC DISK

Peter L. Gonthier, Hope College
Results of our pulsar population synthesis of radio and gamma-ray pulsars from the Galactic disk will be presented. Over the past several years, a program has been developed to simulate pulsar birth, evolution, and emission using Monte Carlo techniques. Our goal is to understand the beam geometry and luminosity models of both radio and gamma-ray emission regions. From our studies of radio pulsars that have clearly identifiable core and cone components, in which we fit the polarization sweep as well as the pulse profiles to constrain the viewing geometry, we develop a model describing the ratio of radio core-to-cone luminosities. In this model, short period pulsars are more cone-dominated than in our previous studies. Using a recently improved radio beam geometry, we present the preliminary predictions for observing these pulsars with FERMI and AGILE. We suggest that the correlations of the radio pulse profile with the gamma-ray profile will constrain high-energy emission regions distinguishing between slot gap and outer gap models.

DARK STARS: A NEW PHASE OF STELLAR EVOLUTION SUPPORTED BY DARK MATTER ANNIHILATION

Paolo Gondolo, Univ. of Utah
The first phase of stellar evolution in the history of the universe may be Dark Stars, powered by dark matter heating rather than fusion. Weakly interacting massive particles can annihilate and provide an important heat source for the first stars in the universe. This talk presents the story of these Dark Stars: how they form, how long they might live, and what they might become at the end of their life.

EMISSION MAPS FROM THE (M)HD CODE AstroBEAR

Jacob Palmer, Rice
An important step in testing numerical simulations is to compare them to observations. In order to do this, the simulations must be able to produce emission maps. I am in the process of adding a post-processing routine to the (M)HD code AstroBEAR that will calculate H-alpha emission, as well as various emission lines of OI, OII, NI, NII, and SII. This talk will cover the methods used to obtain these emission maps from AstroBEAR output and some of the problems encountered. I will also discuss briefly the goal of adding a new cooling routine to AstroBEAR that will take into account more of the microphysics involved in cooling.

NEW RESULTS FROM THE PIERRE AUGER OBSERVATORY

Stefan Westerhoff, Univ. of Wisconsin, Madison
The Pierre Auger Observatory in Malargue, Argentina, is the world's largest detector for the study of the origin of ultrahigh energy cosmic rays. The experiment stretches over 3000 km^2 and measures cosmic rays with energies above 10^18 eV using two complementary detector types: an array of 1600 particle detectors on the ground, and four fluorescence detectors overlooking the ground array from the periphery. The Observatory was recently completed, but data taking started already at the beginning of 2004. Results from the first years of operation include a measurement of the energy spectrum and the chemical composition of ultrahigh energy cosmic rays, as well as first indications that the arrival direction distribution of the highest energy cosmic rays is not isotropic. In this talk, I will review the most recent results, with a special emphasis on the arrival directions and possible correlation with known astrophysical sources.

NEON AND SULFUR IN THE UNIVERSE: OBSERVATIONS WITH SPITZER

Reginald J. Dufour, Rice
Neon and Sulfur are two significant elements produced by alpha-capture onto carbon and oxygen in massive stars and ejected into the ISM as supernovae. Until recently, determination of the abundances of these elements in the ISM of galaxies from optical studies of H II region spectra have been limited by observing only the lines of minority ionization states (Ne++ and S+). With the Spitzer Space Telescope IRS we can observe emission lines of [Ne II], [Ne III], [S III] and [S IV], which are the dominant ionization states in photoionized nebulae. I will report on such observations and results for two spiral galaxies, M83 and M33, and motivation for observations of a third galaxy, NGC 6822.

INVESTIGATING THE SPECTRAL, PHYSICAL, AND KINETIC PROPERTIES OF SAGITTARIUS A* THROUGH GRMHD AND MONTE-CARLO/FOKKER-PLANCK SIMULATIONS

Guy Hilburn, Rice
I will present research which involves simulating the physical and kinetic properties of the accretion disk of Sagittarius A* by use of HARM, a GRMHD code. Electron and photon distributions are then calculated through use of a Monte-Carlo/Fokker-Planck code, allowing for study of the time-dependent emission through synchrotron, bremsstrahlung, and Compton scattering processes, and the evolution of the electron distribution. Conclusions on the nature of electron injection and heating in the region can be drawn from these tools.

SOFT X-RAY EMISSION OF MAGNETARS: RESONANT CYCLOTRON SCATTERING

Zorawar Wadiasingh, Rice
Soft X-ray spectra of soft gamma repeaters and anomalous X-ray pulsars have been customarily fit with an empirical blackbody plus power-law model. Resonant cyclotron scattering (RCS) has recently emerged as a physical model for soft X-ray emission in magnetar candidates. A non-thermal spectrum emerges when blackbody emission from a magnetar's surface is distorted and upscattered through efficient multiple resonant cyclotron scattering with warm non-relativistic plasma in the magnetosphere. I shall discuss the RCS model and its application to soft X-ray spectra of magnetar candidates.

DETECTION OF ZEEMAN SIGNATURE USING THE LSD (LEAST SQUARE DECONVOLUTION) METHOD

Wei Chen, Rice
With the assumption that most spectral lines exhibit Zeeman signatures with almost the same shape and that line intensities add up linearly, I wrote a code, using the least square deconvolution method, to detect Zeeman signatures from the optimal extracted polarization echelle spectra. As a result, we can see clearly the signatures from the resulting profile. In fact, LSD of Stokes I spectra also performs quite well.

Journal Club: DETECTION OF PULSED GAMMA-RAYS ABOVE 25 GEV FROM THE CRAB PULSAR
(MAGIC Collaboration)

Sarah Story, Rice
Since the discovery of gamma-ray emission from pulsars, competing models have been put forth to explain the way particles are accelerated in pulsar magnetospheres and the sites of the gamma-ray emission. The most powerful discriminator between these models is the maximum energy and shape of the high-energy spectral cutoff. This cutoff has been predicted to occur between a few GeV and a few tens of GeV. Thus far, observations in this regime have been limited. EGRET, aboard the CGRO, could not observe above about 5 GeV, and atmospheric Cherenkov telescopes are not sensitive below about 60 GeV. With recent modifications, however, the MAGIC telescope has been able to observe at energies down to 25 GeV, and has detected pulsed emission from the Crab pulsar at that energy with 6.4-sigma confidence. This severely restricts the possible sites of emission in the Crab. More detailed information will be available when new data from Fermi is published later this year.

THE NATURE OF DOUBLE-PEAKED EMISSION LINES IN MWC349A

Lauren Cleeves, Rice
MWC349, a well known emission-line star and the only known natural laser has been a subject of extensive study at the Maria Mitchell Association. During my REU at MMA this summer I became involved with the peculiar object through analysis of the double peaked nature of its Halpha line. Most mm hydrogen recombination lines and quasi-thermal near-IR lines observed in the source appear double-peaked, and this behavior has been ascribed to their formation in an edge-on circumstellar disk. However, the strongly double-peaked character of the Halpha emission line has instead been previously attributed by Hartmann et al. to be self-absorption in the line. We obtained an Halpha spectrum with the 0.9m coude feed telescope at Kitt Peak from which I attempted to reconcile the two different explanations for similar line morphology by showing that the line can indeed be formed in a circumstellar disk, though only if it forms in a very narrow ring. Higher-resolution spectroscopy of other optical and near-IR lines will further clarify the situation and will also provide a probe into disk structure of interesting object.

LABORATORY ASTROPHYSICS AND REAL EQUATIONS OF STATE: THE NEXT CHALLENGES FOR ASTROPHYSICAL MHD SIMULATIONS

Robert Carver, Rice
Magneto-hydrodynamics (MHD) simulation codes are an essential tool in astrophysics research, allowing us to better understand phenomena such as stellar outflows, supernovae, and accretion disks. These MHD codes have until recently focused on simulating ideal gases. As more complicated flow structures and shocks have gained increased interest, these codes have had to evolve. Recently a new field of study, laboratory astrophysics, has become increasingly important in the study and understanding of strophysical MHD flows. To adapt to this new field of study, we have added a real gas equation of state (EOS) capability to an astrophysics MHD code. This talk will focus on the required changes to that code and the results from our first simulation of a laboratory astrophysics experiment.

OBSERVATIONAL AND THEORETICAL INTERPRETATION OF ENERGETIC PARTICLE TRANSPORT IN SOLAR FLARES

Antoun Daou, Rice
The combination of excellent space-based remote sensing, and image reconstruction techniques, as well as improvements in numerical modeling, help enhance our understanding of particle transport in solar flares. A rigorous analysis of flare hard X-ray emission uses the unprecedented spectral and spatial resolution of the RHESSI telescope data in order to better understand the spectral properties of the emitting electron population in solar flares. A forward-fit to the data, using a Fokker-Planck kinetic code, completes this study by numerically modeling the particle transport in phase-space in realistic magnetic geometries and for different particle injection profiles. I will also expose other projects such as filament eruption analysis using STEREO/MLSO, the flare-CME-chromospheric wave relationship, the relationship between microflares and jets, mechatronix engineering, and science outreach efforts.

EXOPLANETS FROM A TO M

John Johnson, IfA, Hawaii
I will present the results of our search for planets around stars at either end of the stellar mass scale, from M dwarfs to "retired" A stars. Our results reveal a strong correlation between stellar mass and planet occurrence, which indicates that planets form roughly 4 times more efficiently around A stars like Vega than around M dwarfs like Proxima Centauri. We also find that planets around A stars all orbit beyond 0.8 AU, which has important implications for our understanding of planet migration, and for the planning of future astrometry and direct imaging missions.