Astrophysics Science Division
Astrophysics Science Division - Seminars & Meetings

Astrophysics Science Colloquium Series
Schedule: April - June 2005

Astrophysics Science Colloquium Series
Schedule: April - June 2005

Through the courtesy of the speakers since 2004, most presentations are available on line.

Future schedules:

  • 2005, Third Quarter

    • Past schedules:

  • 2005, First Quarter
  • 2004, Fourth Quarter
  • 2004, Third Quarter
  • 2004, Second Quarter
  • 2004, First Quarter
  • 2003, Fourth Quarter
    • Time: 3:45 pm (Meet the Speaker at 3:30 pm) - Location: Bldg 2 Conference Room (Ground Floor, Rm 8) - unless otherwise noted.
    To view the abstract of a seminar, click on the title.

    April

    Sunday Monday Tuesday Wednesday Thursday Friday Saturday
           1 2
    3 4 5 - Lev Titarchuk (GSFC) How To Distinguish an X-ray Neutron Star From a Black Hole 6 7 8 9
    10 11 12 - Drake Deming (GSFC) Infrared Radiation from Extrasolar Planets 13 14 15 16
    17 18 19 - David Berge (MPI-K Heidelberg) Observations of Galactic gamma-ray sources with H.E.S.S. 20 - Christine Done (Univ. Durham, UK) Scaling Black Holes 21 22 23
    24 25 26 - David Buote (UC Irvine) Shedding X-Ray Light on Dark Matter 27 28 29 30

    May

    Sunday Monday Tuesday Wednesday Thursday Friday Saturday
    1 2 3 - Tim Hamilton (Shawnee State Univ.) The Quasar Fundamental Plane: the Host-Nucleus Connection 4 5 - Simona Soldi (INTEGRAL Science Data Centre) X-ray observations of AGN with INTEGRAL 6 7
    8 9 10 - Attila Kovacs (Caltech) The View of the Sky through the Eyes of a SHARC 11 12 13 14
    15 16 17 - Demos Kazanas (GSFC) Gamma-Ray Burst Modeling 18 19 - Paola Rebusco (MPA) QPOs and Nonlinear Resonance: What is the Link? 20 21
    22 23 24 - Anton Dorodnitsyn (Space Research Institute, Moscow) Radiation-driven Winds Near BHs 25 26 - Frank H. Shu (National Tsing Hua University, Taiwan) Chaos in Spiral Galaxies 27 28
    29 30 31 - AAS Meeting, No Seminar     

    June

    Sunday Monday Tuesday Wednesday Thursday Friday Saturday
        1 2 3 4
    5 6 7 No Colloquium Scheduled 8 9 10 11
    12 13 14 No Colloquium Scheduled 15 16 17 18
    19 20 21 - Deirdre Horan (VERITAS), Blazars in the VERITAS Era 22 23 24 25
    26 27 28 - Jerry Edelstein (UC Berkeley SSL), Results from the SPEAR Mission 29 30 - Isabelle Grenier (CEA Saclay), The Dark Side of Interstellar Clouds   

    How To Distinguish an X-ray Neutron Star (NS) From a Black Hole (BH): Spectral Index and Quasi-Periodic Oscillation Frequency Correlation in NS and BH sources

    Dr. Lev Titarchuk

    GSFC

    Tuesday, 5 April 2005

    Abstract

    Recent studies have shown that strong correlations are observed between the low frequencies (1-10 Hz) of quasiperiodic oscillations (QPOs) and the spectral power law index of several Black Hole (BH) candidate sources in low hard state, steep power-law (soft) state and in transition between these states. All data points of correlating photon index vs QPO low frequency for GRS 1915+105, XTE 1550-564, 4U 1630-47 and Cyg X-1 will be shown in the talk. In the soft state this index-QPO frequency correlations show the saturation to the photon index 2.7 at high values of the low frequency which was identified as a black hole signature. In this talk I argue that this saturation is not the case at least for NS source 4U 1728-34 for which the index monotonically increases with the low frequency to the values of 6 and higher. This effect has been revealed analyzing almost all available data for 4U 1728-34 in the RXTE data archive. I show the spectral evolution of the Comptonized blackbody spectra when the source undergoes the transition from the hard to soft states. The hard state spectrum is a typical Comptonization spectrum of the soft photon radiation (from the disk and the NS surface) which index 1.8 whereas the soft state spectrum consists of two blackbody components which are only slightly Comptonized. The color disk and NS surface temperatures slightly decreases from 0.93 keV to 0.83 keV and from 2.9 keV to 2.2 keV respectively when the source undergoes the transition from the hard to soft states. Thus one can claim (as expected) that in the NS source the thermal equilibrium is established when the sources goes to the soft state whereas in BH sources because of BH horizon (the drain in system) the equilibrium is never established. The BH emergent spectrum, even in high/soft state, has a power law component. A new method for evaluation of the BH mass using this observable index-frequency correlation will be demonstrated.

    Infrared radiation from Extrasolar Planets

    Dr. Drake Deming

    GSFC

    Tuesday, 12 April 2005

    Abstract

    A class of extrasolar giant planets - the so-called `hot Jupiters' - orbit within 0.05 AU of their primary stars. These planets should be hot and so emit detectable infrared radiation. The planet HD 209458b is an ideal candidate for the detection and characterization of this infrared light because it is eclipsed by the star. This planet has an anomalously large radius (1.35 times that of Jupiter), which may be the result of ongoing tidal dissipation, but this explanation requires a non-zero orbital eccentricity (~0.03), maintained by interaction with a hypothetical second planet. Here we report detection of infrared (24 micron) radiation from HD 209458b, by observing the decrement in flux during secondary eclipse, when the planet passes behind the star. The planet's 24 micron flux is 55 +/- 10 micro-Jy (1 sigma), with a brightness temperature of 1130 +/- 150 Kelvins, confirming the predicted heating by stellar irradiation. The secondary eclipse occurs at the midpoint between transits of the planet in front of the star (to within +/- 7 min, 1 sigma), which means that a dynamically significant orbital eccentricity is unlikely.

    Observations of Galactic gamma-ray sources with H.E.S.S.

    Dr. David Berge

    MPI-K Heidelberg

    Tuesday, 19 April 2005

    Abstract

    H.E.S.S. is a new system of four imaging atmospheric Cherenkov telescopes operating in full strength since December 2003 in Namibia. During its first two years of running, a number of Galactic gamma-ray sources have been observed. The talk will review the most important results, including highlights like spatially resolved gamma-ray emission from shell-type supernova remnants (RX J1713.7-3946 and RX J0852.0-4622) and a gamma-ray survey of the inner part of our Galaxy performed with the H.E.S.S. telescopes in 2004.

    Scaling black holes: Galactic binaries, UltraLuminous X-ray Sources and
    Active Galaxies

    Dr. Christine Done

    Univ. Durham, UK

    Wednesday, 20 April 2005 at 12noon

    *Bldg 2, Room 22*

    Abstract

    Black holes come in a variety of size scales, from the stellar mass objects in binary systems in our own galaxy, to the supermassive black holes which power the Active Galactic Nuclei, with the ultra-luminous X-ray sources lying somewhere between. Theoretically, the accretion flow onto a black hole should be fundamentally similar in all these objects, depending mostly on L/L_Edd, with only a rather weak dependence on black hole mass. I will review recent advances on accretion flows in the galactic objects, show how these can be applied to understand some puzzling aspects of the ultraluminous sources, and then relate them to observations of the supermassive black holes.

    Shedding X-Ray Light on Dark Matter

    Dr. David Buote

    UC Irvine

    Tuesday, 26 April 2005

    Abstract

    I review constraints on dark matter from X-ray observations of galaxies and clusters of galaxies with Chandra and XMM-Newton, and discuss their implications for cosmology. I also briefly discuss our program to study substructure in galaxy clusters and present preliminary constraints supporting the standard cosmological model having both dark matter and dark energy.

    The Quasar Fundamental Plane: the Host-Nucleus Connection

    Dr. Tim Hamilton

    Shawnee State University

    Tuesday, 3 May 2005

    Abstract

    We have found a relationship between a quasar's host galaxy properties and its nuclear luminosity. When nuclear luminosity (optical or x-ray) is plotted against the half-light radius and effective surface magnitude of the host, we find quasars lie on a thin plane within this parameter space. This "fundamental plane" of quasars explains 96% of the variance in the optical sample and 95% in the x-ray sample. Dividing the quasars into radio-loud or radio-quiet, elliptical or spiral classes, the fundamental plane rotates about a fixed axis. The different classes are characterized by different angles, with radio-loudness having the strongest effect. The origins of the quasar fundamental plane may lie in the normal galaxy fundamental plane and the fueling mechanism of the active nucleus. Then this relationship could be used to test active galaxy unification models by comparing the fundamental planes of different AGN types. We have already found a similar plane for Low-Luminosity AGN and can make some initial comparisons. Further work might find application to galaxy evolution.

    X-ray observations of AGN with INTEGRAL and other Satellites

    Simona Soldi

    INTEGRAL Science Data Centre

    Thursday, 5 May 2005 at 1:30pm

    *Bldg 2, Room 8*

    Abstract

    Hard X-ray emission from AGN in crowded fields as well as in isolated regions of the sky can be studied thanks to the imaging and spectral capabilities of the INTEGRAL satellite. We show some of the AGN observed by INTEGRAL during the regular scans of the Galactic Plane. We investigate the shape of the continuum emission and the presence of an exponential cutoff which could be the signature of Comptonization processes. We present then INTEGRAL observations of the high latitude quasar 3C 273, one of the most studied AGN. The observation of June 2004 was performed during a particularly low state of the synchrotron emission with simultaneous multiwavelength observations. We study possible correlations between the X-ray emission as seen by recent and past X-ray satellites and the emission at other frequencies, using and updating the 3C 273's on-line database (http://obswww.unige.ch/3c273/).

    The View of the Sky through the Eyes of a SHARC

    Attila Kovacs

    Caltech

    Tuesday, 10 May 2005

    Abstract

    The SHARC-2 array, developed in collaboration at Caltech and Goddard Space Flight Center, has brought us new images of the sky at 350um with unprecedented quality and depth. Its success owes to its innovative design and a novel approach to sub-mm data reduction, which may be adapted to a range of new large format bolometer arrays in the far-infrared and sub-millimeter. With its 350um 'eyes' it is specially suited to the study of dust in the ISM, which enabled us to pursue various science objectives, both within and outside the boundaries of our own Galaxy. Snapshots of these results will be presented, with a special focus on the study of the distant (z~1-3) universe.

    Gamma-Ray Burst Modeling

    Demos Kazanas

    GSFC

    Tuesday, 17 May 2005

    Abstract

    We report on progress with more detailed calculations based on the 'Supercritical Pile' model of Gamma Ray Bursts (GRB). This model was proposed as a means for converting the energy stored in the form of relativistic protons in the Relativistic Blast Wave (RBW) of a GRB into electrons and then to photons using the photon-proton pair production reaction. It was also argued on the basis of semi-analytical arguments that the threshold of this reaction is responsible for the peak of the GRB nu F nu spectra at ~1 MeV, as required by observation. The name of the model derives from the fact that it depends on certain thresholds very similar to those of a nuclear pile (critical mass). We simulate the spectral and temporal evolution of a GRB by following the distributions of photons, electrons and protons as implied by this model and test for the kinematic and dynamic thresholds which are found in agreement with the analytic estimates. We can now produce, under some simplified assumptions, the evolution of prompt GRB spectra from IR to TeV gamma rays which can be compared to observations.

    QPOs and Nonlinear Resonance: What is the Link?

    Paula Rebusco

    MPA

    Thursday, 19 May 2005

    Time: 1:30pm, Location: Bldg 2, Rm 8

    Abstract

    Kilohertz quasi-periodic variability is observed in the X-ray emission of many Galactic black hole and neutron star sources. Kluzniak and Abramowicz noticed that twin peak HFQPOs are in rational ratios, and that most often the ratio is 3:2. In particular, all four microquasars in which twin peak HFQPOs are observed, show with a rather impressive accuracy the fixed 3:2 ratio. In neutron star sources, where twin peak HFQPOs vary and ratios are not fixed, the ratios cluster very close to 3:2. An analytic calculation was done by considering the possibility of non-linear parametric resonance in nearly circular, nearly Keplerian, nearly equatorial plane motion of test particles around compact objects. The obtained solution confirms the importance of the 3:2 ratio and it agrees with a previous numerical simulation. With a particular choice of tunable parameters, it fits accurately the observational data for Sco X-1. Why the resonant model is so promising? What is still missing?

    Radiation-driven Winds Near BHs

    Anton Dorodnitsyn

    Space Research Institute, Moscow

    Tuesday, 24 May 2005

    Abstract

    The critical aspect of any radiation-driven wind theory is how the momentum is transferred from the radiation field to matter. One of the most favorable mechanism which is believed to work in winds launched from luminous accretion disks is the acceleration of plasma by the absorption of the radiation flux in lines of abundant elements. In a standard theory only gradients of the flow velocity are taken into account in Sobolev approximation, when calculating the radiation pressure force. This theory was developed to explain winds from early-type stars, for which it is in a good agreement with observational data, and now is extensively used to explain winds in AGNs, YSO, and binaries. We report on our recent studies of line-driven winds near compact objects. The crucial difference from the standard case of O-type star wind consists of the following: if a wind is settled not too far from BH (within 50 rg) both velocity gradient and gradient of the gravitational potential should be taken into account when calculating the radiation force. The first stands for the Sobolev effect the latter takes into account gravitational redshifting. We develop a theory of such flows from first principles, considering one dimensional wind from accretion disk around Schwarzschild BH. We show, that taking into account gravitational redshifting can indeed significantly increase the efficiency of acceleration. We discuss the possible connection of our studies with recently detected absorption features in X-ray spectra of some quasars. The observational signature of such flows may have very important implications in future studies of parameters of putative BHs in central engines.

    Chaos in Spiral Galaxies

    Frank H. Shu

    National Tsing Hua University, Taiwan

    Thursday, 26 May 2005

    Time: 10:30am, Location: Bldg 21, Rm 183

    Abstract

    The visible spiral structure of disk galaxies is often quite disorderly, with spurs, branches, and feathers making ragged what theoreticians ideally describe as a two-armed, grand-design, spiral. In the most notorious cases, the blue-light pictures associated with newly formed stars from the gaseous interstellar medium are observationally "flocculent." The mystery deepens when infrared images, such as those from the Spitzer Space Telescope, show that the distribution of older disk stars is often arranged in beautiful two-armed grand-design spirals in the same galaxies where the blue-light images are chaotic. One interpretation is that the irregular nature of population I spiral-structure demonstrates that the dynamics of interstellar gas is largely decoupled, perhaps by numerous supernovae explosions, from the regular gravitational forcing provided by the smooth spiral arms of the background disk stars. In this talk, I advance an older and opposite concept: extra structural features appear not because the gas is decopuled from the background spiral forcing, but because it is too well coupled. In the nonlinear regime, branches, spurs, and feathers can arise, respectively, because of the action of ultraharmonic resonances, reflections of density waves off sharp edges created by nonlinear dredging, and gravitational instabilities - perhaps mediated by magnetic fields - behind the compression zones of galactic shocks. In particular, chaos, or flocculence, is produced in the presence of overlapping resonances. Thus, disorderly effects in galaxies can be due to orderly causes, a well-known result, by now, in other areas of nonlinear dynamics.

    Blazars in the VERITAS Era

    Deirdre Horan

    VERITAS

    Tuesday, 21 June 2005

    Abstract

    Blazars are the main class of active galactic nuclei detected at gamma-ray energies. The EGRET experiment on the Compton Gamma Ray Observatory made firm identifications of 66 blazars and recent results have associated many of the unidentified EGRET sources with known blazars. At higher energies (E>300 GeV), ground-based gamma-ray telescopes have made firm detections of seven blazars with a number of other detections claimed at lower levels of significance. In order to improve our understanding of the gamma-ray emission from blazars, we must detect and study more of these objects at very high energies. I will describe the current observational status of blazars at TeV energies and will discuss search strategies that can be employed to increase the sample at these very high energies. With the next generation of space and ground based gamma-ray observatories coming on line, we can expect to identify and discover many more gamma-ray blazars and significantly improve our understanding of the physics of these powerful objects.

    SPEAR Reveals the Far Ultraviolet Galaxy

    Jerry Edelstein

    UC Berkeley, SSL

    Tuesday, 28 June 2005

    Abstract

    SPEAR has obtained unprecedented hyper-spectral images of diffuse FUV (900-1700) emission from most of the Galaxy. We describe this sounding-rocket, its orbital micro-sat mission, and results. Observations include supernova remnants, super-bubbles and the general ISM. A plethora of spectral lines have been detected from high-energy gas (e.g. HeII, CIII, CIV, OIII, OIV, OVI, and SiIV) and from cooler gas (e.g. SiII, AlII and FeII). The gas is likely not in equilibrium. We detect strong H2 fluorescence emission over the sky that can directly measure the H2 destruction rate and association with NHI and CO. Our maps of the bright FUV continuum, disentangled from the fluorescence contribution, will probe dust properties and the dust-gas association.

    The FUV interstellar radiation field, a controlling agent of stellar formation through the heating of the cold ISM, can be directly measured. The uncertain reddening to bright stars can be tested.

    The Dark Side of Interstellar Clouds

    Isabelle A. Grenier

    Service d'Astrophysique, CEA Saclay

    Thursday, 30 June 2005

    Location: Bldg 2, Room 8 - Time: 1:30pm

    Abstract

    Large amounts of dark gas, not accounted for in the radio HI and CO line surveys, have been found in infrared and gamma rays above 100 MeV, at the interface between the atomic and molecular clouds in the solar neighbourhood. This gas is traced in gamma rays by its interaction with ambient cosmic rays and at mm and sub-mm wavelengths by its dust content. The dark clouds are rather diffuse, with volumes and average volume densities closer to that of the dense HI clouds than the CO cores. The diffuse H2 or dense HI nature of this gas is open. The local measurements imply a total dark-gas mass in the Milky Way at least comparable to the molecular mass detected in CO. The column-densities and large angular extent of the local dark clouds imply severe revisions of the gamma-ray interstellar emission model to high latitudes, therefore of the extragalactic gamma-ray background intensity and of the EGRET source catalogue. New estimates will be discussed.

    Jerry Bonnell

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