Research Events
For the Public
Northwestern University

CIERA Special Seminars 2014-2015

*Tech = Technological Institute (2145 Sheridan Road)
**Db = Dearborn Observatory (2131 Tech Drive)

Fall Quarter 2014

  Date/Time      Visitor Host
  Oct. 23
  Tech F210

Rob Ferdman
    McGill University
    Using pulsars in a Galactic-scale gravitational-wave detector

At the forefront of observation astrophysics is the effort to directly detect gravitational waves (GWs), which remains a "holy grail" in validating Einstein's general theory of relativity. Along with ground-based GW detectors such as Advanced LIGO/VIRGO, pulsar timing has become a serious contender for making the first such detection. This will be done using a so-called Pulsar Timing Array (PTA), which uses the distances between Earth and several millisecond-period pulsars (MSPs) as arms of a Galactic-scale GW detector. It aims to measure the common effect of a stochastic GW background on the pulse arrival times of an ensemble of MSPs, thought to be due to coalescing supermassive black holes at the centers of merging galaxies at high redshifts. PTAs are sensitive to the nanohertz frequency region of the GW spectrum, and are thus complementary to the larger frequency ranges probed by ground-based detectors, which will be sensitive to sources such as merging NS pairs.
This is an international undertaking; the North American wing of this effort, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), uses the Green Bank Telescope (GBT) in West Virginia and the Arecibo Telescope in Puerto Rico to regularly observe over 30 MSPs as part of a PTA. In this talk, I will discuss how we will be able to detect GWs with pulsar timing, and describe recent progress. I will also briefly describe ongoing and future instrumentation that will greatly benefit this work, including the 100-m class CHIME telescope, currently being constructed in British Columbia.

Vicky Kalogera and Fred Rasio
  Oct. 27
  Tech F160

Laura Sampson
    Montana State University, Bozeman
    Testing Relativity with the Parameterized Post-Einsteinian Framework

In the very near future, ground-based detectors and pulsar timing arrays will begin to make regular detections of gravitational waves. Among the most interesting science we will be able to accomplish with these detections is in the area of testing General Relativity. In this talk, I give a brief overview of tests of GR that have been performed to date, and then spend the bulk of the time discussing the new tests we will be able to perform using gravitational waves. In particular, I focus on template-based searches for deviations from GR, using a model-independent tool called the parameterized post-Einsteinian framework.

Vicky Kalogera and Tyson Littenberg
  Nov. 3
  Tech F160

Ondrej Pejcha
    Princeton University
    The Rugged Landscape of the Core-Collapse Supernova Explosions

All massive stars end their lives with core collapse and many as supernova explosions. Despite observations of thousands of supernovae, detailed numerical calculations and theoretical efforts, the mechanism of explosion is poorly understood and perhaps even unknown. By parameterizing the systematic uncertainty in the explosion mechanism and by using spherical quasi-static evolutionary sequences for many hundreds of progenitors over a wide range of metallicities, we study how the explosion threshold maps onto observables - fraction of successful explosions, remnant neutron star and black hole mass functions, explosion energies, nickel yields - and their mutual correlations. Successful explosions are intertwined with failures in a complex but well-defined pattern that is not well described by the progenitor initial mass and is tied to the pre-collapse structure of the progenitor star. Other supernova properties show a similar pattern. Finally, to facilitate better comparison of the theory and the data, we present a new method to extract parameters from supernova light curves and expansion velocities in a statistically correct way.

Vicky Kalogera
  Nov. 24
  Tech F160

Titos Matsakos
    University of Chicago
    Classification of Magnetized Star-Planet Interactions:
    Bow Shocks, Comet-like Tails, and In-spiraling Streams

Stellar irradiation is believed to drive outflows from the surface of close-in exoplanets, a phenomenon that is supported by transit observations of Hot Jupiters. Assuming planetary magnetospheres similar to those of our solar system, such outflows are expected to be magnetized. In addition, the environment of short period orbits consists of the sweeping stellar wind plasma that is known to attain super-sonic velocities. This framework suggests the manifestation of complex magnetized star-planet interactions in systems harboring Hot Jupiters. In this work, we perform a series of parameterized 3D magneto-hydrodynamic numerical simulations in order to provide a classification for the different types of interactions that may occur. We incorporate stellar and planetary outflows that are consistent with detailed physical models and investigate case by case the exhibited dynamics.

Francesca Valsecchi
  Dec. 1
  Tech F160

Benedikt Diemer
    University of Chicago
    The (Non-)Universality of Halo Density Profiles

The density profiles of dark matter halos are an essential input for models of galaxy formation, as well as for the interpretation of numerous observations such as weak and strong lensing signals. The profiles are commonly thought to follow a simple, universal shape, and only depend on two parameters, mass and concentration. Using a large suite of cosmological simulations, I will show that the outer halo density profiles depend on an additional parameter, the mass accretion rate, and present an accurate new fitting formula that takes this dependence into account. I will further discuss the question of universality, and show that the definition of the halo boundary plays a crucial role. Similarly, halo concentrations are usually described as a universal function of mass and redshift. Instead, I will present a model in which concentration depends on an additional parameter: the local slope of the matter power spectrum. I will demonstrate that this model accurately (to better than 10-15%) describes simulated concentrations over a large range of redshifts, halo masses and cosmological parameters, and is in excellent agreement with the recent observations of the CLASH cluster survey.

  Dec. 15
  Tech F160

Doug Watson
    University of Chicago


Winter Quarter 2015

  Date/Time      Visitor Host
  Jan. 28
  Tech F160

Nicholas McConnell
    University of Hawaii

Daniel Angles-Alcazar

For more information, contact: ciera@northwestern.edu

Past CIERA Special Seminars