MARCH 23 - 27, 2015


1:00 pm: Institute for Theory and Computation (ITC) Pizza Lunch. "Magnetized Jets Driven by the Sun: The Structure of the Heliosphere Revisited," Dr. Merav Opher, Boston University, and "Particle Acceleration in Supernova Remnants: The Role of Magnetic Fields," Dr. Patrick Slane, CfA. Phillips Auditorium.


12:30 pm: High Energy Astrophysics Division Lunch Talk. 1) "An 'Espresso' for Ultra-High-Energy Cosmic Rays," Dr. Damiano Caprioli, Princeton University; 2) "Measuring MHD Turbulence in the ISM using Synchrotron Fluctuations," Dr. Blakesley Burkhart, CfA; and 3) "The Whipple Mission: A Discovery Proposal to Study the Outer Solar System and Oort Cloud," Dr. Ralph Kraft, CfA. Phillips Auditorium.

Abstract: 1) The spectrum of cosmic rays (CRs) observed at Earth extends over more than twelve orders of magnitude in energy, up to more than 10^20eV. While Galactic CRs up to about 10^17eV are believed to be produced in supernova remnants, the sources of Ultra-High Energy CRs are much more elusive. We propose that UHECRs above 10^18eV may be produced in relativistic jets of powerful active galactic nuclei (AGNs): "seed" galactic CRs that penetrate the jet sideways are reaccelerated by the "steam" of the relativistic flow via a one-shot mechanism, which we dub the "espresso" mechanism. The typical energy gain is proportional to the square of the jet Lorentz factor, which allows powerful blazars to accelerate UHE iron nuclei up to more than 10^20eV. The chemical composition of UHECRs is predicted to match the composition in the seed region (the CR knee), in agreement with recent observations by the Pierre Auger Observatory and the Telescope Array. 2) Magnetized turbulence is a ubiquitous physical process in the ISM of galaxies from scales of kpc to accretion disks. In general, quantitative studies of turbulence have been hindered in the past. 3) The existence of a large population of rocky and/or icy bodies in the outer Solar system, the Oort cloud, has long been hypothesized based on the orbits of long-period comets and simulations of the formation of the Solar system. There have been no direct detections of these bodies in this region, and given their extremely low fluxes either in reflected Solar light or thermal emission, direct detection is well beyond the capability of any existing or proposed observatory. However, these objects could be detected as they pass across the line of sight to background stars briefly occulting the stellar light. We recently submitted a proposal to NASA's Discovery AO (PI: C. Alcock) to build a mission that would continuously monitor the optical light curves of tens of thousands of stars at 5 Hz to search for occultations by these outer Solar system objects. Over a wide range of object sizes and distances, the occultations are well described by Fresnel diffraction, so that we can constrain the size of and distance to the occulting objects from the light curves. In this presentation, I will outline the science that we will do with this mission, detail the instrumentation, describe the underlying physics of the occultations and what information we obtain from the light curves, and outline SAO's role in this exciting mission. by our lack of theoretical understanding of MHD turbulence especially in the presence of observational constraints. However there has been a recent explosion of new techniques and diagnostics for measuring MHD turbulence in the multiphase ISM. In this talk I will discuss the utility of synchrotron intensity and polarization for measuring turbulent fluctuations in the ISM. In particular one can use polarization gradients and correlation function of synchrotron intensity to back out important flow parameters such as the sonic and Alfven Mach numbers. This has particularly interesting consequences for measuring turbulence in the accretion disk of Sg A* using the Event Horizon Telescope.

1:00 pm: Institute for Theory and Computation (ITC) Seminar. Speakers: Michael Bietenholz, York University, Canada, and HartRAO, South Africa, and Ondrej Pejcha, Princeton University. Topics: To be announced. Pratt Conference Room.


11:00 am: Institute for Theory and Computation (ITC) Colloquium. "Coming of Age in the Dark Sector," Dr. Frank van den Bosch, Yale University. Pratt Conference Room.

Abstract: I present a new, analytical model for the assembly of dark matter halos and their substructure. I demonstrate how the subhalo mass and velocity functions arise from a universal mass accretion history, combined with simple, purely gravitational processes describing the tidal stripping and impulsive heating of subhalos. This comprehension allows straightforward prediction of how subhalo statistics scale with halo mass, redshift and cosmology. The model is in superb agreement with numerical simulations, and allows the construction of thousands of realizations of dark matter halos with superb mass resolution. As an application, I address the Too-Big-To-Fail problem for the satellite galaxies in the Milky Way with statistics of unprecendented signal-to-noise.

4:00 pm: Colloquium. "Gaia Mission Status and Scientific Performance," Dr. Timo Prusti, European Space Agency. Preceded by tea at 3:30 pm. Phillips Auditorium.

Abstract: The commissioning phase of the Gaia satellite was completed in July 2014 and we are well into the first year of routine phase operations out of the nominal 5 year mission. All subsystems are working and the operational parameters have been tuned for optimum science performance. A final upgrade of the on-board detection software is under testing. The aim is to be operational in the final configuration by summer 2015. The magnitude limit of the survey has been set to G=20.7 mag for astrometry and photometry. The spectroscopy magnitude limit is currently G_RVS=16.2 mag, but may be adjusted pending the new on-board software testing. The Science Alerts stream based on photometry has been started while preparations are underway for the first intermediate catalogue release by summer 2016. Examples of Gaia observations will be shown to indicate the scientific power of this ESA cornerstone mission.