SSL Colloquium on Nov. 15th: Sara Buson

5 Years of Fermi-LAT Highlights


The Fermi observatory was launched on June 11th 2008 and consists of two instruments which observe the gamma-ray sky from 8 KeV to over 300 GeV.  The first five years of the mission have seen numerous exciting scientific results, some expected and others unexpected.  Fermi LAT data allowed for the first time high statistics observations of gamma-ray sources of known and diverse classes, of both galactic and extragalactic nature, as well as discovery of new emitters.

I will present an overview of the Fermi mission and the role it plays in current particle astrophysics and cosmology research.  Then I will discuss recent scientific highlights involving several topics and focussing on extra-galactic sources classes.


SSL Colloquium on Nov. 1st: Martin White

Cosmic sound, near and far

Planck and the CMBThe coupling of baryons and photons by Thomson scattering in the early universe leads to a rich structure in the power spectra of the cosmic microwave background photons and the matter.  The study of the former has revolutionized cosmology and allowed precise measurement of a host of important cosmological parameters. The study of the latter constrains the nature of the dark energy believed to be causing the accelerated expansion of the universe. I will discuss recent measurement of cosmic sound by the Planck and BOSS collaborations, and what they have taught us about cosmology.

SSL Colloquium on Oct. 25th: Ashley King

The regulation of Black Hole Winds and Jets Across the Mass Scale 

Black Hole Winds and Jets

We present a study of the mechanical power generated by both winds and jets across the black hole mass scale. We begin with the study of ionized X-ray winds and present a uniform analysis using Chandra grating spectra. The high-quality grating spectra facilitate the characterization of the outflow velocity, ionization, and column density of the absorbing gas. We find that the kinetic power of the winds, derived from these observed quantities, scales with increasing bolometric luminosity as log (L wind, 42/Cv ) = (1.58 ± 0.07)log (L Bol, 42) – (3.19 ± 0.19). This suggests that supermassive black holes may be more efficient than stellar-mass black holes in launching winds, per unit filling factor, Cv . If the black hole binary (BHB) and active galactic nucleus (AGN) samples are fit individually, the slopes flatten to αBHB= 0.91 ± 0.31 and αAGN = 0.63 ± 0.30 (formally consistent within errors). The broad fit and individual fits both characterize the data fairly well, and the possibility of common slopes may point to common driving mechanisms across the mass scale. For comparison, we examine jet production, estimating jet power based on the energy required to inflate local bubbles. The jet relation is log (L Jet, 42) = (1.18 ± 0.24)log (L Bondi, 42) – (0.96 ± 0.43). The energetics of the bubble associated with Cygnus X-1 are particularly difficult to determine, and the bubble could be a background supernova remnant. If we exclude Cygnus X-1 from our fits, then the jets follow a relation consistent with the winds, but with a higher intercept, log (L Jet, 42) = (1.34 ± 0.50)log (L Bondi, 42) – (0.80 ± 0.82). The formal consistency in the wind and jet scaling relations, when assuming that L Bol and L Bondi are both proxies for mass accretion rate, suggests that a common launching mechanism may drive both flows; magnetic processes, such as magnetohydrodynamics and magnetocentrifugal forces, are viable possibilities. We also examine winds that are moving at especially high velocities, v > 0.01c. These ultra-fast outflows tend to resemble the jets more than the winds in terms of outflow power, indicating that we may be observing a regime in which winds become jets. A transition at approximately L Bol ≈ 10-2 L Edd is apparent when outflow power is plotted versus Eddington fraction. At low Eddington fractions, the jet power is dominant, and at high Eddington fractions, the wind power is dominant. This study allows for the total power from black hole accretion, both mechanical and radiative, to be characterized in a simple manner and suggests possible connections between winds and jets. X-ray wind data and jet cavity data will enable stronger tests.


SSL Colloquium on Oct 18th: Michael McDonald

New Insights into Galaxy Cluster Evolution from the South Pole Telescope

Phoenix ClusterWe present the latest results from the South Pole Telescope collaboration, focusing on a large follow-up program to observe the 80 most massive clusters at 0.4 < z < 1.2 with the Chandra X-ray Observatory. This survey has led to the discovery of the exotic Phoenix cluster, which harbors a massive starburst at its center, perhaps fueled by the first bonafide cooling flow. We present the latest results of a multi-wavelength follow-up of the Phoenix cluster, revealing in detail the extreme nature of this system. The full sample of 80+ clusters has provided new clues to the formation of cool cores, and the evolution of the cooling/feedback balance in cluster cores, as we will discuss. Finally, we will describe work in progress to constrain the evolution of the “Universal” temperature, pressure, and entropy profiles as a function of redshift. This work is providing the most detailed look at how the intracluster medium in an ensemble of clusters has assembled and evolved over the past ~8 Gyr.

SSL Colloquium: Christoph Weniger


Christoph Weniger (Max-Planck-Institut für Physik, Germany) will present the SSL colloquium on May 3rd: `Recently, we identified a tentative gamma-ray line in the Galactic center
observations of the Fermi LAT, which would be an unequivocal smoking-gun
signature for dark matter annihilation in the Universe. One year after
discovery, the signature is still as ambiguous as it could be. I discuss
recent experimental and theoretical developments and show how the situation
could be clarified until end of next year.’