Talks & Events
Astronomy Colloquia: 2006
Radiation transfer in neutron star magnetospheres: the binary pulsar and magnetars
This talk will be separated into two parts. First, I will discuss eclipses in the binary pulsar system PSR J0737-3039, where the faster pulsar A is eclipsed once per orbit. A simple model of eclipses based on synchrotron absorption on closed field lines of slower pulsar B reproduces the complicated observed light curve down to intricate details and provides a direct proof of the long-standing assumption of dipolar magnetic fields of neutron stars.
In a second part I will describe a model of non-thermal emission from magetars due to resonant cyclotron scattering of surface radiation in their magnetospheres. Applying the model to anomalous X-ray pulsar 1E 1048.1—5937 gives a fit just as good as less physically motivated “black body plus powerlaw” spectra and allows one to determine density and temperature of the magnetospheric plasma.
Do Extragalactic Cosmic Rays Induce Cycles in Fossil Diversity
Recent work has revealed a highly statistically significant 62 +/- 3-million-year cycle in the number of marine genera. This implies a periodic process extending back 540 My. While astro- and geophysical phenomena may be periodic, no plausible mechanism has been found. The fact that the period of the diversity cycle is so close to the 64 My period of the vertical oscillation of the Solar system relative to the galactic disk is suggestive. We propose that the diversity cycle is caused by the anisotropy of cosmic ray (CR) production in the galactic halo/wind/termination shock and the shielding effect of the galactic magnetic fields. CRs affect the biosphere in a number of ways: they cause DNA damage, mutations and cancer due to increased radiation, affect the atmospheric ozone concentration and UV protection, initiate cloud formation, and can affect climate. The high statistical significance of (i) the phase agreement between maximum excursions of the Sun toward galactic north and minima of the diversity cycle and (ii) the correlation of the magnitude of diversity drops with cosmic ray peak values through all cycles provide solid support for our model.
Searching for protoplanetary fragments in the Sloan Digital Sky Survey
The main asteroid belt represents a significant repository of material left over from the formation of the terrestrial planets. However, the formation and dynamical history of the asteroid belt is complicated, with a significant fraction of asteroids having undergone varying degrees of mechanical and chemical processing. One very important such mechanism is the formation and subsequent disruption of large planetesimals thought to have occured in the early Solar System. Among the most highly processed asteroids are the Vtype. V-type asteroids exhibit absorption bands characteristic of a basaltic (i.e. volcanic) composition, indicating an origin on the crust of large differentiated parent bodies. Until recently all known V-type asteroids were dynamically and compositionally consistent with being fragments of 4 Vesta, the second largest of the asteroids. This talk describes a search for new V-type asteroids, dynamically independent of Vesta, using the Sloan Digital Sky Survey dataset and follow-up observations from Apache Point Observatory. The discovery of new independent V-type asteroids will give us important insights into the timing and processes involved in the formation of planets.
Optically Selected Galaxy Clusters: Update 2006
The Red-Sequence Cluster Surveys (RCS-1, now complete, and RCS-2, ongoing) are a pair of large optical surveys comprising about 1100 square degrees of imaging, designed to find clusters to redshifts beyond one. I will present a number of recent results from these surveys, including the cosmological analysis of RCS-1, and new results on strong lensing in RCS-2. Additionally I will discuss the SDSS cluster catalog, and the future of cluster research within the SDSS.
High Resolution Radio Cosmology Near and Far
Measurements of the angular power spectrum of temperature anisotropies in the Microwave Background strongly support a model in which "primordial" energy density fluctuations generated by inflation-- modified mainly by simple gravity-driven evolutionary processes-- grow to form the structures seen today. While the last scattering surface itself appears to be well-explained by a simple physical picture, important questions remain about the subsequent evolution of large scale structures. For example: what is the state of the present-day mass density field, and where are the baryons? I will discuss two projects with the Green Bank Telescope which address these questions: 30 GHz discrete source observations undertaken to improve the precision of arcminute-scale CMB measurements; and a search for the long millimeter-wave hyperfine transitions of metal ions. I will also briefly describe recent first-light observations with the GBT at 90 GHz and related science prospects in this waveband.
New Challenges in Astrophysical Particle Acceleration
It is nearly a century since Victor Hess understood the nature of GeV cosmic radiation and over sixty years since Pierre Auger demonstrated the presence of PeV particles. The last decade has seen detailed studies of the energy frontier just below a ZeV. In addition imaging of supernova remnants and extragalactic jets by Chandra X-ray Observatory and H.E.S.S. has allowed us to explore astrophysical acceleration sites in detail. High Mach number shock fronts are confirmed as powerful accelerators and it now appears that they also amplify magnetic field and create PeV particles. The manner by which this happens will be discussed. PeV electron acceleration in relativistic jets requires a different mechanism and some possibilities will be explained. Candidate "ZeVatrons" will be reviewed. Finally, prospects for GLAST, which is due for launch next
Fall, will be described.
The Swift Observatory and High Energy Emission from GRB Afterglows
Swift was launched 2004 November 20. Since that time, the Burst Alert Telescope has detected approximately 2 gamma ray bursts (GRBs) per week, and the pointed instruments, including the X-ray Telescope and the Ultraviolet Optical Telescope, have slewed to a large fraction of these bursts with unprecedented speed. The prompt observation of GRB positions has allowed the X-ray telescope to study GRB afterglows at times that are several orders of magnitude earlier than past observations. Many exciting results have emerged, including X-ray afterglow detections of multiple short-hard bursts, ubiquitous flares at late times (100-10000 s) which imply delayed sporadic internal engine activity, a new canonical afterglow light curve that includes the transition from the prompt emission and multiple breaks in the power law-decay slope, very high redshift afterglow measurements, as well as other new results. A summary of these recent observations and their implications will be discussed, with particular emphasis on the emergence of new phenomena in the early X-ray afterglows of long bursts.