Research

Supernovae and their Aftermath: The collapse of a massive star and the resulting supernova explosion are dramatic events which both complete the stellar life cycle and regulate the structure of the Galaxy's interstellar medium (ISM). However, we don't yet fully understand how stars explode; constraints on the many complicated processes which occur during core collapse are desperately needed. Since we rarely see a nearby star go supernova, our focus is on studying the aftermaths of supernova explosions, namely supernova remnants and young neutron stars, and in using these objects to infer the properties of the supernova, the progenitor star, and their surroundings. This work is providing new insights into the micro- and macro-physics of the core-collapse process, on the properties of supernova progenitors, and on the mechanisms which produce the diversity we see in the resulting compact objects.

Magnetic Fields and Turbulence: Magnetism plays a critical role in many areas of astrophysics, because it controls both the bulk flow properties of interstellar gas as well as the motion of individual charged particles. However, we know surprisingly little about the properties of the Galactic magnetic field. We are making a concerted effort to redress this situation, using the Faraday rotation of the diffuse polarized radio background as a new way to study structure and turbulence in magnetized gas. Some of our recent projects include using the power spectra of rotation measures to map the turbulent cascade of ionized gas in the Galactic plane, using the Faraday rotation of background point sources to map out the large-scale magnetic structure of the inner Galaxy, and analyzing polarization data on the Large Magellanic Cloud in order to carry out the most detailed study yet of the magnetic field of an external galaxy. Such data represent a whole new way of studying the ISM, and can allow a comprehensive study of interstellar magnetic fields on scales ranging from sub-parsec turbulence up to global galactic structure.

Pulsar Winds: As a rapidly spinning young neutron star (a "pulsar") slows down, it deposits its enormous reservoir of rotational energy into its environment via a relativistic wind, producing an observable pulsar wind nebula (PWN). PWNe are a rich source of information. Most fundamentally, PWNe provide a direct probe of the high-energy processes through which a neutron star's considerable reservoir of rotational energy is eventually deposited into its environment. Secondly, because PWNe are close enough to be spatially resolved, they provide an excellent laboratory for studying the process through which a rotating compact object couples to its environment, a theme now also emerging in modeling of gamma-ray bursts and their afterglows. Finally, it is important to realize that the presence of a PWN unambiguously points to the presence of a central neutron star, even when the latter cannot be directly detected. PWNe are thus good signposts in the ongoing search for the youngest and most energetic neutron stars. We run a diverse X-ray, radio and optical program focused on using PWNe as probes of the interaction between pulsars and their environments. Through this work, we hope to provide a detailed physical basis for understanding the processes through which pulsars accelerate relativistic particles and interact with their surroundings.

Shocks and Particle Acceleration: On the basis of energetics alone, supernova remnants (SNRs) have long been considered a primary source of cosmic rays below "the knee" (i.e. with energies less than 10^15 electronvolts). First-order Fermi shock acceleration (also called diffusive shock acceleration), in which particles gain energy from scattering back and forth across the shock, has been suggested as the most probable acceleration mechanism in SNR shocks. However, it is only recently that observational evidence has emerged for acceleration in SNRs up to these energies, through the detection of non-thermal X-ray emission and TeV gamma-ray emission from a limited number of SNRs. We are studying those few SNRs which efficiently accelerate cosmic rays, with the aim of understanding what particular conditions and mechanisms are responsible for high-energy particle production.

Next Generation Radio Telescopes: Radio astronomy has a remarkable track record of discovery - including the discovery of pulsars, quasars, the cosmic microwave background, complex molecules in interstellar space, the existence of gravitational waves and the first extra-solar planets. Yet, all this was accomplished with technology largely from the 1960s and 1970s. Astronomers from all over the world are now working toward constructing a next generation radio telescopes, using 21st century hardware and software, the Square Kilometre Array (SKA). The SKA will have extreme sensitivity, be able to image vast fields of view, and to cover huge fractions of the radio spectrum simultaneously. Through these new capabilities, entirely new types of science become possible. Our group is heavily involved in developing the SKA science case, and in building and using SKA prototypes.

Neutron Star Cooling: Neutron stars are macroscopic manifestations of processes that otherwise occur only in individual atomic nuclei. Formed hot in the core collapse that terminates the life of a massive star, they are supported against gravitational implosion by neutron degeneracy pressure. However, details of the interior structure of neutron stars remain poorly understood, largely due to our incomplete understanding of the strong interaction at ultrahigh densities. In the early stages of their lives, energy loss is dominated by neutrino emission. However, the neutrino production rate is highly dependent upon the structure of the interior. In the ``normal'' cooling scenario, neutrino production proceeds primarily via the modified Urca process. The residual heat diffuses from the core to the surface, manifesting itself as blackbody-like emission - modified by effects of any residual atmosphere - which peaks in the soft X-ray band. The rate at which the surface temperature declines depends critically upon the neutrino emission rate, and thus provides constraints on hadronic physics at high densities. We carry out X-ray observations of young neutron stars in order to explore their cooling properties and better understand the interior structure. Our results show that the standard cooling scenario is too slow to explain observations, and that enhanced neutrino cooling in the neutron star interiors is required.

The Interstellar Medium

Low-Frequency Radio Astronomy

Ultraluminous X-ray Sources

X-ray Binaries

Next Generation X-ray Telescopes