Pat Slane

Research

The Explosive Aftermaths of Stars

Supernova Remnants

In the collapse of a massive star, or the thermonuclear ignition of a white dwarf core prompted by either accretion or merger with another white dwarf, the resulting supernova explosion expels a solar mass or more of material enriched in heavy elements synthesized during the stellar evolution and explosive nucleosynthesis. The expelled ejecta drive a collisionless shock into the surrounding medium, creating a hot shell of ISM/CSM material - the supernova remnant (SNR). As this blast wave slows down due to the swept-up material, the increased pressure drives a reverse shock into the ejecta, heating these as well. This creates an inner shell with abundances that reflect the composition of the stellar ejecta. The density structure and composition of the material behind the blast wave provides key information about the environment surrounding the stellar progenitor, including possible modifications from winds or mass ejection events, while the ejecta heated by the reverse shock contain information about the progenitor and details about the explosion mechanism. In addition, the rapid shocks are capable of amplifying magnetic fields and accelerating particles to extremely high energies, producing a significant component of the Galactic cosmic rays.

My research includes studies of the structure and composition of SNRs, their ability to accelerate cosmic rays, and details of their temperature and ionization structure that places constraints on shock heating and temperature equilibration between particle species in the postshock region. These studies incorporate observations from across the electromagnetic spectrum, but are particularly driven by spatially-resolved spectral investigations of the X-ray emission from SNRs.

Pulsars and their Winds

Supernovae from the collapse of massive stars ejecta most of the stellar debris from the progenitor. However, the very central regions often collapse to form an ultra-compact neutron star. Generally, but perhaps not always, these neutron stars have extremely strong magnetic fields and are spinning rapidly, resulting in high electric potentials that accelerate particles and form beams of pulsed emission as the star rotates. The magnetic and particle winds from these pulsars expand into the SNR interior where they are confined by the slower-moving ejecta, leaving an expanding magnetic bubble filled with synchrotron-emitting particles in these so-called composite SNRs. The confinement of these pulsar wind nebulae (PWNe) by the surrounding ejecta defines an outer boundary condition; a termination shock is formed in the far interior, where the pulsar wind is decelerated to gradually match the nebular flow. Magnetic confinement results in jet structures along the pulsar rotation axis.

The emission and structure of PWNe offer the opportunity to study the conversion of rotation into energetic outflows - a theme with importance on many astrophysical scales, but for which the actual spin-down and associated geometry of the wind structure can be uniquely probed in PWNe. In its early evolution, the outer boundary of a PWN drives a shock into the innermost SNR ejecta, providing a direct glimpse at the material formed closest to the core of the explosion.

As the PWN evolves, the SNR reverse shock eventually disrupts the PWN, modifying the energy evolution its relativistic particles, and potentially providing paths for escape. Because most pulsars are born with high velocities imparted by kicks in the explosion, they eventually escape their SNRs, with their winds subsequently confined by pressure from their motion through the ISM. While much fainter at these ages, the absence of the SNR can provide the ability to discern details of the PWN structure in its innermost regions.

My research involves studies of PWNe at all evolutionary stages though multiwavelength observations and hydrodynamical modeling.

Isolated Neutron Stars

While many young neutron stars have the properties of the pulsars described above, X-ray observations of the young Cas A SNR revealed a central star whose properties are vastly different. Their are no detectable pulsations and there is no evident wind nebula. Subsequent observations have revealed similar neutron stars in other systems. Meanwhile, other SNRs reveal central neutron stars whose emission greatly exceeds the power available from the observed spin down, some displaying outbursts that indicate energy release from enormous magnetic fields.

The demographics of neutron stars within SNRs provide our best understanding of the birth properties of these objects, potentially allowing for connections to the details of the supernova explosions and the properties of the progenitor stars. My research in this area has concentrated on searches for neutron stars in SNRs and on measurements of the surface temperatures in comparison with models for their cooling, which place important constraints on the inner structure, for which the equation of state is poorly known due to the extremely high densities.

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