Grants and Contributions:

Grant or Award spanning more than one fiscal year. (2017-2018 to 2022-2023)

The astrophysics of stars and compact objects (neutron stars, white dwarfs and black holes) constrain fundamental physics in crucial and unique ways. Furthermore, over the past several years astronomers and physicists have opened two new windows onto the Universe: gravitational radiation and neutrinos. In the near future astronomers will be able to probe the Universe with x-ray polarimetry. I propose several lines of research that exploit these unique and timely opportunities. In particular I propose a series of studies to probe the properties of weakly interacting particles in stars in globular clusters: both theoretical and observational investigations. Second I propose to develop techniques to constrain the neutron star equation of state using a combination of x-ray polarimetry, timing and spectroscopy including the effects of QED birefringence that produces unique signatures on the polarization of x-ray radiation from neutron stars. The third direction is gravitational. In particular I will explore how electromagnetic observations can best probe the properties of accreting black holes.

Our recent studies of the stellar populations in the core of 47 Tucanae demonstrate how a holistic approach to modelling the data can reap great rewards. Specifically we model the evolution of the stars in the cluster including stellar evolution, dynamics and atmospheres starting from the fundamental physical equations all the way to a prediction of the observed data including the observational biases. To obtain constraints on the underlying physics we have to constrain the astrophysical uncertainties, so we simulate the entire evolution of a star from the main sequence to the oldest white dwarfs and use observations and models of the entire process. We now have similar data for over 50 other globular clusters so we can further mitigate the astrophysical uncertainties by studying the stars in a variety of environments.

Observations of x-ray polarization from neutron stars and black holes will providing estimates of their structure and the magnetic fields that surround these objects. I propose to develop theoretical models of the emission regions of x­ray pulsars (that haven't be updated since the eighties) and the magnetic fields surrounding black holes. The goal is to bring the theoretical models all the way to the observational realm to predict the observed distribution of photon energies, polarization directions and arrival times to understand these objects in the greatest detail.