Title:

The Effect of the Stellar Wind on X-ray Polarization in High Mass Binary Systems

A high-mass X-ray binary system (HMXB) is a compact object that is in a binary orbit with a high mass companion star, the compact object usually being a black hole or neutron star. These are interesting for us to study because we are able to see how matter behaves in the strongest gravitational and magnetic fields in the Universe. The emissions from the innermost regions around the compact object are predominantly X-rays; these X-rays are a good tool to study how matter behaves in these extreme environments. A HMXB are among the brightest X-rays sources in the sky, making them easy to study. We are able to study these systems with tools like X-ray polarimetry, upcoming missions being IXPE in space and XL-Calibur on a balloon, these tools give us precise polarization measurements. The X-ray polarization properties of the compact object has been studied theoretically in great detail, but not much research has looked into what happens to the polarization levels of photons as they interact with the stellar wind. Recent work indicates that scattering in the stellar wind may have an observable effect changing the polarization. Our goal is to take a closer look at the effect of the stellar wind in different systems. To be able to study these systems we implemented a Monte Carlo ray tracing algorithm. Then we track the photons through the stellar wind using a Runge-Kutta integration method in order to find the interaction point. At the interaction point we simulate Compton scattering and photon absorption. In order to test the algorithm, we simulate simple systems where the compact objects are isotropically emitting unpolarized X-rays while orbiting the companion star in a spherically symmetric stellar wind. The results we find will be important to future experiments because the equipment that is now being used is very sensitive to changes in polarization levels. In this paper, I will discuss the raytracing algorithm I implemented, and some first results I obtained with this algorithm.

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