Julia Speicher 
Advisor: Dr. David Ballantyne, School of Physics, Georgia Institute of Technology

Type I X-ray bursts as probes of the neutron star accretion flow

Date: Monday, June 16, 2025 
Time:  11:00 a.m.  
Location: Boggs 1-44 (VizLab)

Zoom link: https://gatech.zoom.us/j/99552032458

Committee Members:

Dr. Tamara Bogdanović, School of Physics, Georgia Institute of Technology

Dr. Nepomuk Otte, School of Physics, Georgia Institute of Technology

Dr. John Wise, School of Physics, Georgia Institute of Technology

Dr. Tod Strohmayer, NASA Goddard Space Flight Center

Abstract:

Neutron stars in low-mass X-ray binaries frequently exhibit Type I X-ray bursts caused by unstable nuclear burning of accreted matter on the neutron star surface. X-ray bursts offer the opportunity to constrain the neutron star radii and masses and hence the equation of state of one of the densest environments in the Universe. However, constraining neutron star parameters requires knowledge of the entire observed emission, which also includes the emission of the neutron star accretion flow. X-ray bursts irradiate it, leading to structural and emission changes. Therefore, understanding the evolution of the accretion flow emission not only allows us to constrain neutron star properties but also to probe its accretion environment.

My PhD dissertation details my work examining the impact of Type I X-ray bursts on the accretion flow of neutron stars. My first project studies the observational impact of an X-ray burst on the corona. Combining an analytical model and a hybrid-emission plasma code reveals how the coronal emission decreases at high X-ray energies and increases at low X-ray energies due to Compton scattering with the burst photons. The project furthermore shows how coronal cooling depends on burst and coronal properties. For my second project, I used a reflection code and post-processed simulation data to calculate how the reflection spectrum of an accretion disk evolves during an X-ray burst. The simulations of the third project examine the effect of neutron star spin on the Poynting-Robertson drag during the burst and demonstrate its importance, regardless of the neutron star's spin. I used this simulation data for the next project to calculate the evolution of the thermal disk emission. The project finds that the enhancement of the thermal disk emission is only weakly correlated with an increase in the mass accretion rate. The dissertation highlights the wealth of observational impact of Type I X-ray bursts on the neutron star's accretion flow, which offers the opportunity to probe its properties and improve the analysis of burst observations.