Collaborative Research: Spectral and Radiation Hydrodynamic Models of Photospheric Radius Expansion X-ray Bursts

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Neutron stars (NS) are the most compact known stellar objects, apart from black holes. When a neutron star is in a binary orbit with another star, and material flows from the star to the neutron star, the NS can undergo an explosive outburst, which often includes the emission of X rays. Type I X-ray bursts are powered by unstable thermonuclear burning of freshly accreted hydrogen- and/or helium-rich material on the surface of a NS in a low-mass X-ray binary. About 20% of bursts are photospheric radius expansion (PRE) bursts, in which the luminosity of the burst is so high that radiation forces drive an optically thick wind that lifts the photosphere (outer shell from which light is radiated) off the NS surface. A research collaboration between the University of Virginia (UVa) and the University of Texas at Arlington will study the physics of PRE X-ray bursts, using a combination of sophisticated computer models to simulate X-ray bursts in 1D spherical symmetry and 2D axisymmetery, both in the Newtonian and general relativistic limits. They will compare the results of their simulations directly with observations. This work will allow astronomers to better understand neutron stars, how they interact with matter, and what is the balance of pressure, volume and temperature within neutron stars. The project will also facilitate STEM research among a number of groups. Two graduate students will be given training in state-of-the-art computational tools. The PIs will incorporate the research into their mentoring of undergraduate researchers from underrepresented groups in STEM fields through one principle investigator’s participation in the VA-NC alliance and Spelman College-UVa Exchange summer research programs and the other’s participation in the Louis Stokes Alliance for Minority Participation Summer Research Academy. The investigators will also draw on the research as part of their public outreach efforts. The proposed research would leverage state-of-the-art numerical simulation tools, combining MESA and Athena++ codes, to provide the most sophisticated simulation of PRE bursts to date. This includes generation of synthetic spectra at different points in the burst to calibrate assumptions commonly used to infer neutron star masses and radii from burst observations. This work is timely due to the wealth of new observations from NASA’s NICER mission, which provide better spectral coverage for later stages of burst evolution and have identified spectral lines in the burst spectra. Spectral modelling will be able to characterize constraints from continuum emission but also aid in the interpretation of these spectral features. These would be among the first calculations to self-consistently solve for the time-dependent solution in full general relativity while exploring the possible inclination dependence through non-spherical calculations.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.