Mark Kennedy
One of the most important challenges in modern Astrophysics is to determine the equation of state for neutron stars. An obvious method of narrowing which equations may be valid is by looking for exceptionally heavy neutron stars (> 2 M☉). For this purpose, the red back and black widow "spider" systems have become increasingly important in recent years, as recent results suggest they contain heavier neutron stars than their other neutron star binary cousins. However, these mass measurements are plagued by observational and methodological biases. This poster will introduce the family of spider systems, how their masses are measured, where the biases come from, and current work in overcoming these biases. Particular focus will be given to the potentially complex heating patterns which are possible on secondary stars surface. These patterns manifest themselves as asymmetries in the optical light curves of these systems, and play an important role in determining the mass of the neutron star primary. I will also show early work on how we are attempting to model the optical spectra of these systems in an attempt to provide more robust mass measurements in the near future, and thus help us to firmly untangle the spiders web.
Slide 2 focuses on the methods behind measuring the binary system's inclination. We see different sides of the companion star as it orbits the neutron star. Because the neutron star is heating up the companion star, a large temperature difference exists between the side which is facing the neutron star (the day side) and the side facing away (the night side). Since these sides are at different temperatures, this means the light curve (a plot of brightness versus time) varies. Modelling this light curve allows us to determine the system's inclination. However, if the energy distribution on the companion star is not symmetric about the day side of the companion, then determining the inclination becomes complicated, and tends to lead to an overestimate of the neutron star mass. We have created a model which allows for diffusion and convection of energy across the star's surface in order to account for these asymmetries in the light curve. Fitting systems this model to data has lowered the estimate of a neutron star mass in a spider from 2.2 solar masses to 1.6 solar masses. This has consequences for the ability of spiders to limit the neutron star equation of state.
Slide 3 focuses on measuring the radial velocity of the companion star in these systems. The optical spectra of these systems are dominated by absorption features from the companion star. By tracking these absorption features over an orbital period of the system and seeing how much they move, we can estimate the companions radial velocity. However, this assumes that the absorption lines all come close to the centre of mass of the companion. This is not necessarily true, especially for systems which are being heated significantly by the neutron star, as absorption features are incredibly temperature dependent. We have built a model which allows us to generate accurate synthetic spectra of the companions assigning every cell on our model companions surface an emergent spectrum. By integrating these over the surface of the star, we can create model spectra of the system at any orbital phase. which eliminates this bias. By comparing these models to data, we can measure radial velocities with an error of 0.2%, which then feeds into an overall improvement in the accuracy of the mass measurement for a spider system.
Slide 4 concludes the poster. It summarizes that measuring neutron star masses is tricky, but with more and more detailed models being developed to account for asymmetries in light curves and create detailed model spectra, the reported values are becoming more and more trustworthy.