Q. Luce | Searcharxiv

arXiv2025

Stellar physics at sub-nanoradian angular resolution

Many stars -- if they could be imaged with enough angular resolution -- would exhibit features expected from theory but not possible to extract from spectra. We may group these by increasing complexity as follows. First, smooth variations in brightness across the surface, resembling solar limb darkening but much more prominent and involving more processes in stars with fast spin or external tides. Next, there are periodic features: not only oscillations, but also convective cells and starspots, which appear to transit across a star as its spins, and exoplanets that really do transit across the star. Then, there are transients like flares. Current optical interferometers provide synthetic apertures of a few hundred metres and angular resolutions down to about nanoradian ($\simeq 0.2\,$milliarcsecond), enough to resolve some of the above features on the nearest upper main-sequence stars, giants and supergiants. Ongoing projects aims to km-scale synthetic apertures, enough to measure the radius of the nearest white dwarf. In this White Paper we briefly discuss what could be observed with synthetic apertures over $\sim20\,$km -- resolving detail on white dwarfs at the level currently possible on supergiants.

astro-ph.IM↗

arXiv2023

The Use of the Signal at an Optimal Distance from the Shower Core as a Surrogate for Shower Size

When analysing data from air-shower arrays, it has become common practice to use the signal at a considerable distance from the shower axis ($r_\text{opt}$) as a surrogate for the size of the shower. This signal, $S(r_\text{opt}$), can then be related to the primary energy in a variety of ways. After a brief review of the reasons behind the introduction of $r_\text{opt}$ laid out in a seminal paper by Hillas in 1969, it will be shown that $r_\text{opt}$, is a more effective tool when detectors are laid out on a triangular grid than when detectors are deployed on a square grid. This result may have implications for explaining the differences between the flux observed by the Auger and Telescope collaborations above 10\,EeV and should be kept in mind when designing new shower arrays.

astro-ph.IM↗