Bright polarised x-ray flashes from dense plasmas

Creating a plasma dominated by strong-field QED (SFQED) effects is a major goal of new multi-PW laser facilities. This is motivated by the fact that the fundamental dynamics of such plasmas is poorly understood and plays an important role in the electrodynamics of extreme astrophysical environments such as pulsar magnetospheres. The most obvious observable for which such a regime has been reached is the production of a bright flash of x-rays, but distinguishing this from other sources of hard x-rays (e.g., bremsstrahlung) is a major challenge. Here we show that the photons from the X-ray flash are highly polarised, as compared to the unpolarised background, i.e., polarisation is an indicator that the SFQED plasma has really produced. For a laser of intensity ${10}^{21}$ Wcm${}^{-2}$ impinging on a solid Al target, the photons of the flash with energy $>10$\thinspace keV are $>65\mathrm{\%}$ polarised.