HARPO, a gaseous TPC as a gamma telescope and polarimeter: First measurement in a polarised photon beam between 1.7 and 74 MeV

Gamma-ray astronomy enables the exploration of the non-thermal emission and magnetic field configuration of objects such as active galactic nuclei (AGN), gamma ray bursts (GRB) and pulsars. Presently, there is a sensitivity gap between 1MeV and 100MeV. Additionally, there is no polarisation measurement above 1MeV, although such a measurement could shed light on emission processes. A gaseous detector can achieve a much better angular resolution in the MeV-GeV range than the current/past telescopes that use tungsten converters, thanks to the reduced multiple scattering of the electrons and positrons. This translates to a greatly improved point source sensitivity and gives access to the linear polarisation of the photons through the azimuthal angle of the pair. The HARPO time projection chamber (TPC) is a high angular resolution telescope for gamma-ray polarimetry. It was installed in a polarised gamma-ray beam at NewSUBARU in Japan in 2014. Data were taken at photon energies from 1.7MeV to 74MeV, and with different polarisation configurations. The experimental setup of the TPC and the photon beam are described. The first results from the beam campaign are shown.