We analyse the performance of a gas time projection chamber (TPC) as a high-performance gamma-ray telescope and polarimeter in the e + e − pair-creation regime. We use data collected at a gammaray beam of known polarisation. The TPC provides two orthogonal * philippe.gros at in2p3.fr projections (x, z) and (y, z) of the tracks induced by each conversion in the gas volume. We use a simple vertex finder in which vertices and pseudo-tracks exiting from them are identified.We study the various contributions to the single-photon angular resolution using Monte Carlo simulations, compare them with the experimental data and find that they are in excellent agreement. The distribution of the azimuthal angle of pair conversions shows a bias due to the non-cylindrical-symmetric structure of the detector. This bias would average out for a long duration exposure on a space mission, but for this pencil-beam characterisation we have ensured its accurate simulation by a double systematics-control scheme, data taking with the detector rotated at several angles with respect to the beam polarisation direction and systematics control with a non-polarised beam.We measure, for the first time, the polarisation asymmetry of a linearly polarised gamma-ray beam in the low energy pair-creation regime. This sub-GeV energy range is critical for cosmic sources as their spectra are power laws which fall quickly as a function of increasing energy.This work could pave the way to extending polarised gamma-ray astronomy beyond the MeV energy regime.

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Current γ-ray telescopes suffer from a gap in sensitivity in the energy range between 100 keV and 100 MeV, and no polarisation measurement has ever been done on cosmic sources above 1 MeV. Past and present e + e − pair telescopes are limited at lower energies by the multiple scattering of electrons in passive tungsten converter plates. This results in low angular resolution, and, consequently, a drop in sensitivity to point sources below 1 GeV. The polarisation information, which is carried by the azimuthal angle of the conversion plane, is lost for the same reasons.HARPO (Hermetic ARgon POlarimeter) is an R&D program to characterise the operation of a gaseous detector (a Time Projection Chamber or TPC) as a high angularresolution and sensitivity telescope and polarimeter for γ rays from cosmic sources. It represents a first step towards a future space instrument in the MeV-GeV range.We built and characterised a 30cm cubic demonstrator [SPIE 91441M], and put it in a polarised γ-ray beam at the NewSUBARU accelerator in Japan. Data were taken at photon energies from 1.74 MeV to 74 MeV and with different polarisation configurations.We describe the experimental setup in beam. We then describe the software we developed to reconstruct the photon conversion events, with special focus on low energies. We also describe the thorough simulation of the detector used to compare results. Finally we will present the performance of the detector as extracted from this analysis and preliminary measurements of the polarisation asymmetry.This beam-test qualification of a gas TPC prototype in a γ-ray beam could open the way to high-performance γ-ray astronomy and polarimetry in the MeV-GeV energy range in the near future.γ rays are produced in some of the most violent objects in the Universe, such as pulsars, active galactic nuclei (AGN) and γ-ray bursts (GRBs). The field of γ-ray astronomy is concerned with the study of these non-thermal emissions, thus allowing us to gain an understanding of the detailed nature of these extreme objects.Below ≈1 MeV Compton telescopes (γe − → γe − ) are highly performant, but the Compton cross-section, and therefore the telescopes sensitivity, decreases with photon energy. Above ≈1 GeV pair telescopes (γZ → Ze + e − ) are highly performant, but they use high-Z converter plates (tungsten in Fermi's Large Area Telescope, Fermi-LAT) which severely degrade their resolution at low energy. The gap in resolution and sensitivity in the MeV-GeV range hinders the study of many broad-band astrophysical emitters. This includes for instance blazars, which are postulated to comprise a sequence, the so-called blazar sequence [], whose properties could be biased by the absence of sensitive observations in the MeV-GeV range. It is also difficult to distinguish sources in crowded regions of the sky, such at the galactic plane. To a large extent, the MeV-GeV sensitivity gap [1] is an angular resolution issue. The angular resolution of pair telescopes can be improved, from the Fermi-LAT's ≈ 5 • at 100 MeV [2] to 1 − 2 ...

Access to the photon polarisation in the 1-100 MeV energy range is a challenge for the next generation of space telescopes. The current telescopes in space are almost blind in this energy range, mainly due to the degradation of the angular resolution of e+e-pair and due to elastic scattering in the matter. Pair-conversion detector technologies as gaseous detectors are a promising alternative to the technologies based on tungsten-converter/thin-sensitive-layer stacks such as COS-B/EGRET/Fermi-LAT, firstly to improve the single-photon angular resolution and secondly for the polarisation information. The use of a time projection chamber (TPC) as a target and a tracking detector will improve by up to one order of magnitude the single-photon angular resolution (0.5 • @100 MeV) with respect to the Fermi-LAT (5 • @100 MeV), and by up to a factor of three with respect to what can be expected for silicon detectors (1.0-1.5 • @100 MeV). With such a good angular resolution, a TPC can close the sensitivity gap at the level of 10 −6 MeV/cm 2 .s) between 3 and 300 MeV despite having e lower sensitive mass. Furthermore, this good single-track angular resolution allows us to measure the linear polarisation fraction. The HARPO (Hermetic ARgon POlarimeter) detector prototype that we built is a high pressure (0.5-4 bar) low pile-up and low-diffusion gas detector. We will present the results of its highstatistics characterisation in the 1.7-74 MeV fully-polarised and non-polarised gamma-ray beam provided by the BL01 line at NewSUBARU. The excellent value of the polarisation asymmetry dilution factor that we measured opens the possibility of having a polarimeter in space working in the MeV-GeV energy range. In conclusion, we will present the design of a balloon-flight prototype ST3G (Self-Triggered Time projection chamber as a Gamma-ray Telescope) which is being developed. We will discuss its expected performance.

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