1. HARPO: Gamma ray telescope and polarimeter in MeV-GeV range Philippe GROS LLR, École Polytechnique for the HARPO collaboration 04/12/2015 Atelier accéleration de particules dans les sources astrophysiques de haute énergie IPAG, Grenoble

2. Motivation for a MeV-GeV telescope
Working principles of the HARPO detector
First results from polarised gamma ray beam

3. Gamma source sensitivity 100keV-100MeV gap
Disclaimer: this is a personal representation of estimations extracted from different presentations for illustrative purposes

4. HARPO Demonstrator scintillators micromegas +GEM readout strips
field cage

5. Electronics Readout of strips
AFTER electronics (developed at CEA for T2K experiment)
Trigger
PM readout with PARISROC chip (Omega)
Micromegas signal read through preamp and CFD
Trigger built with FPGA on PMm2 card

6. TPC: photon conversion
The incoming photon interacts with the gas and decays into an electron-positron pair
γ 𝑍 + → 𝑒 + 𝑒 − 𝑍 + γ 𝑒 − → 𝑒 + 𝑒 − 𝑒 −

7. TPC: Gas ionisation Ar++e-
The electron and positron travel through the gas (mostly Argon) and ionises it, freeing many electrons and positive ions
This takes a few nanoseconds

8. TPC: Drift and Readout
The electrons drift along the electric field in a few microseconds.
The are amplified and read out on the readout plane

9. Polarisation measurementφ
The azimuthal angle φ is related to the polarisation direction
𝑑Γ 𝑑ϕ ∝1+Α𝑃cos 2 ϕ− ϕ 0

10. Beam test
Demonstrator built and tested in polarised photon beam in NewSUBARU, Japan
13 Energy points, 1.6 to 74MeV, ~60Mevents
Laser
gamma beam
electron beam

11. Events

12. Events Nucleus recoil

13. Events triplet conversion

14. Reconstructed event

15. Very preliminary results
Ongoing analysis
Effects of polarisation visible down to 11MeV
Reconstruction needs improvement
Large systematic effects
low reconstruction efficiency
No optimal fit yet

16. Conclusions Many low energy gamma telescopes are being studied
A gaseous tracker would give the best resolution
Polarimetry in the pair regime is possible!

17. Questions? The HARPO team

18. Science case Blazar (most sources seen by Fermi)
E spectrum described by both leptonic and hadronic models
In gamma range
Leptonic unpolarised
Hadronic polarised
example of HBL
H. Zhang and M. Böttcher,
A.P. J. 774, 18 (2013)