GRB Polarimetry with POLAR. Merlin Kole

Transcription

1 GRB Polarimetry with POLAR Merlin Kole Alsatian X-ray Workshop, November 13th, 2017

2 POLAR on the TG : TG-1 Space Lab launched TG-2 Chinese Space Lab launched in September 2016 POLAR launched with the TG-2 POLAR is the only foreign payload on TG-2 Aim: Measuring the polarization of up to 100 Gamma-Ray Bursts Energy Range: kev 2 / 18

3 Gamma-Ray Burst Polarimetry Many models exist with many different predictions on the polarization parameters See for example: Kenji Toma arxiv: Polarization holds information on emission process, e.g. synchrotron emission or photospheric emission or... Also on emission region, strong magnetic fields? Highly ordered? 3 / 18

4 Gamma-Ray Burst Polarimetry Previous measurements were performed GRB Instr./Sat. Pol. (%) Remark A COSI < 46% low statistics A GAP/IKAROS Constant Pol. Angle A GAP/IKAROS 70 ± 22 Constant Pol. Angle A GAP/IKAROS 27 ± 11 Pol. Angle changes by RHESSI 80 ± 20 systematics RHESSI systematics A IBIS/INTEGRAL IBIS/INTEGRAL A IBIS/INTEGRAL 4/43 ± 25 Changing Angle and Degree A SPI/INTEGRAL 98 ± 33 Inconsistent with IBIS BATSE/CGRO BATSE/CGRO 35 - Most measurements performed by non-dedicated instruments Non of the measurements is really constraining Required: A large catalog of GRB constraining measurements 4 / 18

5 Compton Polarimetry Azimuthal scattering angle dependence on polarisation Figure of merit = µ 100: Modulation measured for a 100% polarized incoming beam Second figure of merit = MDP: Minimum level of polarization distinguishable from an unpolarized flux with (typical) 3σ certainty ( dσ dω = r2 o E 2 E 2 E 2 E + E ) E 2 sin2 θ cos 2 φ. (1) 5 / 18

6 POLAR design POLAR uses a segmented scintillator array to measure the Compton scattering angle In total 1600 plastic scintillators, mm, EJ-248M Plastic scintillators optimize the cross section for Compton scattering in the kev energy range Plastic scintillators allow for a relatively large effective area, with low mass of 30 kg Small granularity of the scintillator array results in high angular resolution, high sensitivity for polarization measurements 6 / 18

7 The Final Product Relatively large effective area Small pixels allows for high precision scattering angle measurements Uniform effective area gives us a large Field of View We see half of the sky and perform polarimetry for sources within 1/3rd of the sky Full description of the instrument recently published: N. Produit et al. arxiv: / 18

8 POLAR on the TG-2 TG-2 Chinese Space Lab launched on September 15th 2016 Operational time of 2 years 8 / 18

9 Calibration Energy Spectra Instrument contains 4 weak Na 22 sources Back-to-back photons are used for calibration Spectra in flight match those measured on ground and from simulations 9 / 18

10 Do we see photons? Crab illustrates our timing capabilities Timing shown precision shown to be below 1 ms Crab can serve as energy calibration source and potential polarization measurement target 10 / 18

11 Do we see GRBs? Total of 49 GRBs reported to community Light curves on: ch/polar/lc/ Approx. 10 of these GRBs have an MDP below 30% 11 / 18

12 Instrument Calibration Careful on-ground calibration was performed Detailed simulations were built to reproduce the measured results in great detail Tests for different energies and different angles Example response to a 110 kev beam of 30 off-axis 12 / 18

13 Instrument Calibration II Instrumental/geometrical effects can be removed with unpolarized beam For on-ground results we can use data, for flight we rely on simulations Results from beam test are promising, although we have a systematic error Details in paper: M. Kole et al. arxiv: In-orbit we have better energy calibration, but unknown items around the instrument 13 / 18

14 In-orbit Performance Currently understanding all the last details of instrument behavior For example studying background induced polarization 14 / 18

15 Study of first GRBs Detailed study of an extreme example: A A was long, measured by Fermi-GBM, and 77 off-axis for POLAR Studying spectral response together with GRB analysis experts at MPE (J. Greiner and J.M. Burgess) Starting to study the measured and simulated (unpolarized) modulation for this Once this works all others will follow shortly 15 / 18

16 Future of POLAR Bad news: HVPS problem occurred in April 2017 Slowly trying to recover the instrument Good news: Polarization studies are progressing fast First results expected early next year Developing POLAR2, to be launched in approx. 5 years. 16 / 18

17 Summary and Outlook POLAR is a Compton based polarimeter designed to study GRB emission in the kev energy range Instrument successfully launched We can see the Crab and GRBs, from about 10 good polarization measurements are expected (MDP below 30%) Expect first polarization results early next year, more will follow soon after Item currently not taking data, hopefully start again soon Or wait for POLAR / 18

18 Thank you for your attention! 18 / 18