Enrico Costa IASF-Rome/INAF

1. X-Ray Polarimetry. A new window for High Energy astrophysics Enrico CostaIASF-Rome/INAF Tsinghua Global Vision Lectures Beijing December 6 2011

2. Timing: (Geiger, Proportional Counters, MCA, in the future Silicon Drift Chambers) Rockets, UHURU, Einstein, EXOSAT, ASCA, SAX, XMM, Chandra. Imaging:Pseudo-imaging (modulation collimators, grazing incidence optics + Proportional Counters, MCA, CCD in the future DepFET) Rockets, SAS-3, Einstein, EXOSAT, ROSAT, ASCA, SAX, Chandra, XMM, INTEGRAL, SWIFT, Suzaku. Spectroscopy:Non dispersive (Proportional Counters, Si/Ge and CCD, Bolometers in the future Tranition Edge Spectrometers) Dispersive: Bragg, Gratings. Rockets, Einstein, EXOSAT, HEAO-3, ASCA, SAX, XMM, Chandra, XMM, INTEGRAL, Suzaku,. Polarimetry: (Bragg, Thomson/Compton, in the future photoelectric and subdivided compton) Rockets, Ariel-5, OSO-8 Measurements in X-ray Astronomy

3. Astrophysics: • Non thermal emission processes producing intrinsically polarized photons. • Deviation from spherical geometry of the matter close to the emitting regions polarizing by transfer in a varietyof situations and classes of sources: jets, accretion disks and columns, reflection, archeoastronomy, etc. What can Polarimetry Test? • Fundamental Physics: • Matter in extreme magnetic fields • Matter in strong gravity fields • Quantum gravity effects

4. A vasttheoreticalliteraturepredicts a wealthofresultsfromX-ray Polarimetry Big Hopes Meager Results Polarimetry would add to energy and time two further observable quantities (the amount and the angle of polarization) constraining any model and interpretation: a theoretical/observational break-through (Mészáros, P. et al. 1988). In 40 years only one positive detection of X-ray Polarization: the Crab (Novick et al. 1972, Weisskopf et al.1976, Weisskopf et al. 1978) P = 19.2 ± 1.0 %;  = 156.4o± 1.4o The swing on the polarization vector of photon trajectories near a black hole was long ago suggested (Connors, Piran & Stark,1980) as another diagnostic; but this is still not feasible because X-ray polarimeters are far from capable of detecting the few percent polarization expected (Rees, 2001).

5. A window not yet disclosed THE TECHNIQUES ARE THE LIMIT! ConventionalX-raypolarimeters are cumbersome and have low sensitivity New technicalsolutions are arriving The sameyearof the pessimistic statement by Martin Rees a newinstrumentwasdevelopedwith the potentialityof a dramaticimprovement in bothsensitivity and controlofsystematics. A new Era forX-ray Polarimetry isaboutto come (maybe….)

6. The conventional formalism

7. The first limit: In polarimetry the sensitivity is a matter of photons MDP is the Minimum Detectable Polarization RS is the Source rate, RB is the Background rate, T is the observing time μ is the modulation factor: the modulation of the response of the polarimeter to a 100% polarized beam Source detection > 10 photons Source spectral slope > 100 photons Source polarization > 100.000 photons Caution: the MDP describes the capability of rejecting the null hypothesis (no polarization) at 99% confidence. For a significant meaurement a longer observation is needed. For a confidence equivalent to the gaussian 5σ the constant is higher 4.29→7.58

8. Bragg diffraction  Bragg diffraction from a crystal can be exploited to measure the degree and the angle of polarization P of a photon beam. A Bragg crystal reflects the radiation at an energy that depends on the lattice spacing and on the incidence angle according to the Bragg law. θ θ Bragg law. A crystal oriented at 45o to an incident linearly polarized x-ray beam acts as a perfect polarization analyzer. At 45o only the component of polarization perpendicular to the incidence plane is reflected. By rotating the crystal around the direction of the incoming beam the counting rate of the reflected beam is modulated by the beam polarization. It is a narrow band technique but has a high modulation factor.

9. OSO-8 satellite with a dedicated Bragg polarimeter 468 graphite mosaic crystals were mounted to the two sector of parabolic surface of revolution. Mosaic spread of 0.8o Band-pass = 40 eV (2.62 keV) Bragg angles allowed between 40o and 50o Overall band-pass 400 eV (2.62 keV)  = 0.94 Projected crystal Area = 2 x 140 cm2 ; Detector area = 2 x 5 cm2 ; FOV= 2o B = 2 x 3 10-2 counts/s in each order (pulse shape analysis + anti-coincidence) Precision measurement: of X-ray polarization of the Crab Nebula without pulsar contamination (by lunar occultation, Weisskopf et al.,1978). P = 19.2 ± 1.0 %;  = 156.4o± 1.4o (2.6 keV) P = 19.5 ± 2.8 %; 152.6o ± 4.0o (5.2 keV) OSO-8 satellite (top) and polarimeter (bottom) 67 % and 99 % confidence contour. The radial scale is the polarization in percent

10. X-ray polarimetry with Thomson scattering φ θ is the angle of scattering. φ is the azimuthal angle, the angle of the scattered photon with respect to the electric vector of the incident photon. At 90o of angle of scattering (θ) the modulation factor is 100 % since there are not photons diffused along the electric field.

11. SXRP (Stellar X-ray Polarimeter) • A step forward in the sensitivity was done devising and building a polarimeter based on Bragg diffraction and Thomson scattering in the focus of a large X-ray telescope. • Photons coming from the SODART telescope are diffracted by a thin mosaic graphite crystal at 2.6 keV and 5.2 keV creating a secondary focus. The photons at E > 5 keV that do not satisfy the Bragg condition pass through and are diffused around by a lithium scatterer. 4 position sensitive proportional counters detect simultaneously the radiation. SXRP is in rotation around the telescope axis. • Bragg diffraction saves the images and is more sensitive at low flux, Thomson scattering provides better sensitivity at large fluxes but the image is lost. • 4 x 100 cm2 imaging proportional counter • Composite window thickness : • 150 m for Thomson scattered photons • 50 m for Bragg diffracted photons, ø = 3.3 cm ) • Graphite mosaic cristal (50 m thick) • Lithium scatterer 7 cm long and Ø = 3 cm encapsulated in 150 m thick beryllium case • Rotary motor for the ensamble detector/analyser at 1 rpm T=105 s. Kaaret et al., SPIE 1989, Soffitta et al., NIM A, 1998

12. From Bragg/Thomson to Photoelectric The turning point of X-Ray Astronomy was the launch of Einstein satellite that first introduced the X-ray Optics. The dramatic increase in sensitivity for the detection of faint sources and the capability to resolve extended source with imaging detectors in the focus of grazing incidence telescopes, that do not require rotation, made the mismatching in the sensitivity of polarimeters, and on the requirements to the payload (rotation) unsustainable. Polarimeter was disembaked from Einsten and Chandra and not accepted on XMM. The only big mission that included a polarimeter was Spectrum-X-Gamma with SXRP. SRG was never launched and SXRP concludes the era of traditional polarimeters. The new Era is based on photoelectric polarimeters and finely subdivided scattering polarimeters.

13. Modern polarimeters dedicated to X-ray Astronomy exploit the photoelectric effect resolving most of the problems connected with Thomson/Bragg polarimeter. The exploitation of the photoelectric effect was attempted very long ago, but only since 2001 it was possible to devise photoelectric polarimeters mature for a space mission. An X-ray photon directed along the Z axis with the electric vector along the Y axis, is absorbed by an atom. The photoelectron is ejected at an angle θ (the polar angle) with respect the incidentphotondirection and at an azimuthal angle φ with respect to the electricvector. If the ejected electron is in ‘s’ state (as for the K–shell) the differential cross sectiondepends on cos2 (φ),thereforeitispreferentiallyemitted in the direction of the electricfield. Itisanidealanalyzerof the polarization. Being the cross sectionalwaysnull for φ = 90o the modulationfactor µ equals 1 for anypolar angle. Heitler W.,The Quantum Theory of Radiation Costa, Nature, 2001 β =v/c By measuring the angular distribution of the ejected photelectrons (the modulation curve) it is possible to derive the X-ray polarization.

14. X-ray polarimetry with a Gas Pixel Detector GEM electric field X photon (E) conversion GEM gain collection pixel PCB E a 20 ns To efficiently image the track at energies typical of conventional telescopes IASF-Rome and INFN-Pisa developed the Gas Pixel detector. The tracks are imaged by using the charge. The principle of detection A photon cross a Beryllium window and it is absorbed in the gas gap, the photoelectron produces a track. The track drifts toward the multiplication stage that is the GEM (Gas Electron Multiplier) which is a kapton foil metallized on both side and perforated by microscopic holes (30 um diameter, 50 um pitch)and it is then collected by the pixellated anode plane that is the upper layer of an ASIC chip. Costa et al., 2001, Bellazzini et al.2006, 2007 Polarization information is derived from the angular distribution of the emission direction of the tracks produced by the photoelectrons. The detector has a very good imaging capability. Costa et al., 2001

15. The real implementation of a working GPD prototype. A sealed polarimeter has been built since some years and has been extensively tested, with thermal-vacuum cycles, it has been vibrated, irradiated with Fe ions and calibrated with polarized and unpolarized X-rays. The GPDs under test was filled with 1) 20-80 He-DME 1 bar, 1cm. 2) pure DME 0.8 bar, 1 cm. 3) Ar DME 60-40 2 atm 2 cm. DME = (CH3)2O 60 µm/√cm diffusion

16. Not only MonteCarlo: Our predictions are based on data Eachphotonproduces a track. From the track the impact point and the emission angle of the photoelectronisderived. The distribution of the emission angle is the modulation curve. Muleri et al. 2007 Impact point The modulation factor measured 2.6 keV, 3.7 keV and 5.2 keV has been compared with the Monte Carlo previsions. The agreement is very satisfying. By rotating the polarization vector the capability to measure the polarization angle is shown by the shift of the modulation curve. Present level of absence of systematic effects (5.9 keV). Bellazzini 2010 Soffitta et al., 2010

17. Two approaches Drift direction z The photons enter parallel with respect to the readout plane. The photons enters perpendicularly with respect to the readout plane.

18. Blachamberk et al., 200 X-ray polarimetry with a micropattern Time Projection Chamber High efficiency Not an imager Black 2007 The photons enter along Z, the readout strips run also along Z. The GEM multiply the charge. The charge is then collected by the 1-d strip detector. The signal in each strip is connected to a waveform digitizer and by using its timing characteristics the information the other coordinate is derived. TThis method allows for decoupling the drift length that blurs the image and decreases the modulation factor from the absorption depth that controls the efficiency. Since the origin of the time is not known the TPC is not an imager.

19. GRAVITY and EXTREME MAGNETISMS SMALL EXPLORER GEMS GEMS is a NASA mission that will measure the X-ray linear polarization from selected sources in an energy range between 2-10 keV. The flight is scheduled to be in 2014. Selected by NASA on June 2009 as the 13th of small explorer. The GEMS mission hosts deployable telescopes (Suzaku Mirrors) to arrive at a focal length of 4.5 m. The payload consisted initially of three TPC polarimeters now reduced to two for budget and schedule reasons. (Swank 2010, Yahoda 2010).

20. Engineering Model vibrated. The polarimeter will have a depth of 78 mm x 4 with four aligned micro-strip detector and a pressure of ¼ of atmosphere (equivalent to 8 Atm/cm). The track image can be distorted because the procedure to measure the two projections of the track is different (time and space). The GEMS satellite, in order to eliminate the incidence of these effect, will rotate with respect to the source direction at a speed of 1 rotation each 10 min that is enough slow to not degrade the star-tracker response and enough fast to accomplish many rotations within a single observation (100 rotations for 105 s of observation).

21. The sensitivity to polarization of GEMS will allow to detect the expected degree of polarization from many X-rays sources being a factor of 100 better than the sensitivity of OSO 8. GEMS has a sensitivity of 1 % (MDP) for a flux of 10 mCrab with 3.3 105 s (Jahoda et al. 2010 corresponding for a flux of 1 mCrab source and 105 s at a MDP of 5.7 %). The GEMS primary mission will last 9 months. Additional 15 months of observation are possible on a competitive base on a Guest Observer program.

22. The missions where the GPD was proposed either are waiting after a phase A completed or were not selected or evolved in missions without anymore a polarimeter on-board. POLARIX Costa et al., ExpAst 2010 IXO NHXM Bookbinder, SPIE, 2010 Tagliaferri et al, ExpAst 2010

23. Implementation of X-ray polarimetry with GPD in proposed missions: - POLARIX (ASI small mission, fasa A completed) 3 Jet-X optics (3,5 m FL, 20 ‘’ HED 500 cm2 @ 2 keV, HEW=(20’’)) 3 GPD (1-cm, 1-Atm, He-DME 20-80) MDP 12 % in 105 s for 1 mCrab source (2-10 keV) 3.8 % in 105 s for 10 mCrab source (2-10 keV) • NHXM • (Proposed ESA M3 Mission not selected) • 1 of 4 Multi-layer optics (Pt-C) (10 m FL) • 2 GPD : 1-cm, 1-Atm, He-DME (LEP) (2-10 keV); • 3-cm 3-Atm Ar-DME (MEP) (6-35 keV) • MDP: • LEP 9.7 % in 105 s for 1 mCrab source (2-10 keV) • 3.1 % in 105 sfor 10 mCrab source (2-10 keV) • MEP13 % in 105 s for 1 mCrab source (6-35 keV) • 4.1 % in 105 for 10 mCrab source (6-35 keV) • In study (HEP, Compton scattering) • MDP 7.2 % for 10 mCrab in 105 s (20-80 keV) Costa, et al., Exp Ast 2010 Tagliaferri et al.i, Exp Ast 2010; Soffitta et al. SPIE 2010 - IXO (ESA/NASA/JAXA Large Mission Evolved in Athena with no polarimeter on-board) Area= 2.5 m2 FL = 20 m HEW= 5’’ XPOL: MDP 1 % 1 mCrab 105 s.

24. A detector more tuned on hard X-rays for NHXM The simulations suggested a mixture of Ar (80%) DME (20%) with 3 cm absorption gap and 3 atm pressure. We name it Medium Energy Polarimeter First Prototype working (2 cm 2 Atm) The MEP prototype in the IASF-Rome facility. MEP detector is working apparently well. It is a good Proportional Counter. Unfortunately it broke soon after this testing. Anyway we ar foresaw further changes. A larger detector for better control of the electric field and to exclude background produced on the walls is in construction.

25. The new window: GEMS and after GEMS • The task to disclose the window of X-ray polarimetry is up to GEMS. • Which Astrophysics can GEMS do and what cannot? • Which ideas should drive the design and implementation of future missins with polarimetric capabilities? • Let us make a short review of the most interesting topics of High Energy astrophysics that can be solved with polarimetric measurements. The major issues are: • Imaging • Higher Energy • Wide Field

26. Positive measurement: of X-ray polarization of the Crab Nebula without pulsar contamination (by lunar occultation, Weisskopf et al., 1978).P = 19.2 ± 1.0 %  = 156.4o± 1.4o The only polarized source already known known XEUSp.s.f. Butthisisonly the averagemeasurement The structureismuch more complex! Toperform separate polarimetry ofdetailsof the major structuresweneedimaging! f.o.v. PSR NW jet SE jet Innertorus Outertorus Howturbulentis the field? Howpolarizedis the PSR? Morover we know from AGILE (confirmed by Fermi) that the Crab (not the PSR) is varying on the scale of days at E>100 MeV. These corresponds to a physical region on the arcsecond timescale! Tavani et al. 2011

27. Accretion physics: magnetic NS Spectropolarimetry may constraint the accretion geometry. Electron (in strong B) or proton (in extreme B) cyclotron lines may be searched for and studied in detail (a target for GEMS) Meszaros et al. (1988)

28. NHXM: Cyclotron lines with 100 ks of observation with MEP The extension of polarimetry to Hard X-Ray can allow for a detailed study of cyclotron lines. All detected lines are above 10 keV. Photoelectric polarimetry extended to higher energies (such as in NHXM) or good quality compton polarimetry can allow for a direct exploration of the cyclotron resonances. Here we need the high energy

29. Vacuum birefringence in strong B Soft Gamma Repeaters and Anomalous X-ray Pulsars are interpreted in the frame of the Magnetar theory (Thompson & Duncan 1993): neutron stars with extreme magnetic fields. For high B, a yet-to-be-confirmed QED effect (vacuum birefringence, Heisenberg & Euler 1936) becomes important, significantly changing the dependence on the phase and the energy of the polarization, providing a test of the magnetar paradigm and a probe of strong-field QED van Adelsberg & Lai 2006

30. Vacuum birefringence in strong B Soft Gamma Repeaters and Anomalous X-ray Pulsars are interpreted in the frame of the Magnetar theory (Thompson & Duncan 1993): neutron stars with extreme magnetic fields. For high B, a yet-to-be-confirmed QED effect (vacuum birefringence, Heisenberg & Euler 1936) becomes important, significantly changing the dependence on the phase and the energy of the polarization, providing a test of the magnetar paradigm and a probe of strong-field QED van Adelsberg & Lai 2006

31. Vacuum birefringence in strong B Soft Gamma Repeaters and Anomalous X-ray Pulsars are interpreted in the frame of the Magnetar theory (Thompson & Duncan 1993): neutron stars with extreme magnetic fields. For high B, a yet-to-be-confirmed QED effect (vacuum birefringence, Heisenberg & Euler 1936) becomes important, significantly changing the dependence on the phase and the energy of the polarization, providing a test of the magnetar paradigm and a probe of strong-field QED van Adelsberg & Lai 2006

32. Vacuum birefringence in strong B Soft Gamma Repeaters and Anomalous X-ray Pulsars are interpreted in the frame of the Magnetar theory (Thompson & Duncan 1993): neutron stars with extreme magnetic fields. For high B, a yet-to-be-confirmed QED effect (vacuum birefringence, Heisenberg & Euler 1936) becomes important, significantly changing the dependence on the phase and the energy of the polarization, providing a test of the magnetar paradigm and a probe of strong-field QED Proton cyclotron E=0.63B14 keV Magnetars are week. Large area would help. Lai et al. 2009

33. SgrB2 is a giantmolecularcloud at 100pc projecteddistancefromSgrA An extreme case of reflection? The spectrumof SgrB2 is pure reflection spectrum Reflectionofwhat? No brightenough source isthere The emissionfrom SgrB2 isextended and brighter in the direction ofSgrA,Murakami 2001 Rashid Sunyaev suggested that SgrB2 is reflecting the emission from the Black Hole in SgrA as it was a few hundred years ago. Integral Image of GC, Revnivtsev 2004

34. X-ray polarimetry can definitively proof or reject the hypothesis that SgrB2 is reflecting today the X-rays generated from the galactic center in the past: SgrB2 should be highly polarized with the electric vector perpendicular to the line connecting the two sources WasourGalaxy Center an AGN a fewhundredsyears ago? (Churazov 2002) From the polarization degree it is possible to derive the correct distance with respect to the GC and therefore the time when SgrA* was active. Angular constraints on the source illuminating SgB2 and Sgr C Imaging needed! NHXM MEP T= 500 ks;

35. Polarization of accretion disks In general polarimetry gives the orientationofanaccretion disk on thsky The first complete theory of polarization from an accretion disk as a function of optical thickness and view angle was given by Sunyaev & Titarchuk, 1985

36. Testing GR in strong field: bending of light in a galactic Black-Hole The Polarization angle from an accretion disk in the ‘Newtonian’ case is either parallel to the major axis of the sky-projected disk (positive) or parallel to the sky-projected disk symmetry axis (negative) A,B,C,D, refers to different viscosity regimes and soft-X-ray photons which are the seeds (Lightman & Shapiro, 1976). But if the field is strong enough the polarization properties altered by gravitational effects. The polarization plane rotates continuously with energy because of General Relativistic effects. This is a signature of the presence of a black-hole (Stark& Connors, Connors& Stark, 1977, Connors, Piran & Stark, 1980). This effect is definitely measurable. are

37. A modern approach to the same problem Two days observation with NHXM (not much different for POLARIX or HXMT). If the Dovciak, Muleri, Karas, Goosman and Matt model (MNRAS 2008) is correct the observational consequences are impressive. Notice that similar results have been found by Schnittman and Krolik (ApJ 2009) and by Li (ApJ 2005). Of course GRS1915+105 is a very favourable case because of high inclination of the disk to the observer. But it is known (from radio jets) and this is an excellent constraint. Likely GEMS will be the first to perform this measurement.

38. Different, independent methods (line profile, continuum shape, rotation of the polarization angle) can be applied simultaneously to estimate the black hole spin (“concordance model”). A broad band measurement and simultaneous spectroscopy and polarimetry (such as NHXM). Stronggravity and black hole spin a=0 a=1 Fabian et al. (2000) Dovciak et al. (2008) a=0.,0.5, 0.9, 0.999 Li et al. (2005)

39. Could we afford very performing imaging (IXO or better): Polarimetry of extended Jets in AGNs and Glactic BHs Western jet of XTE J1550-564 and the PSF of XPOL. MDP is 4.4 % with 1 Ms Jet of M87 and the knot A with the PSF of XPOL. MDP is 6 % of 200 ks The X-ray polarization measurements can extend the synchrotron emission in jets also at X-rays- At the knots of M87 the optical polarization has a minimum may be because of shocks waves that enhance X-rays but randomize the magnetic fields. X-ray polarimetry can proof it also at X-rays. But a large area (>1 m2) and high angular resolution (<15”) is required: IXO or possibly better as XEUS was supposed to be.

40. Can polarimetry test Quantum Gravity theories? Quantum Gravity should be effective on the Planck Energy scale (EQG=1019 GeV). One of the major approach to quantization of Gravity is the Loop QG that predicts birefringence effects. The result is a difference of light velocity for the two states of circular polarization: V+=χ[1+c(E/EQG)n] V-=χ[1-c(E/EQG)n] Highly polarized, distant sources may be used to test this predictions. UV measurements and the Crab nebula detection already give χ<10-4. Much tighter constraints are required before ruling out this solution.

41. Which source for QG test? GRB afterglows are likely the best candidate because can be studied up to Z=4. But we do not really know how polarized they are, and the polarization could be energy dependent. Moreover they require a dedicated strategy of satellite operations Blazars are likely a good alternative. Even though those bright enough to beprofitably used are closer than GRBs (Z<1), they should be highly polarized and the dependence on energy should be moderate or absent in the X-Rays. Poutanen, 1994

42. A soft revolution: The modern scattering polarimeters The polarimetry based on Compton scattering is the oldest one. It resulted much less effective than Bragg polarimetry. But the present perspectives are good. It works as well aboard balloons and therefore things can move independently from the long term planning of satellite missions. Scattering polarimetry based on a passive scattering material (as foreseen in SXRP and implemented in Rhessi) is nowadays less attractive because of the high background. A low energy one is under study in India (Paul 2010). The modern techniques of light read-out such as pixelled photomultipliers, avalanche photodiodes, Silicon photomultipliers and the availability of fast inorganic scintillators (such as LaBr3) or high Z semiconductors affords for finely subdivided instruments. Active scattering polarimeters where both the scatterer and the absorber are detectors in coincidence are already operative or planned. The scattering polarimeters already existing or planned are (McConnel 2010).

43. X-calibur: focal plane polarimeter with a plastic scintillator scatterer and CZT absorber in the focus of a multilayer optics Some experiments POLAR. Plastc scintillator as both scatterer and absorber GRAPE. Scatterers of plastic scintillator and absorbers of CsI. GRAPE has observed the Crab in last sammer. We are waiting for the results. GAP aboard IKAROS satellit. A cylinder of plastic Scintillator surrounded by CsI.

44. A further focal plane polarimeter for even higher energy based on Compton scattering has been investigated. A photon Compton scatters by a low-Z scintillator and it is absorbed by high-Z detector. Our concept: a high energy polarimeter originally for NHXM Simulated modulation curve for 10 cm length BC404 as scatterer (5 mm diameter) and LaBr3 as the absorber at 5 cm distance at 35 keV. Efficiency of LEP, MEP and HEP. LEP efficiency arrive at 10 keV. It is smaller than MEP efficiency that arrives at 35 keV compensating the decreasing mirror efficiency to arrive at a similar sensitivity. The HEP efficiency covers the rest of the energy band where the multilayer optics are effective. (Soffitta SPIE 2010) Based on simulation MDP 7.2 % for 10 mCrab in 105 s (20-80 keV)

45. A new perspective: Wide field polarimetry Polarimeters have been developed mainly as pointed instruments in combination with a collimator or in the focus of a telescope. Both photoelectric polarimeters and compton polarimeters can be used to see a wide field of view. The sensitivity is lower because of the huge diffuse background embarked. Moreover the control of systematics is a hard task. An umpolarized beam impinging off-axis will produce a modulated signal. This modulation must be accounted to disentingle the modulation deriving from the polarization. But the ambition to detect polarization of Gamma-Ray Bursts makes this effort worthwile. The actual sensitivity will depend on the hability to perform this correction by software tricks, by accurate calibration, starting from a good design.

46. GAP: GRB100826A a first result? This could be the first detection of polarization of a GRB since the INTEGRAL detection are much less controlled

47. What from polarimetry of GRBs? 14 years after the discovery of the afterglow we understand many things about the afterglow and many less about the prompt burst. Polarimetry of the optical afterglow is typically very low of the order of 1-2%. The only case of possible high polarization GRB090102 ~10%(Steele 2009) is for a very quick follow up and could be more connected to the prompt than to the afterglow. Some models predict a high polarization only around the so called break, a change of slope possibly connected to the slowing of the fire-ball prducing the effect that the cone of relativistic collimation arrives to include the whole jet. If the emission is synchrotron the detection of X-ray polarization, that can be expected to be of the same order, is hopeless, especially if we want to have a time resolved measurement. It would require m2 area and very fast repointing. Much ado about nothing.

48. The prompt Polarimetry of the prompt GRB is more promising and is probably the major improvement we can expect to understand the physics of the process and, in particular, the structure of the magnetic field.It should be combined with low energy sensitivity. From the discussion in Lazzati (2010) we derive the following table. In general we should think to large areas in order to be capable to perform pulse resolved polarimetry.

49. Conclusion GEMS will allow for polarimetry of some representative objects, mainly galactic. Future experiments should foresee extended performances, especially the higher energies and imaging. Polarimeters and spectrometers abord the same satellite could result very effective. END