Enrico Costa IASF-Rome/INAF

Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3

Enrico CostaIASF-Rome/INAF

Instruments and Methods of Astronomical X-ray polarimetry

polarimetry

COST ACTION MP1104 – Meeting WG-3

Warsaw May 7-9 2012


Measurements in X-ray Astronomy

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, Astro-H, ???.

Polarimetry: (Bragg, Thomson/Compton, in the future photoelectric and subdivided compton) Rockets, Ariel-5, OSO-8


What can Polarimetry Test?

Astrophysics: Astrophysics:

• Non thermal emission processes producing Non thermal emission processes producing intrinsically polarized photons.intrinsically polarized photons.

• Deviation from spherical geometry of the Deviation from spherical geometry of the matter close to the emitting regions matter close to the emitting regions polarizing by transfer in a varietypolarizing by transfer in a variety of of situations and classes of sources: jets, situations and classes of sources: jets, accretion disks and columns, reflection, accretion disks and columns, reflection, archeoastronomy, etc.archeoastronomy, etc.

Fundamental Physics:Fundamental Physics:

• Matter in extreme magnetic fieldsMatter in extreme magnetic fields

• Matter in strong gravity fieldsMatter in strong gravity fields

• AxionsAxions

• Quantum gravity effectsQuantum gravity effects


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).

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).

In 40 years only one positive detection of X-ray Polarization: the Crab 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) (Novick et al. 1972, Weisskopf et al.1976, Weisskopf et al. 1978) P = 19.2 P = 19.2 ± 1.0 ± 1.0 %; %; = 156.4 = 156.4oo ± 1.4± 1.4o

A vast theoretical literature predicts a wealth of results A vast theoretical literature predicts a wealth of results from X-ray Polarimetryfrom X-ray Polarimetry


A window not yet disclosed

THE TECHNIQUES ARE THE LIMIT!THE TECHNIQUES ARE THE LIMIT!

Conventional X-ray polarimeters are cumbersome and Conventional X-ray polarimeters are cumbersome and have low sensitivityhave low sensitivity

New technical solutions are arrivingNew technical solutions are arriving

The same year of the pessimistic statement by Martin The same year of the pessimistic statement by Martin Rees a new instrument was developed with the Rees a new instrument was developed with the potentiality of a dramatic improvement in both potentiality of a dramatic improvement in both sensitivity and control of systematics.sensitivity and control of systematics.

A new Era for X-ray Polarimetry is about to come A new Era for X-ray Polarimetry is about to come (maybe….)(maybe….)


The conventional formalism


Or

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But in a paper of (experimental) X-ray polarimetry it is rare to find Stokes parameters.

The point is that circular polarization in practice is not measurable. Actually with strong permanent magnets the absorption coefficients for the two sates of circular polarization is different in a thin (20-30 eV) range. This is compatible with X-ray microcalorimeters resolution but the sensitivity would be poor and the set-up extremely complex, since the readout of a microcalorimeter is a squid.


The first limit: In polarimetry the sensitivity is a matter of photons

Source detection > 10 photonsSource spectral slope > 100 photonsSource polarization > 100.000 photons

MDP is the Minimum Detectable Polarization

RS is the Source rate, RB is the Background rate, T is the observing timeμ is the modulation factor: the modulation of the response of the polarimeter to a 100% polarized beam

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


Bragg diffraction

Bragg diffraction from a crystal can be exploited to measure the degree and the angle of

polarization P of a photon beam.

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45sin2 1

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.

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.

Bragg law.

θθ


Flown Polarimeters

Rocket, 1972

Ariel 5, 1974-1975

OSO-8, 1975-1978


OSO-8 satellite (top) and polarimeter (bottom)

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 with a dedicated Bragg polarimeter

67 % and 99 % confidence contour. The radial scale is the polarization in percent


φ

At 90o of angle of scattering (θ) the modulation factor is 100 % since there are not photons diffused along the electric field.

X-ray polarimetry with Thomson scattering

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.


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 Kaaret et al., SPIE 1989, Soffitta et al., NIM A, 1998

T=105 s.


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.


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Heitler W.,The Quantum Theory of Radiation

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 incident photon direction and at an azimuthal angle φ with respect to the electric vector.

If the ejected electron is in ‘s’ state (as for the K–shell) the differential cross section depends on cos2 (φ), therefore it is preferentially emitted in the direction of the electric field. It is an ideal analyzer of the polarization.

Being the cross section always null for φ = 90o the modulation factor µ equals 1 for any polar angle.

ββ =v/c =v/c

By measuring the angular distribution of the ejected photelectrons (the modulation curve) it is possible to derive the X-ray polarization.

Costa, Nature, 2001


GEM electric field

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.

pixel

GEM

20 ns

a E

X photon (E)

PCB

conversion gain

collection

The principle of detection

X-ray polarimetry with a Gas Pixel Detector

Costa et al., 2001

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.

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.

Costa et al., 2001, Bellazzini et al.2006, 2007


• Generation (photoelectron + Auger)

• Propagation (SS_MOTT)

• Creation and diffusion of primary ionization (Maxwell, Garfield, Magboltz)

• Gas multiplication

• Digitization

• Pixel Representation

The photoelectrons create tracks in the gas

Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3Enrico Costa-IASF-Roma INAF

• Peaking time: 3-10 Peaking time: 3-10 s, externally adjustable;s, externally adjustable;• Full-scale linear range: 30000 electrons;Full-scale linear range: 30000 electrons;• Pixel noise: 50 electrons ENC;Pixel noise: 50 electrons ENC;• Read-out mode: asynchronous or Read-out mode: asynchronous or synchronous;synchronous;• Trigger mode: internal, external or self-Trigger mode: internal, external or self-trigger;trigger;• Read-out clock: up to 10MHz;Read-out clock: up to 10MHz;• Self-trigger threshold: 2200 electrons (10% Self-trigger threshold: 2200 electrons (10% FS);FS);• Frame rate: up to 10 kHz in self-trigger Frame rate: up to 10 kHz in self-trigger modemode (event window);(event window);• Parallel analog output buffers: 1, 8 or 16;Parallel analog output buffers: 1, 8 or 16;• Access to pixel content: direct (single pixel) Access to pixel content: direct (single pixel) or serial or serial (8-16 clusters, full matrix, region of interest);(8-16 clusters, full matrix, region of interest);• Fill fraction (ratio of metal area to active Fill fraction (ratio of metal area to active area): 92%area): 92%))

ASIC features


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.

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.

DME = (CH3)2O

60 µm/√cm diffusion


Not only MonteCarlo: Our predictions Not only MonteCarlo: Our predictions are based on dataare based on data

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.

Each photon produces a track. From the track the impact point and the emission angle of the photoelectron is derived. The distribution of the emission angle is the modulation curve.

By rotating the polarization vector the capability to measure the polarization angle is shown by the shift of the modulation curve.

Muleri et al. 2007

Soffitta et al., 2010

Present level of absence of systematic effects (5.9 keV).Bellazzini 2010

Impact point


Two approaches

The photons enters perpendicularly with respect to the readout plane.

The photons enter parallel with respect to the readout plane.

Drift direction

z


X-ray polarimetry with a micropattern Time Projection

ChamberHigh efficiency Not an imager

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.

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.

Black 2007


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).


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).

Engineering Model vibrated.

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).

The track image can be distorted because the procedure to measure the two projections of the track is different (time and space).


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.


Costa et al., ExpAst 2010

POLARIX

NHXM

Tagliaferri et al, ExpAst 2010

IXO

Bookbinder, SPIE, 2010

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.


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)

Costa, et al., Exp Ast 2010- 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 s for 10 mCrab source (2-10 keV)

MEP 13 % 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)

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.


A detector more tuned on hard X-rays for NHXMThe 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.


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


A past future: the Focal Plane of

XEUS

A telescope of 5 mA telescope of 5 m22. The baseline includes XPOL a polarimeter in the . The baseline includes XPOL a polarimeter in the focusfocus

CenA 0.6 %

NGC4151 0.7 %

NGC5548 0.8 %

MCG6-30-15 1.2 %

Circinus Galaxy 2.8 %

IC429A 0.7 %

Fairall 9 1.6 %

Mk501 (outburst) 0.5 %

MK421 0.7 %

3C273 0.9 %


CenA 0.6 %

NGC4151 0.7 %

NGC5548 0.8 %

MCG6-30-15 1.2 %

Circinus Galaxy 2.8 %

IC429A 0.7 %

Fairall 9 1.6 %

Mk501 (outburst) 0.5 %

MK421 0.7 %

3C273 0.9 %

MDP for AGN (105 s) with a Conservative ME Configuration

Enrico Costa - COSTA_Varsaw_120507_XRay


Why imaging?: e.g. the CrabPositive measurement: of X-ray Positive measurement: of X-ray polarization of the Crab Nebula polarization of the Crab Nebula without pulsar contamination without pulsar contamination (by lunar occultation, Weisskopf (by lunar occultation, Weisskopf et al., 1978).et al., 1978).

P = 19.2 P = 19.2 ± 1.0 ± 1.0 % %

= 156.4= 156.4oo ± 1.4± 1.4o

But this is only the average But this is only the average measurement The structure is much measurement The structure is much more complex! To perform separate more complex! To perform separate polarimetry of details of the major polarimetry of details of the major structures we need imaging!structures we need imaging!

PSRPSR

NW jetNW jet

SE jetSE jet

Inner torusInner torus

Outer torusOuter torus

f.o.v.f.o.v.

XEUSpXEUSp.s.f..s.f.

How turbulent is the field? How polarized is the PSR?How turbulent is the field? How polarized is 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


Jet of M87 and the knot A with the PSF of XPOL. MDP is 6 % of 200 ks

Western jet of XTE J1550-564 and the PSF of XPOL. MDP is 4.4 % with 1 Ms

Could we afford very performing imaging (IXO or better):

Polarimetry of extended Jets in AGNs and Glactic BHs

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.


FROM IXO TO ATHENA

IXO had a single telescope with 4 instruments on a sliding device. ATHENA has 2 telescopes with 2 fixed instruments. The polarimeter was dropped.

Enrico Costa - COSTA_Varsaw_120507_XRay

Eventually JUICE was selected and ATHENA was dropped as a whole.


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).


Some experiments X-calibur: focal plane polarimeter with a plastic scintillator scatterer and CZT absorber in the focus of a multilayer optics

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.


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.

Based on simulationMDP 7.2 % for 10 mCrab in 105 s (20-80 keV)

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.

(Soffitta SPIE 2010)


Are other techniques ruled out?Diffraction

has still a role. Especially at lower energies, where neither photoelecric or scattering can work. Artificial crystals with graded spacing have been proposed. Herman Marshall (MIT) has proposed such a polarimeter that could be very interesting for pulsars <1keV

FiltersThe natural filters for X-rays are bragg crystals. The imits (narrow band and need for rotation of both thediffractor and the detector) have been already shown. Other filters have been of no use soo far. But Nigel Bunnister from George Fraser group at Leicester University has proposed (Exp.Astronomy 2006) filters based on dichroic properties of plate-like crystals grown from materials including large molecules and Br. The filter is effective in a narrow band (10eV) around the edge K of Br (13 keV) The hope is to extend the technique don to 2.7 keV and possibly to .8. In terms of sensitivity is not better than bragg but it could be mounted in front of a microcalorimeter and could do polarimetry almost for «free» (rotation of the filter and not of the whole detector.


Something we need

Instrumentation:More support to R&D (SiPMTs, GPD, Filters, ..)

TheoryCelestial calibration sources an in particular: We need a bright unpolarized source!

POLITICSESA after L1 selection (Juice) has decideded to follow the recommendation to continue the technical developments in view of a future X-ray observatory. ESA should support not only the optics the spectroscopy and imaging but also polarimetry.


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.


GAP: GRB100826A a first result?

This could be the first detection of polarization of a GRB since the INTEGRAL detection are much less controlled


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.


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.


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

Enrico Costa IASF-Rome/INAF

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