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Enrico Costa IASF-Rome/INAF

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1 Enrico Costa IASF-Rome/INAF
Instruments and Methods of Astronomical X-ray polarimetry polarimetry Enrico Costa IASF-Rome/INAF COST ACTION MP1104 – Meeting WG-3 Warsaw May

2 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

3 What can Polarimetry Test?
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 variety of situations and classes of sources: jets, accretion disks and columns, reflection, archeoastronomy, etc. Fundamental Physics: Matter in extreme magnetic fields Matter in strong gravity fields Axions Quantum gravity effects

4 Big Hopes Meager Results
A vast theoretical literature predicts a wealth of results from X-ray Polarimetry 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! Conventional X-ray polarimeters are cumbersome and have low sensitivity New technical solutions are arriving The same year of the pessimistic statement by Martin Rees a new instrument was developed with the potentiality of a dramatic improvement in both sensitivity and control of systematics. A new Era for X-ray Polarimetry is about to come (maybe….)

6 The conventional formalism

7 Or 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.

8 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 > 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

9 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.

10 Flown Polarimeters Rocket, 1972 OSO-8, Ariel 5,

11 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)  = Projected crystal Area = 2 x 140 cm2 ; Detector area = 2 x 5 cm2 ; FOV= 2o B = 2 x 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

12 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.

13 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

14 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.

15 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 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. 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.

16 The principle of detection
X-ray polarimetry with a Gas Pixel Detector 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 GEM electric field 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. pixel GEM 20 ns a E X photon (E) PCB conversion gain collection 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

17 The photoelectrons create tracks in the gas
Generation (photoelectron + Auger) Propagation (SS_MOTT) Creation and diffusion of primary ionization (Maxwell, Garfield, Magboltz) Gas multiplication Digitization Pixel Representation

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

19 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) He-DME 1 bar, 1cm. 2) pure DME 0.8 bar, 1 cm. 3) Ar DME atm 2 cm. DME = (CH3)2O 60 µm/√cm diffusion

20 Not only MonteCarlo: Our predictions are based on data
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. Not only MonteCarlo: Our predictions are based on data 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

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

22 X-ray polarimetry with a micropattern Time Projection Chamber
High efficiency Not an imager Black 2007 Blachamberk et al., 200 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.

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 m. The payload consisted initially of three TPC polarimeters now reduced to two for budget and schedule reasons. (Swank 2010, Yahoda 2010).

24 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).

25 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 s (Jahoda et al 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.

26 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

27 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 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 % in 105 s for 1 mCrab source (2-10 keV) 3.1 % in 105 s for 10 mCrab source (2-10 keV) MEP % 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.

28 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.

29 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

30 A past future: the Focal Plane of XEUS
CenA % NGC % NGC % MCG % Circinus Galaxy 2.8 % IC429A % Fairall % Mk501 (outburst) 0.5 % MK % 3C % A telescope of 5 m2. The baseline includes XPOL a polarimeter in the focus

31 MDP for AGN (105 s) with a Conservative ME Configuration
CenA % NGC % NGC % MCG % Circinus Galaxy % IC429A % Fairall % Mk501 (outburst) 0.5 % MK % 3C % Enrico Costa - COSTA_Varsaw_120507_XRay

32 Why imaging?: e.g. the Crab
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 XEUSp.s.f. But this is only the average measurement The structure is much more complex! To perform separate polarimetry of details of the major structures we need imaging! f.o.v. PSR NW jet SE jet Inner torus Outer torus 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

33 Could we afford very performing imaging (IXO or better):
Polarimetry of extended Jets in AGNs and Glactic BHs Western jet of XTE J 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.

34 Eventually JUICE was selected and ATHENA was dropped as a whole.
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. Eventually JUICE was selected and ATHENA was dropped as a whole. Enrico Costa - COSTA_Varsaw_120507_XRay

35 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).

36 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.

37 Our concept: a high energy polarimeter originally for NHXM
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. 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)

38 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 Filters The 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.

39 Something we need Instrumentation:
More support to R&D (SiPMTs, GPD, Filters, ..) Theory Celestial calibration sources an in particular: We need a bright unpolarized source! POLITICS ESA 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.

40 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.

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

42 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 GRB ~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.

43 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.

44 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

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