Paolo Soffitta IASF-Rome/INAF

1. X-Ray Polarimetry Paolo SoffittaIASF-Rome/INAF

2. In polarimetry 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 Source detection > 10 photons Source spectral slope > 100 photons Source polarization > 100.000 photons

3. 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.

4. Flown Bragg Crystal Polarimeters Rocket, 1971 OSO-8, 1975-1978 Ariel 5, 1974-1975

5. 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

6. 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.

7. Thomson Polarimeters • They were the first experiment to be flown on-rockets • Rocket flight on April 1969 to search for polarization n the Crab Nebula (Wolff, 1970). • Rocket flight on July 1969 to search for polarization in Sco X-1 (Angel et al., 1969). • Rocket flight on 1971 (larger version) in combination with a Bragg polarimeter (Novick et al. 1972) Each Lithium block and detector was : 5 cm x 5 cm and 12.7 cm height surrounded by proportional counters. Only upper limits: P < 27 % at 99 % confidence (Wolff et al., 1970) on Crab. Loss of telemetry reduced the significance of the ’71 flight-data.

8. 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

9. 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 tempted very long ago, but only since five-ten years 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. 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.

10. Hard X-ray photo imaging The photoelectric effect can be exploited by imaging the track produced by a photoelectron in gas Back in 1994 the optical image produced during multiplication in gas of a photoelectron was collected by a CCD at hard X-ray (54 keV). Austin et al., SPIE, 1994

11. 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

12. ASIC features 105600 pixels 50 μm pitch • Peaking time: 3-10 ms, externally adjustable; • Full-scale linear range: 30000 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%) The chip is self-triggered and low noise. It is not necessary to readout the entire chip since it is capable to define the sub-frame that surround the track. The dead time downloading an average of 1000 pixels is 100 time lower with respect to a download of 105 pixel.

13. 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

14. IASF-Rome facility for the production of polarized X-rays Close-up view of the polarizer and the Gas Pixel Detector Facility at IASF-Rome/INAF keV Crystal Line Bragg angle 1.65 ADP(101) CONT 45.0 2.01 PET(002) CONT 45.0 2.29 Rh(001) Mo Lα 45.3 2.61 Graphite CONT 45.0 3.7 Al(111) Ca Kα 45.9 4.5 CaF2(220) Ti Kα 45.4 5.9 LiF(002) 55Fe 47.6 8.05 Ge(333) Cu Kα45.0 9.7 FLi(420) Au Lα45.1 17.4 Fli(800) Mo Kα44.8 Aluminum and Graphite crystals. Capillary plate (3 cm diameter) Spectrum of the orders of diffraction from the Ti X-ray tube and a PET crystal acquired with a Si-PiN detector by Amptek PET (Muleri et al., SPIE, 2008)

15. 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

16. More energies, more mixtures Pure DME (CH3)2O Modulation curve at 2.0 keV μ = 13.5% We performed measurement at more different energies and gas mixtures. (Muleri et al., 2010).

17. 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.

18. Two approaches The photons enter parallel with respect to the readout plane. The photons enters perpendicularly with respect to the readout plane.

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 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 (Yahoda 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. GEM electric field X photon (E) conversion GEM gain collection pixel PCB E a 20 ns X-ray polarimetry with a Gas Pixel Detector To efficiently image the track IASF-Rome and INFN-Pisa developed the Gas Pixel detector since 2001. The gas allows for having tracks enough long to be imaged. 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. Polarization information is derived from the angular distribution of the tracks produced by the photoelectrons, imaged by a finely subdivided gas detector. Costa et al., 2001

23. 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

24. 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.

25. 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.

26. What can be measured by imaging polarimetry ?. What can be explored at higher energies ?.

27. Spectro-imaging polarimetry of PWNe Imaging polarimetry is fundamental to probe the magnetic field topology. Since X-ray emitting electrons have short synchrotron lifetimes, X-rays provide a much cleaner view of the inner regions, limiting the risk of superposition effects along the line of sight. The Crab Nebula is the only source in which the polarization degree has been measured so far in X-rays (P=19%, Weisskopf et al. 1978, ApJ 220, L117) confirming the synchrotron nature of its emission. NHXM LEP position resolution Spacially resolved X-ray polarimetry can map the magnetic fields in the pulsar wind nebulae like the Crab helping to verify models of generation of its different features .

28. NHXM: Cyclotron lines with 100 ks of observation with MEP

29. 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 Was the GC 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 NHXM MEP T= 500 ks;

30. STRONG GRAVITY in AGNs : Matter and radiation close to the black-hole experience General and Special Relativity Effects called ‘Strong Gravity’ that in AGNs may also manifest through time dependent polarization variability due to time dependent reflection of the primary emission from the accretion disk. The time dependent polarization variability depends on the observed intensity and on the spin of the black-hole. In case of MCG-6-30-15 the characteristic variability of the Iron line and of the continuum (Miniutti & Fabian, 2004) suggested that the source of primary emission originates in a small region on the black-hole spinning axis. The observed variability is due to a variation of the height of the source with a variation of the gravitational effects. Since the height controls the direct emission at infinite, the reflected fraction from the disk and the incidence angle, the polarization vary with the source height (and therefore with the observed intensity) in a way that depends on the spin of the black-hole and on the inclination. Being the expected polarization larger at larger energy (for the larger albedo Compton, the absence of Fe line and the smaller contribution of the direct emission) it can be studied with the MEP above 8 keV to verify the model. Simulated polarimetry of MCG 6-30-15, (500 ks) with MEP in NHXM 20-50 keV 10-20 keV 6-10 keV 2-10 keV Polarization degree at infinite as a function of the source height (Dovĉiak, et al., 2011)

31. IXO: 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 (Pinchera et al., in preparation) 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.

32. 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. 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)

33. For a non focal plane experiment to arrive to a large area for sensitive polarimetry with also energy resolution by using Compton scattering, a subdivided and two phase polarimeter [scatterer (low-Z) and absorber (high-Z)] must be devised. • Single PM tube for each 5 cm x 5 cm x 4 cm NA102 scatterer and 7 cm x 1 cm x 9 cm NaI(Tl) absorber •  25 %, Modular implementation Phoswich-type collimator. Field of View of 14.5o MDP = 7 % in 5.5 hours in 30-700 keV for Crab Gunji et al., 1994 • The apparatus is divided in an array of small detecting units in form of a fiber-like scintillators. • A photon scattered by a low Z-fiber-like scintillator pass across a number of similar fibers until is absorbed by a high-Z fiber. A good sensitiviity can be reached. • Far-away fibers are fed to a single channel of a multi-wire PM tube to detect coincidence events. •  = 50% .MDP = 1.6 % in 20-200 keV in 5.5 hours for Crab A = 1000cm2. • MDP sensitive the low energy threshold. Single cell: 37-plastic scintillator fibers (2mm large) pins, 5 cm thick, 24-semi-exagonal sticks (CsI or the faster YAP). The system is made by replicas of such cells. (Costa et al., 1993, 1995)

34. END