1. A small mission featuring an imaging x-ray polarimeter with high sensitivity Martin C. Weisskopf a, Luca Baldini b, Ronaldo Bellazzini b, Alessandro Brez b, Enrico Costa c, Richard Dissly d, Ronald F. Elsner a, Sergio Fabiani c, Giorgio Matt e, Massimo Minuti b, Fabio Muleri c, Stephen L. O’Dell a, Michele Pinchera b, Brian Ramsey a, Alda Rubini c, Carmelo Sgro’ b, Paolo Soffitta c, Gloria Spandre b a NASA Marshall Space Flight Center, Huntsville, AL 35805-1912, USA; b INFN-Pisa, Pisa, Italy; c IAPS/INAF, Rome, Italy; d Ball Aerospace &Technologies Corp., United States; e Dipartimento di Fisica ”E. Amaldi” Universita’ degli Studi Roma Tre, Rome, Italy; 2013/08/25 SPIE Optics + Photonics, San Diego 25-29 August 2013 1 Speaker: Paolo Soffitta IAPS/INAF

2. The situation of X-ray polarimetry today SPIE Optics + Photonics, San Diego 25-29 August 2013 22013/08/25 Worse than ever Not only paucity of available data with respect to imaging/timing and spectroscopy but also : GEMS was discontinued for programmatic reason. IXO descooped in Athena and then Athena+ without a polarimeter in the focal plane

3. 2013/08/25 SPIE Optics + Photonics, San Diego 25-29 August 2013 3 Do not forget that many years ago ….. SXRP the Stellar X-Ray Polarimeter was part of the Spectrum-X-Gamma mission to be flown in a reasonably short time and While waiting for the results of SRG, any possible mission of polarimetry was hold in stand-by. Kaaret et al., 1989, Tomsick et al., 1997, Soffitta et al., 1998

4. 2013/08/25 SPIE Optics + Photonics, San Diego 25-29 August 2013 4 Thanks to the approval of GEMS and XPOL on IXO, the expectations for new data, have stimulated the theoretical researches on polarimetry. This came out as powerful diagnostic of physics of the inner regions of compact objects and of extreme gravitational and magnetic fields. A number of topics has been also dig on polarimetry as diagnostics of fundamental physics : Strong gravity in galactic black hole & AGN. QED in superstrong B field of magnetars. Quantum Gravity effects. Axion-like-particle search.

5. 1. Is the measurement really feasible at the declared level of sensitivity ? 2. Is the measurement credible ? 3. Is the measurement meaningful ? Question 1) and 2) are mainly related to the systematic effects. Any removal of systematic is not for free in terms of statistics. Background can be an issue for polarimetry of faint sources. 2013/08/25 SPIE Optics + Photonics, San Diego 25-29 August 2013 5 Some doubts always make expectations from X-ray polarimetry less compelling: Bragg diffraction at 45 o around the arrival direction is not very sensitive but has small systematic effects (TBC).

6. Here we present a mission concept based on imaging X-ray polarimetry that answer to these doubts 2013/08/25 SPIE Optics + Photonics, San Diego 25-29 August 2013 6 X-ray optics with 30’’ of HEW with a high effective area in the 2-6 keV energy band. A Gas Pixel Detector filled with He-DME based mixture Imaging possible Low Background Small systematics

7. Three light telescopes 2013/08/25 SPIE Optics + Photonics, San Diego 25-29 August 2013 7

8. The detector Gas Pixel Detector Polarimetry - Imaging - Timing - Spectroscopy 2013/08/25 SPIE Optics + Photonics, San Diego 25-29 August 2013 8

9. 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 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 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 cos 2 (φ), therefore it is preferentially emitted in the direction of the electric field. Being the cross section always null for φ = 90 o the modulation factor µ equals 1 for any polar angle. β =v/c By measuring the angular distribution of the ejected photoelectrons (the modulation curve) it is possible to derive the X-ray polarization. Costa, Nature, 2001 2013/08/25 SPIE Optics + Photonics, San Diego 25-29 August 2013 9

10. GEM electric field Polarization information is derived from the angular distribution of the emission direction of the tracks produced by the photoelectrons. 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 IAPS- 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 2013/08/25 SPIE Optics + Photonics, San Diego 25-29 August 2013 10

11. ASIC features 105600 pixels 50 μm pitch Peaking time: 3-10  s, externally adjustable; Peaking time: 3-10  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 synchronous; Read-out mode: asynchronous or synchronous; Trigger mode: internal, external or self-trigger; Trigger mode: internal, external or self-trigger; Read-out clock: up to 10MHz; Read-out clock: up to 10MHz; Self-trigger threshold: 2200 electrons (10% FS); Self-trigger threshold: 2200 electrons (10% FS); Frame rate: up to 10 kHz in self-trigger mode Frame rate: up to 10 kHz in self-trigger mode (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) or serial Access to pixel content: direct (single pixel) 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 area): 92%) 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 10 5 pixel. 2013/08/25 SPIE Optics + Photonics, San Diego 25-29 August 2013 11

12. SPIE Optics + Photonics, San Diego 25-29 August 2013 1) The track is recorded by the PIXel Imager 2) Baricenter evaluation 3) Reconstruction of the principal axis of the track: maximization of the second moment of charge distribution 4) Reconstruction of the conversion point: major second moment (track length) + third moment along the principal axis (asymmetry of charge release) 5) Reconstruction of emission direction: pixels are weighted according to the distance from conversion point. Tracksreconstruction Tracks reconstruction 2013/08/2512

13. A new prototype with an extended GEM for better drift field uniformity Mixture fillingHe 20% + DME 80% 1 bar Gas cell thickness 1 cm GEM50 um pitch, 50 um thick, 88 x 88 mm OLD NEW SPIE Optics + Photonics, San Diego 25-29 August 2013 Same window, same ASIC but a much larger GEM, with the addition of a large Guard Ring and field forming frames. 2013/08/2513

14. The calibration facility keV Crystal Line Bragg angle 1.65 ADP(101) CONT 45.0 2.01 PET(002) CONT 45.0 2.29 Rh(001) Mo L α 45.3 2.61 Graphite CONT 45.0 3.7 Al(111) Ca K α 45.9 4.5 CaF 2 (220) Ti K α 45.4 5.9 LiF(002) 55 Fe 47.6 8.05 Ge(333) Cu K α 45.0 9.7 FLi(420) Au L α 45.1 17.4 Fli(800) Mo K α 44.8 Facility at IASF-Rome/INAF Close-up view of the polarizer and the Gas Pixel Detector Capillary plate (3 cm diameter) Aluminum and Graphite crystals. Spectrum of the orders of diffraction from the Ti X-ray tube and a PET crystal acquired with a Si-PiN detector by Amptek (Muleri et al., SPIE, 2008) PET 2013/08/25 SPIE Optics + Photonics, San Diego 25-29 August 2013 14

15. An X-ray polarimeter with spectroscopic capabilities Pure DME (CH 3 ) 2 O μ = 13.5% Modulation curve at 2.0 keV (Muleri et al., 2010).

16. The imaging properties of the GPD. IAPS/INAF laboratory : Very narrow pencil beam. Detector shifts : 300  m Location accuracy : 30  m (rms) Half Energy Width : 93  m Panter X-ray facility (MPE, Germany): JET-X (Telescope, same as Swift, ~ 1mm/arcmin) Focal Length (3.5 m) JET-X HEW (4.5 keV, 4.5 keV) : 18’’ JET-X + GPD (HEW) : 23.2’’ (394  m ) Spiga et al., 2013, Fabiani et al. 2013 See Talk 8859-22 : Soffitta et al. tomorrow Imaging properties are mainly driven by the optics.

17. Absence of spurious modulation New Design 125 kcounts: Modulation factor: ~50% Spurious modulation measured: ~0.54% Spurious polarization measured: ~1% MDP 99% with  =50% and 125 kc: ~ 2.3% Old design more than 10 6 counts Check on the control of absence of systematics with Fe 55 source at the moment (5.9 keV). Many celestial source are expected to be highly polarized, however : To measure 2 % of polarization (modulation of ~1 %) the systematics should be understood/eliminated at level well below 1%.

18. SPIE Optics + Photonics, San Diego 25-29 August 2013 Environ. background spatial distribution of a GPD 2-cm; 2-bar filled with an ArDME gas mixture. Modulation curve of the background that suggests that most of the background comes from the walls. He-DME filled GPD with the new design. The Fe 55 collimated source on the corner allows for preventing time-out of the electronics. OLD DESIGN. Walls close to the ASIC NEW DESIGN. Walls far from the ASIC edge. Projection on X of a slice excluding Fe 55.. Projection on Y of a slice excluding Fe 55. Modulation curve Polarimetry 2013/08/2518

19. 2013/08/25 SPIE Optics + Photonics, San Diego 25-29 August 2013 19 The mission concept Three telescopes Three GPDs Extensible bench for the three telescopes. Slow rotating (detector or satellite) Weisskopf et al., 2008

20. 2013/08/25 SPIE Optics + Photonics, San Diego 25-29 August 2013 20 The characteristic of the mission Polarization sensitivity is evaluated by taking into account energy dependent modulation factor, efficiency, effective area, source spectra. Imaging capabilities are evaluated on the base of the PSF of the optics and the effect of inclined penetration. The intrinsic contribution of the GPD is negligible. The field of view is due to 1.5 x 1.5 cm 2 current active area of the GPD Spectral resolution (FWHM) is that of a good proportional counter as the GPD is. Time resolution could be better due to the hold time and the spread due to drift of photoelectron tracks (< 1 m s). Background is evaluated in the overall HEW by using OSO-8 NeCO 2 Prop. Count. Goal 15’ Togliere accuratezza 2 us 8us e’ un arrotondamento per riparmiare bit

21. 2013/08/25 SPIE Optics + Photonics, San Diego 25-29 August 2013 21 The needed Payload resources Mass budget Power budget In bolds are mass and power derived from phase A studies (POLARIX and IXO)

22. 2013/08/25 SPIE Optics + Photonics, San Diego 25-29 August 2013 22 Telemetry budget and observation strategies. Observation strategies : Always photon by photon transmission (If X band available) On-board processing for relatively bright sources and S-band Interleaved long observation of faint (1-10 mCrab source) and shorter observation of bright (> 60 mCrab) source with 2 Gbyte on-board memory and downloading in more than one passage.

23. The Minimum Detectable Polarization 2013/08/25 SPIE Optics + Photonics, San Diego 25-29 August 2013 23 MDP (99%) = m= Modulation factor S = source rate (c/s); B = background rate (c/s); T = observing time (s)

24. What measurements are possible with this mission concept 2013/08/25 SPIE Optics + Photonics, San Diego 25-29 August 2013 24 Sensitive polarimetry of point sources. Space resolved polarimetry of extended sources. Crowded fields especially of faint sources.

25. 2013/08/25 SPIE Optics + Photonics, San Diego 25-29 August 2013 25 Space resolved polarimetry of Pulsar Wind Nebulae (PWN) The morphology of PWNs are complex. The OSO-8 integrated measure of the position angle is tilted with respect to jets and torus axis What is role of the magnetic field (turbulent or not ?) in accelerating particles and form structures ? The measure of the pulsar polarization is facilitated. Study of birefringence by comparing the Crab pulsar X-ray and optical phase resolved polarization. OSO-8

26. 2013/08/25 SPIE Optics + Photonics, San Diego 25-29 August 2013 26 Space resolved polarimetry of Supernovae remnants. Example of meaningful observation when made space resolved in 10 6 s but in 10 regions. Angular resolved polarimetry (4-6 keV) in average the power law is 22% of total emission The role of the synchrotron emission with respect to thermal bremsstrahlung The role and the morphology of the magnetic fields T = 10 5 s

27. 2013/08/25 SPIE Optics + Photonics, San Diego 25-29 August 2013 27 In some cases complex fields are better studied with an imaging polarimetry experiments. Polarimetry of molecular clouds in the galactic center but also : Polarimetry of clusters Polarimetry of Ultraluminous X-ray sources in other Galaxies. Sgr B2 10 6 s P = 40 % 1 s (angle) = 4.4 o

28. 2013/08/25 SPIE Optics + Photonics, San Diego 25-29 August 2013 28 Point sources Acceleration mechanisms in jet-sources: Blazars and galactic m Quasars. Blazar X-ray polarimetry can be also exploited for placing upper limits on Quantum Gravity on statistical base (Mrk 421 MDP 3% 10 5 s) m Quasars allow studies of Strong Gravity through the rotation of polarization plane with energy plane Propagation in extreme magnetic fields : White Dwarf, millisecond pulsars, X-ray binaries and magnetars Magnetars allow also studies of QED through vacuum resonance and birefringence. Scattering in aspherical environment : X-ray binaries in hard state (corona) and radio quiet AGNs (ionization cones & torus) A radio-quiet AGN like MCG-6-30-15 can be used to decide if the distortion of the iron line is due to reflection from the disk by Strong Gravity or to a clumpy absorption.

29. 2013/08/25 SPIE Optics + Photonics, San Diego 25-29 August 2013 29 MCG 6-30-15; T = 2 10 6 s The detection is sufficient to disentangle the models. The residual systematic effects must be kept small.

30. 2013/08/25 SPIE Optics + Photonics, San Diego 25-29 August 2013 30 Point sources and science goals within reach

31. END 2013/08/25 SPIE Optics + Photonics, San Diego 25-29 August 2013 31