Instrumentation and future missions

Instrumentation and future missions Sergio Fabiani INAF IAPS (Roma) Alsatian Workshop on X-ray Polarimetry, 13 th -15 th November 2017, Strasburg, France

PHYSICS Emission processes : cyclotron, synchrotron, non-thermal bremmstrahlung [Westfold, 1959; Gnedin & Sunyaev, 1974; Rees, 1975] Scattering on aspherical accreting plasmas : disks, blobs, columns [Sunyaev & Titarchuk, 1985; Mészáros, P. et al. 1988, Sazonov 2002] Vacuum polarization and birefringence through extreme magnetic fields (QED effects) [Gnedin et al., 1978; Ventura, 1979; Mészáros & Ventura, 1979] Gravitational Fields: GR effects [Stark and Connors 1977, Connors et al. 1980, Dovciak et al. 2008, Li et al. 2008, Schnittman and Krolik 2009] Quantum Gravity [Gambini & Pullin 1999] Why X-ray polarimetry in Astronomy? Search for axion-like particles [Bassan et al. 2010, Perna et al. 2014] SOURCES Acceleration phenomena Pulsar wind nebulae SNRs Jets Solar Flares Emission in strong magnetic fields Magnetic cataclysmic variables Accreting millisecond pulsars Accreting X-ray pulsars Magnetar Scattering in aspherical situations X-ray binaries Radio-quiet AGN X-ray reflection nebulae

POLARIMETRY BASICS Polarimeter = Analyser + Detector Analyser : For analysing different angles of polarization Detector : To detect photons for each angle Unpolarized radiation flat response Polarized radiation Modulated response

POLARIMETRY BASICS MODULAITON FACTOR (measured with 100% pol. rad.) Polarization Degree Minimum Detectable Polarization At 99% of confidence level [Weisskopf et al. 2010, Stromayer & Kallman 2013] R : source rate B : background rate T : integration time

POLARIMETRY BASICS For fitting also the equation: M = N (1 + A cos(2 φ φ 0 ) Polarization = A/µ Stokes parameters P= Q^2+U^2 I f 0 = 1 2 atan Q U No V no circular polarization with present techniques

POLARIMETRY a hard work For polarimetry is crucial to define properly the detector geometry a not well conceived geometry can originate large systematics hexagonal geometry for sensitive elements (pixels, scintillating bars) is preferable than a squared one Rotation can be used to reduce this systematics, but it can be a problem for the integration of the polarimeter on the bus Scattering and photoelectric polarimeters, whose signal depends on the azimuthal response, show a spurious modulation if the incoming beam of radiation is inclined with respect to the detector axis (the azimuthal symmetry is broken). This effect is larger for larger inclinations and starts to be relevant for inclination of some degrees. Typically a correction is applied by comparing this modulation curve to the on axis one (Jonetoku et al. 2011, PASJ). The theoretical treatment of this effect, with the correction to apply, is described in Muleri 2014, ApJ

POLARIMETRY BASICS Scientific goal Sources < 1keV 1-10keV > 10 kev Acceleration phenomena PWN yes (butabsorption) yes yes Emission in strong magnetic fields Scattering in aspherical geometries SNR no yes yes Jet (Microquasars) yes (butabsorption) yes yes Jet (Blazars) yes yes yes Solar Flares difficult (large thermal and lines) difficult(large thermal and lines) WD yes (butabsorption) yes difficult AMS no yes yes X-ray pulsator difficult yes (no cyclotron?) yes Magnetar yes (better) yes no Corona in XRB & AGNs difficult yes yes (difficult) X-ray reflection nebulae no yes (long exposure) yes Fundamental Physics QED (magnetar) yes (better) yes no GR (BH) no yes no QG (Blazars) difficult yes yes Axions(Blazars,Clusters) yes? yes difficult yes

POLARIMETRY BASICS

Bragg diffraction E = nhc 2d 1 sin 45 q o Bragg law. θ θ Given a crystal (d 1 ) and an energy, diffraction happens for a q angle that verifies the formula. Very narrow energy band (few ev ) possibly increased by using brent crystals With a diffraction angle of 45 the outcoming beam is 100% polarized orthogonally with respect to the incidence plane By rotating the crystal around the beam axis, the outcoming radiation flux is modulated because the polarized component is alternatively the s and p component (modulation period twice the rotation period) Extremely inefficient to measure polarization of continuous spectra, but allows to analyse photons at energies starting from less than 1 kev

Bragg diffraction SOLPEX : SOLar spectroscopy and Polarimetry EXperiments for solar flares Emission lines < 10 kev [Doschek 2002] Thermal bremsstrahlung up to 20 kev (possibly low polarized if Maxwellian distribution of velocity is perturbed by plasma expansion along magnetic field lines due to heat propagation) [Emslie & Brown 1980] Shibata et al.1995 Moreover: Scattering of radiation therefore still polarization Non-thermal bremsstrahlung >20 kev (highly polarized up to 40%) [Zharkova et al. 2010] The hard X-rays are the «natural» target for polarimetry However the non-thermal component can be large even at low energy in the early impulsive phase of flares This will be the target of B-POL (Bragg POLarimeter) in the SOLPEX experiment to be mounted on the KORTES platform on board the Nauka Russian module on the ISS (STĘŚLICKI et al. 2015,Proc. IAU Symposium No. 320, 2015)

Bragg diffraction SOLPEX : SOLar spectroscopy and Polarimetry EXperiments for solar flares F.O.V. ~ 2x2 arcmin Crystal Si 111 bent crystal (85.5mm x 31 mm) at the Brewster angle ~45 Radius of curvature: 610.0 mm Spectral range: 3.940-4.505 Å Radiation detector: CCD Rotation: 1 rev./s Pointed using pin-hole image STĘŚLICKI et al. 2015,Proc. IAU Symposium No. 320, 2015 On board the ISS: Easy access and maintenance, no power limitation 16 eclipses in 24 h, pointing limitations due to ISS motion, only ~10 min of uninterrupted observation per orbit possible

Bragg diffraction LAMP : Lightweight Asymmetry and Magnetism Probe Micro-satellite mission concept dedicated for astronomical X-ray polarimetry and is currently under early phase study (China) Goals: thermal emission from the surface of pulsars and synchrotron emission produced by relativistic jets in blazars. Segmented paraboloidal multilayer mirrors with a collecting area of about 1300 cm 2 to focus 250 ev X-rays onto a position sensitive detector at the focal plane. She et al. 2015, SPIE Compared with natural crystals, multilayer mirrors allow: to choose, to some extent, the energy of interest, to shape it as a paraboloid and to focus the incident beam onto a tiny spot such that the signal to noise ratio is maximized

Bragg diffraction LAMP : Lightweight Asymmetry and Magnetism Probe Reflection angle varies along the mirror and thickness changes to match the Bragg law for 250 ev photons anywhere on the mirror. Focal plane detectors: CCD is a problem for a low-cost micro-satellite Gas detector (GPD as a simple imaging detector, not polairmeter) There are 4 X-ray dim isolated neutron stars, 3 rotation-powered pulsars, and a couple dozens of blazars can be detected with an MDP below 10% with an exposure of 10 6 seconds She et al. 2015, SPIE

Bragg diffraction REDSoX: Rocket Experiment Demonstration of a Soft X-ray Polarimeter Polarization is analysed by means of: Critical Angle Transmission (CAT) gratings that disperses radiation by matching the Bragg condition at the first order of Laterally graded multilayer mirrors (LGMLs) that illuminate CCD detectors that determine the intensity Three coated multilayer mirrors placed 120 apart allow to measure three Stokes parameters (I, Q, and U ) at any time without instrument rotation - Srikanth Panini Singam talk on multilayer mirrors for soft X-ray polarimetry - Marshall talk on A Soft X-ray Polarimeter Marshall et al. 2017, SPIE Günther et al. 2017, SPIE Egan et al. 2017, SPIE

Photoelectric polarimetry f : azimuthal angle q : polar angle b : orbital asymmetry factor = 2 for s orbitals (perfect pol. analyser) < 2 other cases Photo-electron emission direction most probable is parallel to the electric vector of the photon The photoelectron track length in gas for this energy range is of the order of mm so a GAS detector is a good choice for this technique

Photoelectric polarimetry Two alternative technologies Gas Pixel Detector (GPD) Time Projection Chamber (TPC) Enrico Costa talk IXPE (NASA/ASI) Approved XIPE (ESA) In competition M4 GEMS (NASA) Cancelled PRAXyS (NASA) Not approved

Photoelectric polarimetry Gas Pixel Detector Polarimetry, but not only! Energy Spectrum : DE FWHM /E ~16% kev @ 5.9 kev ( E) Timing information Imaging Track analysis first step Track analysis second step

Photoelectric polarimetry IXPE IXPE : Set of three mirror module assemblies (MMA) focus x-rays onto three corresponding focal plane detector units SPIE Proceedings Weisskopf et al. 2016, Soffitta et al. 2017 SPIE Proc. >> Mission Overview Sgrò et al. 2017 >> GPD polarimeter details Muleri et al. 2017 >> Calibration activities Fabiani et al. 2017 >> More details about imaging for calibration

Photoelectric polarimetry extp Zhang et al. 2016, SPIE Primary goals EoS of matter at supra-nuclear density QED effects in highly magnetized star Accretion in the strong-field regime of gravity Polarimetry focusing array baseline: Observational thechniques Simultaneous spectral-timing (Si SDD)-polarimetry (GPD) Energy range 0.5-30 kev Framework Selected background mission in the Strategic Priority Space Science Program of the Chinese Academy of Sciences since 2011. Consortium: China, European and USA institutions 2 telescopes HEW 30 (15 ) A Eff =250 cm 2 at 2 kev F.O.V = 12 arcmin Energy band 2-10 kev Sensitivity 5 μcrab for 10 4 s

Photoelectric polarimetry GPD towards hard X-rays Higher efficiency: High Z gas mixtures (ex. He -> Ar) Thicker absorption gap If source dominated: For the Sun the high flare flux allows polarimetry without telescopes (but flare integrated polarization) To resolve the flare fratures an angular resolution of at least 10 arcsec is needed Some past proposal : HXR Astroph.: Tagliaferri et al. 2012, Exp.Ast. Solar Flartes (ESA S1, ESA-CAS): Berrilli et al. 2015, SPIE JATIS Fabiani et al. 2013, Mem. S.A.It. GPD for HXR IXPE like GPD

Scattering Polarimetry E E 20 kev a threshold for an efficient scattering regime pol. vec. Photoelectric absorption Coherent scattering Incoherent Scattering Scatterer/absorber coincidence for a low background polarimeter

Scattering Polarimetry Higher modulation factor at q=90 scattering angle The modulation factor maximum is lower for higher energy of the scatterd radiation But scattering function S(q,Z) takes into account the interaction with atoms in matter and thus suppresses forward scattering q, Z

Scattering Polarimetry Many different designs Thomson: light passive scatterer made of Li/Be (low photo-absorption to exploit coherent scattering) Compton: 1 Phase: Scatterer and Absorber made of the same low Z material. 2 Phase: Low Z scatterer (higher scattering prob. Than photoelectric absorption) and High Z absorber to maximize the absorption probability Focal plane: pointed observation, large effective area depends on the optics Non focal plane: large F.O.V (ex. GRB obs.) or pointed observations if collimated (PoGO family)

Scattering Polarimetry Typical critical parameters for scattering polarimeters are: background rejection if large sensitive volumes are involved. Mitigation by means of scatterer/absorber coincidence (but it is intrinsic of the Compton scattering technique), anticoincidence, shielding and a careful estimation of sensitive volumes needed - For Thomson polarimeters background is a very critical issue Scintillation light cross-talk Mitigation by means of a careful choice and application of the wrapping Scintillating element light loss (ex. from the edges). Mitigation by means of a careful choice of the wrapping and optical contact between the interfaces towards the light sensor

Scattering Polarimetry Many different designs Underlined are flown polarimeters Thomson POLIX (non focal plane) Paul et al. 2010, 2016 SPR-N (non focal plane) Zhitnik et al., 2006 1 Phase polarimeters POLAR (non focal plane) [Xiao et al. 2017] Talk by Merlin Kole PoGOLite, PoGO+ (non focal plane) [Chauvin et al. 2016] Talk by Mette Friis and Victor Mikhalev 2 Phase polarimeters X-Calibur (focal plane) [Endsley et al. 2015, Beilike et al. 2014] PolariS (focal plane) [Hayashida et al. 2016] GRAPE (non focal plane) [Kishimoto et al. 2007] PHENEX (non focal plane) [Gunji et al. 2008] GAP (non focal plane) [Yonetoku et al. 2011] SPHiNX (non focal plane) [Xie Talk] PINGUIN-M (non focal plane) [Kotov et al. 2011] PING-P (non focal plane) [Kotov et al. 2016]

Thomson scattering X-ray polarimeter made with X-ray proportional counters XPoSat launch is planned in 2019. (Raman Research Institute (RRI) for a small satellite mission of ISRO) Energy range 5-30 kev Scattering Polarimetry POLIX Paul et al. 2010, 2016

Scattering Polarimetry POLAR Space-born polarimeter launched on September 15 th 2016 as part of the Chinese spacelab TG-2 Optimized for 50-500 kev GRBs prompt polarimetry Talk by Merlin Kole Detector module structure 64 PS bars (5.8 5.8 176 mm 3 each) readout by a 64 channel MAPMT (Hamamatsu H8500) POLAR flight model The full instrument consists of 25 identical modules.

Scattering Polarimetry SPHiNX SPHiNX (Segmented Polarimeter for High energy X-rays) Swedish satellite (50 kg), Japan contributes detector and ground station Phase-A/B1, waiting for the selection by the end 2017 Scientific goals: GRB Energy range 50-500 kev, A eff 70 cm 2 Scatter: Plastic + PMT Absorber: GAGG (or BGO) +APD (or SiPM) Fei Xie talk Thesis by Erik Ahlberg and Samin Hasan Optimising a small satellite for hard X-ray polarisation studies of gamma ray Bursts, (KTH) http://conf2nd.coreu.hiroshima-u.ac.jp/wpcontent/uploads/sites/3/2017 /02/HiromitsuTakahashi.pdf

Scattering Polarimetry from PoGOLite to POGO+ PoGOLite is a pathfinder balloon-born experiment flown in 2013 Energy range 20-240 kev From this experience the design was optimized to PoGO+ flown in summer 2016 Chauvin et al. 2017, Nat., Shedding new light on the Crab with polarized X-rays Talk by Mette Friis and Victor Mikhalev

Scattering Polarimetry HXR polarimeter for POLARIS Hard x-ray imaging polarimeter for the proposed PolariS small mission (JAXA) A coarse imaging capability 1-2 arcmin depends on the pitch of the scattering element readout separately Based on the PHENEX prototype flown on a balloon born experiment in 2006 (Gunji et al. 2008) Scatterer: squared matrix of plastic scintillator rods few mm of base side Absorber: elements CsI(Ti) Read out: PMT CsI Plastic Hayashida et al. 2016 Gunji et al. 2008

Scattering Polarimetry PING-P In the path of the past (see Tindo s articles of 70) and recent tradition of Russian solar X-ray polarimetry: SPR-N/CORONAS-F [Zhitnik et al., 2006, 2014] (Thomson scattering) PINGUIN-M/CORONAS-PHOTON [Kotov et al., 2011]) (Compton scattering) PING-P is a Compton scattering polarimeter part of the PING-M experiment for studying solar X-ray activity on board the Interhelioprobe mission planned to launch after 2025 Figure: PING-M polarimeter (PING-P) in comparison with other polarimeters: PINGUIN-M, SPR-N (Zhitnik et al., 2006) and RHESSI (McConnell et al., 2002).

Scattering Polarimetry PING-P 3 organic scintillators as scatterers in coincidence with 6 CsI(Ti) absorber Effective area about 2.5 cm 2, Energy range: 20-150 kev Minimal measurable polarization degree for a X class solar flare is about 1% Kotov et al. 2016, Adv. Sp. Res.

Scattering Polarimetry CdTe/CZT Currently under exploration new different configuration based on CdTe/CZT detectors for high-energy polarimetry. They includes 2D and 3D CZT/CdTe spectroscopic imagers with coincidence readout logic to handle scattering events and to perform simultaneously polarisation, spectroscopy, imaging, and timing measurements Particularly interesting is the study for the development of Laue lenses that would allow a wide band high energy band-pass Miguel Moita talk

Conclusions Missions and detectors X-ray polarimetry is starting to be a crowded field from the point of view of theoretical studies and new instrument ideas, actual designs and real detectors From the early observations in the 70 a new series of pathfinder experiments balloon-born or on board small satellites has been launched and is still planned for the near future A critical mass in the scientific community starts to be relevant indeed Europe, USA, Japan, China, India are involved in many different projects IXPE represents the first step towards larger projects