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X-ray polarimetry at INAF

Paolo SoffittaIAPS/INAF (Rome, Italy)

IAPS/INAF : Enrico Costa, Sergio Fabiani , Fabio Muleri , Alda Rubini, Paolo Soffitta.

INFN-Pisa : Ronaldo Bellazzini, Alessandro Brez, Massimo Minuti, Michele Pinchera, Gloria Spandre.

Why X-ray Astrophysical Polarimetry ?

Polarization from celestial sources may derive from:• Emission processes themselves: cyclotron, synchrotron, non-thermal bremsstrahlung (Westfold, 1959; Gnedin & Sunyaev, 1974; Rees, 1975 • Scattering on aspherical accreting plasmas:

disks, blobs, columns. (Rees, 1975; Sunyaev & Titarchuk, 1985; Mészáros, P. et al. 1988)

• Vacuum polarization and birefringence through extreme magnetic fields

(Gnedin et al., 1978; Ventura, 1979; Mészáros & Ventura, 1979)

2.1 Acceleration phenomena

– Jets

– Pulsar Wind Nebulae

– Supernova Remnants

– mQSOs

– Blazars & Radiogalaxies

– Magnetic reconnection

- solar flares

2.2 Emission in strong magnetic fields

– Accreting White Dwarfs

– Millisecond X-ray pulsars

– Accreting X-ray pulsars

Astrophysics with XIPE

2.3 Scattering in aspherical situations

– X-ray binaries

– Radio-quiet AGNs

– X-ray reflection nebulae

Fundamental physics with XIPE

3.1 QED in strong magnetic fields

3.2 General Relativity in extreme gravity fields

3.4 Search for axion-like particles

3.3 Quantum Gravity

Measurement of the polarization of the radiation.

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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 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 cos2 (φ), therefore it is preferentially emitted in the direction of the electric field.

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

β =v/c

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

Costa, Nature, 2001

The Galactic Center Black Hole Laboratory

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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

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

Granada, 19-22, Nov.2013

SPIE Optics + Photonics, San Diego 25-29 August 2013

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1) The track is collected by the ASIC

2) Baricenter evaluation (using all the triggered pixels)

3) Reconstruction of the principal axis of the track: maximization of the second moment of charge distribution

4) Reconstruction of the conversion point: third moment along the principal axis (asymmetry of charge distribution to select the lower density end) + second moment (length) to select the region for conversion point determination).

5) Reconstruction of emission direction: (maximization of the second moment with respect to the conversion point ) but with pixels weighted according to the distance from it.

Tracks reconstruction

2013/08/25

The Galactic Center Black Hole Laboratory

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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.

Granada, 19-22, Nov.2013

1.5 cm

The Galactic Center Black Hole Laboratory

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The real implementation of a working GPD prototype.

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

Titanium Frame

9 cm

Beryllium window

Electronics

Granada, 19-22, Nov.2013

Weight of the GPD + Lab Electronics = 2 kgPower Consumption of the GPD + Lab Electronics = 5 W

HE-DME mixture: sensitive range 2-10 keV

The Galactic Center Black Hole Laboratory

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IASF-Rome facility for the production of polarized X-rays.

keV Crystal Line Bragg angle1.65 ADP(101) CONT 45.02.01 PET(002) CONT 45.02.29 Rh(001) Mo Lα 45.32.61 Graphite CONT 45.03.7 Al(111) Ca Kα 45.94.5 CaF2(220) Ti Kα 45.45.9 LiF(002) 55Fe 47.68.05 Ge(333) Cu Kα 45.0

9.7 FLi(420) Au Lα 45.1 17.4 Fli(800) Mo Kα 44.8

Facility at IASF-Rome/INAFClose-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

Granada, 19-22, Nov.2013

The Galactic Center Black Hole Laboratory

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Not only MonteCarlo: Our predictions are 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

Granada, 19-22, Nov.2013

More energies, more mixtures

We performed measurement at more different energies and gas mixtures.

Pure DME (CH3)2O

μ = 13.5%

Modulation curve at 2.0 keV

(Muleri et al., 2008, 2010).

The imaging properties of the GPD.

IAPS/INAF laboratory :Very narrow pencil beam.Detector shifts : 300 m.Position resolution : 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) : 18’’ JET-X + GPD (HEW) : 23.2’’ (394 m )

Spiga et al., 2013, Fabiani et al. 2013

Imaging properties are mainly driven by the optics.

A Gas Pixel Detector for higher energies (6-35 keV)

Ar-DME 2-atm; 2-bar

Efficiency (dashed) and modulation Factor (solid) with Monte Carlo and measurement for the low energy (2-10 keV) polarimeter and medium energy (6-35 keV) polarimeter.

Riunione Nazionale Astronomia X 15-16/11/2012 P. Soffitta

Triggered by the effective area at high energy up to 80 keV of the mirror foreseen for NHXM but exploiting the heritage of previews works on Compton Polarimetry. We re-started such activity.

Angular depandance of Compton effect.

Costa et al. NIM 1995 Soffitta et al., SPIE 2010

Compton Polarimetry

Riunione Nazionale Astronomia X 15-16/11/2012 P. Soffitta

Riunione Nazionale Astronomia X 15-16/11/2012 P. Soffitta

By using GEANT 4 and a Monte Carlo specifically developed at this purpose we evaluated the tagging efficiency as a function of energy by using the two measured values at 22 keV and 60 keV. The sensitivity estimation on the right performed for a configuration similar to that of the experimental laboratory set-up is based on an experimental measurement of the efficiency.

NHXPM GEMS Energy range(keV) Energy range (keV) LEP (2-10) (2-10) MEP (6-35) HEP (20-80) not included in the propos. Angular resolution LEP (15 ‘’) 12 arcmin MEP (20 ‘’)

The laboratory measurements confirm the anticipation of the Monte Carlo simulation.

MDP HEP

GEMS : MDP is 0.01 for a 10 mCrab source with an observation of 3.3 x 105seconds. For NHXM LEP it would take around 106 s.

NHXM GEMS Energy range(keV) Energy range (keV) LEP (2-10) (2-10) MEP (6-35) HEP (20-80) not included in the propos. Angular resolution LEP (15 ‘’) 12 arcmin MEP (20 ‘’)

Different Scenarios (Concepts)

Polarimetry is an [almost] undisclosed domain of X-ray Astronomy.It can be performed, with guaranteed results and with a large discovery space, in many different scenarios.

1) Baseline. Photoelectric Polarimetry with at 2-10keV GPD (imaging focal plane) for a:Small (POLARIX, IXPE, XIPE, …)Medium (NHXM-LEP)Large (XEUS, IXO)

2) Extended versionsExtend the band of GPD to higher energies 5-35 keV (NHXM-MEP)Non imaging focal plane scattering polarimeter (NHXM-HEP)3) Descooped versionsArray of GPDs with collimator both LEP and/or MEP

4) Side versions Polarimetry of transients (GRB,SGR) with Wide Field Instruments Polarimetry of solar flares

All these concepts produce valuable results (but costs and throughput are not the same)

Possibili collaborazioni con la Cina.

Universita’ Tsinghua (Beijing) Pi Prof. Hua Feng. Contributo italiano Gas Pixel detector come imager e contatori di fotoni ma con finestra sottile.

Pulsar X isolate e Blazars.

ESA-CAS joint mission

• Call end 2014 two years study four years implementation• launch 2021

Payload requirement :

- Mass 60 kg - Power 50 Watts - Satellite weight 250 kg

XIPE non puo’ essere riproposto con gli specchi di JET-X (70 kg ognuno). Proponiamo XILPE (XIPE Light) in cui I mandrini di JET-X possono essere riutilizzati per realizzare un payload entro i limiti :

XIPE : Enrico Costa, Paolo Soffitta (IAPS/INAF). Ronaldo Bellazzini (INFN-Pisa), Hua Feng (Tsinghua University); Wang (Tonji University, Shanghai).

Spiga et al., 2014

Risorse Scientific capability

Energy range 2-10 keV for polarimetryPolarization sensitivity 20 % at 1 mCrab 105 s

Imaging capability (overall) 23’’ HEW, 15’ x 15’ FoVSpectroscopic capability 20 % at 6 keV

Timing 8 µs, 10 s dead timeBackground (point source) 20 nCrab

Crab rate 47 c/sTelemetry 29 kbit (typical 0.4 Crab)

Payload Power Watt

GPD + Filter Wheel (FW) 2 (typical)8 (peak FW)

Mirror Thermal control 10 (peak)

Back End Electronics 12

Control Electronics 16

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Payload Volume GPD + Filter Wheel + Box 23.0 cm x 16.5 cm x 18.0 cm

(L,L,H) Bee (20 cm max from GPD) 19.0 cm x 13.8 cm x 10.8 cm

(L,L,H)Control Electronics (anywhere) 28.5 cm x 11 cm x 21 cm (L,L,H)Mirror 30 cm (diameter); 60 cm (length)Focal length 350 cm

Payload Mass (kg)GPD + Filter wheel + Box 3.3X-ray Telescope 15Back End Electronics 1.6Control Electronics 5Optical bench and Telescope Tube

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Abbiamo inoltre una presentazione di SEEPE Solar energetic emission and particle explorer. Siming Liu (PMO), Paolo Soffitta (IAPS/INAF), Ronaldo Bellazzini (INFN-Pisa), Robert Wimmer (Kiel)

Una missione pensata per essere complementare a solar-orbiter ma senza ottiche a bordo.

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XTP (Possible) Instruments. Prima versione

High-energy Collimated Array (1-100 keV)

Low-energy Collimated Array (0.5-15 keV)

High-energy Focused Array (1-100 keV)

All Sky Monitor (5-300 keV) Polarization Observation

Telescope (2-10 keV)

Low-energy Focused (0.5-10 keV)

4 m focal length

SDD/CZT

SDD/CZT

CCD

SCD

GEM

CZT

Past collaboration with China for X-ray polarimetry HXMT