X-ray Polarimetry - INAF-IASF-Roma

X-Ray Polarimetry
Paolo Soffitta
IASF-Rome/INAF
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

In polarimetry sensitivity is a matter of
photons
MDP is the Minimum Detectable Polarization
R S is the Source rate
R B 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
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

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.
2d 1
sin
n
E
nhc
2d
1
sin 45
Bragg o law.
θ
θ
A crystal oriented at 45 o to an incident linearly polarized x-ray beam acts as a perfect polarization
analyzer. At 45 o 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.
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

Flown Bragg Crystal Polarimeters
Rocket, 1971
OSO-8, 1975-1978
Ariel 5, 1974-1975
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

OSO-8 satellite with a dedicated Bragg polarimeter
OSO-8 satellite (top) and
polarimeter (bottom)
468 graphite mosaic crystals were mounted to the two sector of
parabolic surface of revolution.
Mosaic spread of 0.8 o Band-pass = 40 eV (2.62 keV)
Bragg angles allowed between 40 o and 50 o
Overall band-pass 400 eV (2.62 keV)
= 0.94
Projected crystal Area = 2 x 140 cm 2 ; Detector area = 2 x 5 cm 2 ; FOV=
2 o B = 2 x 3 10 -2 counts/s in each order (pulse shape analysis + anticoincidence)
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 %; = 156.4 o 1.4 o (2.6 keV)
P = 19.5 2.8 %; 152.6 o 4.0 o (5.2 keV)
67 % and 99 % confidence
contour. The radial scale is
the polarization in percent
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

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 90 o of angle of scattering (θ) the modulation factor is 100 %
since there are not photons diffused along the electric field.
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

Thomson Polarimeters
They were the first experiment to be flown on-rockets
1. Rocket flight on April 1969 to search for polarization n the Crab Nebula (Wolff, 1970).
2. Rocket flight on July 1969 to search for polarization in Sco X-1 (Angel et al., 1969).
3. 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.
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

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.
Kaaret et al., SPIE 1989,
Soffitta et al., NIM A, 1998
• 4 x 100 cm 2 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=10 5 s.
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

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.
Heitler W.,The Quantum Theory of Radiation
Costa, Nature, 2001
sin
cos
5
Z
2 mc 2
7 2 2
2
β =v/c
4 2
r o 4
4
137 h 1 cos
By measuring the angular distribution of the ejected
photelectrons (the modulation curve) it is possible to derive
the X-ray polarization.
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.
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

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
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

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
A photon cross a Beryllium window and it is absorbed in the gas gap, the GEM electric field
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.
X photon (E)
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.
conversion
gain
collection
20 ns
pixel
GEM
a E
PCB
Costa et al., 2001
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

ASIC features 105600 pixels 50 μm pitch
• Peaking time: 3-10 s, 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 10 5 pixel.
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

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.
DME = (CH3) 2 O
3) Ar DME 60-40 2 atm 2 cm.
60 µm/√cm diffusion
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

IASF-Rome facility for the production of
polarized X-rays
Facility at IASF-Rome/INAF
Close-up view of the polarizer and the Gas Pixel Detector
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
Capillary plate
(3 cm diameter)
PET
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)
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

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.
Impact point
Not only MonteCarlo: Our predictions are
based on data
Muleri et al. 2007
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.
Soffitta et al., 2010
Present level of absence of
systematic effects (5.9 keV).
Bellazzini 2010
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

More energies, more mixtures
Pure DME (CH 3 ) 2 O
Modulation curve at 2.0 keV
We performed measurement at more different
energies and gas mixtures.
μ = 13.5%
(Muleri et al., 2010).
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

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.
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

Two approaches
The photons enters perpendicularly
respect to the readout plane.
with
The photons enter parallel with respect to
the readout plane.
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

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 13 th 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).
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

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 10 5 s of observation).
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

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 10 5 s
(Yahoda et al. 2010 corresponding for a flux of 1 mCrab source and 10 5 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.
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

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.
GEM electric field
X photon (E)
conversion
gain
collection
pixel
GEM
PCB
Polarization information is derived from the angular distribution of the
tracks produced by the photoelectrons, imaged by a finely subdivided gas
detector.
20 ns
a E
Costa et al., 2001
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

POLARIX
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 onboard.
Costa et al., ExpAst 2010
NHXM
IXO
Bookbinder, SPIE, 2010
Tagliaferri et al, ExpAst 2010
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

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 cm 2 @ 2 keV, HEW=(20’’))
3 GPD (1-cm, 1-Atm, He-DME 20-80)
MDP 12 % in 10 5 s for 1 mCrab source (2-10 keV)
3.8 % in 10 5 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 10 5 s for 1 mCrab source (2-10 keV)
3.1 % in 10 5 s for 10 mCrab source (2-10 keV)
Costa, et al., Exp Ast 2010
MEP 13 % in 10 5 s for 1 mCrab source (6-35 keV)
4.1 % in 10 5 for 10 mCrab source (6-35 keV)
In study (HEP, Compton scattering)
MDP 7.2 % for 10 mCrab in 10 5 s (20-80 keV)
Tagliaferri et al.i, Exp Ast 2010;
- IXO
Soffitta et al. SPIE 2010
(ESA/NASA/JAXA Large Mission Evolved in Athena with no polarimeter on-board)
Area= 2.5 m 2 FL = 20 m HEW= 5’’ XPOL: MDP 1 % 1 mCrab 10 5 s.
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

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.
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

What can be measured by imaging polarimetry ?.
What can be explored at higher energies ?.
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

Spectro-imaging polarimetry of PWNe
NHXM LEP position resolution
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.
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 .
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

NHXM: Cyclotron lines with 100 ks of
observation with MEP
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

Was the GC an AGN a few hundreds years ago?
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 (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;
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

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)
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

IXO:
Polarimetry of extended Jets in AGNs and Glactic BHs
Jet of M87 and the knot A with the
PSF of XPOL. MDP is 6 % of 200 ks
Western jet of XTE J1550-564 and the PSF
of XPOL. MDP is 4.4 % with 1 Ms
(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.
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

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.
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.
Simulated modulation curve for 10 cm length BC404 as
scatterer (5 mm diameter) and LaBr 3 as the absorber at 5
cm distance at 35 keV.
(Soffitta SPIE 2010)
Based on simulation
MDP 7.2 % for
10 mCrab in 10 5 s
(20-80 keV)
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

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 14.5 o
Gunji et al., 1994
MDP = 7 % in 5.5 hours in 30-700 keV for Crab
• 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.
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)
•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 = 1000 cm 2 .
• MDP sensitive the low energy threshold.
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011

END
Paolo Soffitta The Extreme and Variable High Energy Sky Chia Laguna 19-21/09/2011