Transcript Instruments and Methods of Astrophysical X-ray

Instruments and Methods of
Astronomical X-ray polarimetry
polarimetry
Enrico Costa
IASF-Rome/INAF
COST ACTION MP1104 – Meeting
WG-3
Warsaw May 7-9 2012
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
Measurements in X-ray Astronomy
Timing:
(Geiger, Proportional Counters, MCA, in the future Silicon Drift Chambers)
Imaging:
Rockets, UHURU, Einstein, EXOSAT, ASCA, SAX, XMM, Chandra.
Pseudo-imaging (modulation collimators, grazing incidence optics +
Proportional Counters, MCA, CCD in the future DepFET)
Rockets, SAS-3, Einstein, EXOSAT, ROSAT, ASCA, SAX, Chandra,
XMM, INTEGRAL, SWIFT, Suzaku.
Spectroscopy: Non dispersive (Proportional Counters, Si/Ge and CCD,
Polarimetry:
Bolometers in the future Tranition Edge Spectrometers)
Dispersive: Bragg, Gratings.
Rockets, Einstein, EXOSAT, HEAO-3, ASCA, SAX, XMM,
Chandra, XMM, INTEGRAL, Suzaku, Astro-H, ???.
(Bragg, Thomson/Compton, in the future photoelectric and subdivided
compton)
Rockets, Ariel-5, OSO-8
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
What can Polarimetry Test?
Astrophysics:
• Non thermal emission processes producing
intrinsically polarized photons.
• Deviation from spherical geometry of the matter
close to the emitting regions polarizing by transfer
in a variety of situations and classes of sources:
jets, accretion disks and columns, reflection,
archeoastronomy, etc.
Fundamental Physics:
•
Matter in extreme magnetic fields
• Matter in strong gravity fields
• Axions
• Quantum gravity effects
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
Big Hopes Meager Results
A vast theoretical literature predicts a wealth of results from
X-ray Polarimetry
Polarimetry would add to energy and time two further observable quantities
(the amount and the angle of polarization) constraining any model and
interpretation: a theoretical/observational break-through (Mészáros, P. et al.
1988).
In 40 years only one positive detection of X-ray Polarization: the Crab
(Novick et al. 1972, Weisskopf et al.1976, Weisskopf et al. 1978) P = 19.2
± 1.0 %;  = 156.4o ± 1.4o
The swing on the polarization vector of photon trajectories near a black
hole was long ago suggested (Connors, Piran & Stark,1980) as
another diagnostic; but this is still not feasible because X-ray
polarimeters are far from capable of detecting the few percent
polarization expected (Rees, 2001).
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
A window not yet disclosed
THE TECHNIQUES ARE THE LIMIT!
Conventional X-ray polarimeters are cumbersome and have low
sensitivity
New technical solutions are arriving
The same year of the pessimistic statement by Martin Rees a
new instrument was developed with the potentiality of a
dramatic improvement in both sensitivity and control of
systematics.
A new Era for X-ray Polarimetry is about to come (maybe….)
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
The conventional formalism
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
Or
N    C  D cos 2(   0 )  F sin 2(   0 )
Q
D

,U  
F

But in a paper of (experimental) X-ray polarimetry it is rare to
find Stokes parameters.
The point is that circular polarization in practice is not
measurable. Actually with strong permanent magnets the
absorption coefficients for the two sates of circular polarization
is different in a thin (20-30 eV) range. This is compatible with
X-ray microcalorimeters resolution but the sensitivity would be
poor and the set-up extremely complex, since the readout of a
microcalorimeter is a squid.
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
The first limit: In polarimetry the 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: the modulation of the response of the polarimeter to a 100% polarized beam
Source detection > 10 photons
Source spectral slope > 100 photons
Source polarization > 100.000 photons
Caution: the MDP describes the capability of rejecting the null hypothesis (no
polarization) at 99% confidence. For a significant meaurement a longer observation is
needed. For a confidence equivalent to the gaussian 5σ the constant is higher 4.29→7.58
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
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.
2d1sin    n
nhc
E
Bragg law.
2d1 sin 45o
θ
θ
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.
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
Flown Polarimeters
Rocket, 1972
OSO-8, 1975-1978
Ariel 5, 1974-1975
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
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 0.8o 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 + 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 %;  = 156.4o ± 1.4o (2.6 keV)
P = 19.5 ± 2.8 %; 152.6o ± 4.0o (5.2 keV)
67 % and 99 % confidence
contour. The radial scale is
the polarization in percent
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
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.
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
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
T=105 s.
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
Kaaret et al., SPIE 1989,
Soffitta et al., NIM A, 1998 • Rotary motor for the ensamble detector/analyser
at 1 rpm
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
From Bragg/Thomson to Photoelectric
The turning point of X-Ray Astronomy was the launch of
Einstein satellite that first introduced the X-ray Optics.
The dramatic increase in sensitivity for the detection of faint
sources and the capability to resolve extended source with
imaging detectors in the focus of grazing incidence telescopes,
that do not require rotation, made the mismatching in the
sensitivity of polarimeters, and on the requirements to the
payload (rotation) unsustainable. Polarimeter was disembaked
from Einsten and Chandra and not accepted on XMM.
The only big mission that included a polarimeter was SpectrumX-Gamma with SXRP. SRG was never launched and SXRP
concludes the era of traditional polarimeters.
The new Era is based on photoelectric polarimeters and finely
subdivided scattering polarimeters.
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
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 attempted very long ago, but only since 2001 it was
possible to devise
photoelectric polarimeters mature for a space
mission.
An X-ray photon directed along the Z axis
Heitler W.,The Quantum Theory of Radiation
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.
Costa, Nature, 2001
2
Z 5 mc 2  4 2

 cos2  
2
sin

r
4
4
 o 137  h 
1   cos 
7
β =v/c
2
By measuring the angular distribution of the ejected
photelectrons (the modulation curve) it is possible to derive
the X-ray polarization.
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. It is an ideal analyzer of the
polarization.
Being the cross section always null for φ =
90o the modulation factor µ equals 1 for
any polar angle.
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
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, theGEM 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
conversion
gain
collection
GEM
pixel
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
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
PCB
aE
20 ns
The photoelectrons create tracks in the gas
• Generation (photoelectron +
Auger)
• Propagation (SS_MOTT)
• Creation and diffusion of primary
ionization (Maxwell, Garfield,
Magboltz)
• Gas multiplication
• Digitization
• Pixel Representation
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
ASIC features
• 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%)
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
Enrico Costa-IASF-Roma INAF
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
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
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.
Not only MonteCarlo: Our predictions are
based on data
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.
Soffitta et al., 2010
Present level of absence of
systematic effects (5.9 keV).
Bellazzini 2010
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
Two approaches
Drift
direction
z
The photons enters perpendicularly
respect to the readout plane.
with
The photons enter parallel with respect to
the readout plane.
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
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.
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
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).
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
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).
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
The sensitivity to polarization of GEMS will allow to detect the expected degree of polarization
many X-rays sources being a factor of 100 better than the sensitivity of OSO 8.
from
GEMS has a sensitivity of 1 % (MDP) for a flux of 10 mCrab with 3.3 105 s
(Jahoda 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.
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
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
on-board.
Costa et al., ExpAst 2010
IXO
NHXM
Bookbinder, SPIE, 2010
Tagliaferri et al, ExpAst 2010
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
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 s for 10 mCrab source (2-10 keV)
Costa, et al., Exp Ast 2010
MEP 13 % 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)
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.
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
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.
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
The new window: GEMS and after GEMS
The task to disclose the window of X-ray polarimetry is
up to GEMS.
Which Astrophysics can GEMS do and what cannot?
Which ideas should drive the design and implementation
of future missins with polarimetric capabilities?
Let us make a short review of the most interesting
topics of High Energy astrophysics that can be solved
with polarimetric measurements. The major issues are:
• Imaging
• Higher Energy
• Wide Field
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
CenA
0.6 %
NGC4151
0.7 %
NGC5548
0.8 %
MCG6-30-15
1.2 %
Circinus Galaxy
2.8 %
IC429A
0.7 %
Fairall 9
1.6 %
Mk501 (outburst)
0.5 %
MK421
0.7 %
3C273
0.9 %
A past future: the
Focal Plane of XEUS
A telescope of 5 m2. The baseline includes XPOL a polarimeter in the
focus
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
MDP for AGN (105 s) with a
Conservative ME Configuration
CenA
0.6 %
NGC4151
0.7 %
NGC5548
0.8 %
MCG6-30-15
1.2 %
Circinus Galaxy
2.8 %
IC429A
0.7 %
Fairall 9
1.6 %
Mk501 (outburst)
0.5 %
MK421
0.7 %
3C273
0.9 %
Enrico Costa Enrico Costa – X-Ray
– COST MP1104 – Warszaw May 7-9 2012 WG3
COSTA_Varsaw_120507_XRay
Why imaging?: e.g. the Crab
Positive 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
XEUSBut this is only the average
p.s.f. measurement The structure is much
f.o.v.
PSR
more complex! To perform separate
polarimetry of details of the major
structures we need imaging!
How turbulent is the field? How polarized is the PSR?
NW jet
SE jet
Inner torus
Outer torus
Morover we know from AGILE (confirmed by
Fermi) that the Crab (not the PSR) is varying
on the scale of days at E>100 MeV. These
corresponds to a physical region on the
arcsecond timescale!
Tavani et al. 2011
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
Could we afford very performing imaging (IXO or better):
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
The X-ray polarization measurements can extend the synchrotron emission in jets also at Xrays- 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. But a large area (>1 m2) and high angular resolution (<15”) is required:
IXO or possibly better as XEUS was supposed to be.
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
FROM IXO TO ATHENA
IXO had a single telescope with 4
instruments on a sliding device.
ATHENA has 2 telescopes with 2
fixed instruments. The
polarimeter was dropped.
Eventually JUICE was
selected and ATHENA
was dropped as a whole.
Enrico Costa Enrico Costa – X-Ray
– COST MP1104 – Warszaw May 7-9 2012 WG3
COSTA_Varsaw_120507_XRay
A soft revolution:
The modern scattering polarimeters
The polarimetry based on Compton scattering is the oldest one. It resulted much
less effective than Bragg polarimetry. But the present perspectives are good. It
works as well aboard balloons and therefore things can move independently from the
long term planning of satellite missions.
Scattering polarimetry based on a passive scattering material (as foreseen in SXRP
and implemented in Rhessi) is nowadays less attractive because of the high
background. A low energy one is under study in India (Paul 2010).
The modern techniques of light read-out such as pixelled photomultipliers, avalanche
photodiodes, Silicon photomultipliers and the availability of fast inorganic
scintillators (such as LaBr3) or high Z semiconductors affords for finely subdivided
instruments. Active scattering polarimeters where both the scatterer and the
absorber are detectors in coincidence are already operative or planned.
The scattering polarimeters already existing or planned are (McConnel 2010).
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
Some experiments
X-calibur: focal
plane polarimeter
with a plastic
scintillator
scatterer and CZT
absorber in the
focus of a
multilayer optics
POLAR. Plastc scintillator
as both scatterer and
absorber
GRAPE. Scatterers
of plastic scintillator
and absorbers of
CsI.
GRAPE has
observed the Crab in
last sammer. We are
waiting for the
results.
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
GAP aboard
IKAROS
satellit.
A cylinder of
plastic
Scintillator
surrounded by
CsI.
Our concept: 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 LaBr3 as the absorber at 5
cm distance at 35 keV.
(Soffitta SPIE 2010)
Based on simulation
MDP 7.2 % for
10 mCrab in 105 s
(20-80 keV)
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
Are other techniques ruled out?
Diffraction
has still a role. Especially at lower energies, where neither photoelecric or
scattering can work. Artificial crystals with graded spacing have been
proposed. Herman Marshall (MIT) has proposed such a polarimeter that
could be very interesting for pulsars <1keV
Filters
The natural filters for X-rays are bragg crystals. The imits (narrow band
and need for rotation of both thediffractor and the detector) have been
already shown. Other filters have been of no use soo far. But Nigel
Bunnister from George Fraser group at Leicester University has proposed
(Exp.Astronomy 2006) filters based on dichroic properties of plate-like
crystals grown from materials including large molecules and Br. The filter
is effective in a narrow band (10eV) around the edge K of Br (13 keV)
The hope is to extend the technique don to 2.7 keV and possibly to .8. In
terms of sensitivity is not better than bragg but it could be mounted in
front of a microcalorimeter and could do polarimetry almost for «free»
(rotation of the filter and not of the whole detector.
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
Something we need
Instrumentation:
More support to R&D (SiPMTs, GPD, Filters, ..)
Theory
Celestial calibration sources an in particular:
We need a bright unpolarized source!
POLITICS
ESA after L1 selection (Juice) has decideded to follow the
recommendation to continue the technical developments in view of a
future X-ray observatory. ESA should support not only the optics
the spectroscopy and imaging but also polarimetry.
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
A new perspective: Wide field polarimetry
Polarimeters have been developed mainly as pointed instruments in
combination with a collimator or in the focus of a telescope.
Both photoelectric polarimeters and compton polarimeters can be
used to see a wide field of view.
The sensitivity is lower because of the huge diffuse background
embarked. Moreover the control of systematics is a hard task.
An umpolarized beam impinging off-axis will produce a modulated
signal. This modulation must be accounted to disentingle the
modulation deriving from the polarization.
But the ambition to detect polarization of Gamma-Ray Bursts
makes this effort worthwile. The actual sensitivity will depend on
the hability to perform this correction by software tricks, by
accurate calibration, starting from a good design.
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
GAP: GRB100826A a first result?
This could be the first detection of polarization of a
GRB since the INTEGRAL detection are much less
controlled
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
What from polarimetry of GRBs?
14 years after the discovery of the afterglow we understand many
things about the afterglow and many less about the prompt burst.
Polarimetry of the optical afterglow is typically very low of the
order of 1-2%. The only case of possible high polarization
GRB090102 ~10%(Steele 2009) is for a very quick follow up and
could be more connected to the prompt than to the afterglow.
Some models predict a high polarization only around the so called
break, a change of slope possibly connected to the slowing of the
fire-ball prducing the effect that the cone of relativistic
collimation arrives to include the whole jet.
If the emission is synchrotron the detection of X-ray polarization,
that can be expected to be of the same order, is hopeless,
especially if we want to have a time resolved measurement. It
would require m2 area and very fast repointing. Much ado about
nothing.
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
The prompt
Polarimetry of the prompt GRB is more promising and is probably
the major improvement we can expect to understand the physics of
the process and, in particular, the structure of the magnetic
field.It should be combined with low energy sensitivity. From the
discussion in Lazzati (2010) we derive the following table.
In general we should think to large areas in order to be capable to perform pulse
resolved polarimetry.
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3
Conclusion
GEMS will allow for polarimetry of some representative
objects, mainly galactic.
Future experiments should foresee extended performances,
especially the higher energies and imaging.
Polarimeters and spectrometers abord the same satellite
could result very effective.
END
Enrico Costa – X-Ray – COST MP1104 – Warszaw May 7-9 2012 WG3