grb polarization study with using czt-imager of...
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GRB polarization study with GRB polarization study with using CZT-Imager of using CZT-Imager of
AstrosatAstrosat
Tanmoy ChattopadhyayTanmoy Chattopadhyay
Physical Research LaboratoryPhysical Research Laboratory
On behalf of Astrosat-CZTI TeamOn behalf of Astrosat-CZTI Team
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GRB Prompt Emission Polarization GRB Prompt Emission Polarization Key Scientific AspectsKey Scientific Aspects
Nm/Nd > 30% with Pol=.2 .7 SO Model
Nm/Nd < 15% SR / CD Model
Pol > .7 CD model
Emission mechanism of prompt emission is still unknown
Synchrotron radiation in globally ordered B (SO)?
Synchrotron radiation in small scale random B (SR)?
Inverse Compton scattering of soft photons (CD)?
Xray polarization of prompt emission may lift the degeneracy in the models
Toma et al 2008
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Hard X-ray Polarimetry with Hard X-ray Polarimetry with Astrosat - CZT-ImagerAstrosat - CZT-Imager
pixelated CZT detectors
4 x 4 cm2 (16 x 16 pixel)
5 mm thick
Total 64 modules (16 in 4 quadrants)
Orbotech CZT modules
Pixilated detectors can measure polarization
Compton scattering in one pixel and absorption of the scattered photon in another pixel constitute the azimuthal angle histogram
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Observed Modulation for Polarized Observed Modulation for Polarized BeamBeam
Geometry used in simulation
Polarization expt with CZTI using Ba133 (356 keV) source
Chattopadhyay et al 2014 Vadawale et al 2015
Hard X-ray Polarimetry with CZTIHard X-ray Polarimetry with CZTI
CZTI does have
polarization measuremnt capability
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CZT-Imager : An Open Detector in CZT-Imager : An Open Detector in Hard X-raysHard X-rays
Transparent collimators and supporting structures of CZTI at energies > 100 keV
Ideal large area detector to detect transcient sources and GRBs
CZTI detected a bright GRB at 60 deg offaxis angle on 6th October, 2015
GRB position from SWFT BAT
Fluence ~ 10^(5) erg/cm^2
T90 ~ 35 sec in 100 – 300 keV
Talk by Vikas (details on the spectral and temporal studies)
GRB20151006A
Crab Crab
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Prospects of CZTI GRB Polarimetry Prospects of CZTI GRB Polarimetry
Filtering conditions
– 100 – 220 keV
– Neighboring pixels
– E_sc/E_dep < 6
No GRB in double pixel events for < 100 keV !!!
No GRB in double pixel events for < 100 keV !!!
Double pixel events (selection criteria) do represent Compton events
Polarization ??? less systematic and distinct response for off-axis sources
GRB20151006A
Crab Crab
GRB20151006AGRB20151006A
Detection of GRB in double pixel events !!!
Detection of GRB in double pixel events !!!
No GRB in double pixel for nonneighboring pixels !!!
No GRB in double pixel for nonneighboring pixels !!!
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Prospects of CZTI GRB Polarimetry Prospects of CZTI GRB Polarimetry GRB20151006AGRB20151006A
Azimuthal Angle distribution before and after GRB
Azimuthal Angle distribution during GRB ~ 35 sec
Requires Simulated azimuthal distribution for the same GRB for unpolarized/polarized case
Azimuthal Angle distribution for GRB (background subtracted)
Azimuthal Angle distribution for GRB (background subtracted)
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Geant-4 Simulation for Geant-4 Simulation for GRB20151006AGRB20151006A
Zeroth level of mass model in Geant4
Spectral parameters as input for source photons in simulation (pow index 1.47 at energy > 100 keV)
Position of the source from Swift BAT
Unpolarized Case
22 deg pol ang 90 deg pol ang
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Polarization for GRB151006APolarization for GRB151006A
Qualitative?Qualitative?
YesYes
Qualitative?Qualitative?
YesYes
Quantitative?Quantitative? Requires detailed analysisRequires detailed analysisQuantitative?Quantitative? Requires detailed analysisRequires detailed analysis
Library of Azimuthal angle distribution for polariozation degree from 20 Library of Azimuthal angle distribution for polariozation degree from 20 – 100 % with pol angle 0 – 180 deg with 10 deg bin – 100 % with pol angle 0 – 180 deg with 10 deg bin
– Integral kind of analysis (laurent et al 2011; Gotz et al Integral kind of analysis (laurent et al 2011; Gotz et al 2009,2014; Mcglynn et al 2009)2009,2014; Mcglynn et al 2009)
Correct the azimuthal distribution by unpolarized distribution and fit Correct the azimuthal distribution by unpolarized distribution and fit cos^(phi) function to obtain modulation factor and therefrom cos^(phi) function to obtain modulation factor and therefrom polarizationpolarization
– Conventional polarimetry methodConventional polarimetry method
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Polarization for GRB151006APolarization for GRB151006A
Cos^(phi) function does not Cos^(phi) function does not fit the modulation curve well fit the modulation curve well
for offaxis sources (Muleri for offaxis sources (Muleri 2014) 2014)
corrected modulation curve corrected modulation curve for simulated distribution (90 for simulated distribution (90
deg pol)deg pol)
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Polarization for GRB151006APolarization for GRB151006A
Cos^(phi) function does not Cos^(phi) function does not fit the modulation curve well fit the modulation curve well
for offaxis sources (Muleri for offaxis sources (Muleri 2014) 2014)
GRB20151006GRB20151006A is highly A is highly polarized polarized (>70%)(>70%)
Requires further Requires further analysis --- stay analysis --- stay
tunedtuned
corrected modulation curve corrected modulation curve for simulated distribution (90 for simulated distribution (90
deg pol)deg pol)
corrected modulation corrected modulation curve for curve for
GRB20151006AGRB20151006A
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Prospects for GRB polarimetry Prospects for GRB polarimetry with CZTIwith CZTI
3 sigma polarization detection for 5 3 sigma polarization detection for 5 times brighter GRBs times brighter GRBs
– GRB Polarization measurement in every GRB Polarization measurement in every 7-8 months 7-8 months
GRB20151006A
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ThanksThanks
CZTI has polarization measurement capability --- tested quite CZTI has polarization measurement capability --- tested quite extensively in Lab with polarized and unpolarized radiation --- extensively in Lab with polarized and unpolarized radiation --- gives further confidence in any claim unlike INTEGRAL/RHESSIgives further confidence in any claim unlike INTEGRAL/RHESSI
GRB20151006A has shown high polarization --- further work GRB20151006A has shown high polarization --- further work needed to measure polarization needed to measure polarization
Genuine polarization signature of GRB201506A certainly proves Genuine polarization signature of GRB201506A certainly proves that CZTI will be able to measure polarization for brighter GRBs --- that CZTI will be able to measure polarization for brighter GRBs --- many interesting science problems to be addressed !!!many interesting science problems to be addressed !!!