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

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

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

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

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

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 !!!

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)

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

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

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)

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

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

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 !!!