Hard X-ray Polarimetrywith

Astrosat-CZTISantosh Vadawale

Physical Research Laboratory, Ahmedabad

2nd CORE-U International Conference: Cosmic Polarimetry from Micro to Macro Scales

17-18 February 2017, Hiroshima

On behalf of the CZTI team

CZTI Team

Ø TIFR: A. R. Rao, M. K. Hinger, A. P. K. Kutty, J. P. Malkar, Milind Patil, Rakesh Khanna, Yash Bhargava, Vikas Chand, Debdutta Paul

Ø IUCAA: Dipankar Bhattacharya, Varun Bhalerao, G. C. Dewangan, Ranjeev Misra, Ajay Vibhute, Vedant, Shrikant

Ø PRL: Santosh Vadawale, Mithun N. P. S., Tanmoy Chattopadhyay (currently at PSU)

Ø VSSC: S. Sreekumar, P. Vinod, Essy Samuel, Priya

Ø SAC: Arvind Singh, Tanul

Astrosat Project Team

Astrosat Mission Team

OutlineØ Astrosat

§ Introduction

§ CZT-Imager

§ CZTI as GRB detector

Ø X-ray Polarimetry with CZTI

§ Experimental confirmation

§ Analysis details

§ Crab results

§ GRB results

Ø Summary

Astrosat

for Multi-wavelength Astronomy

Collaborative project of

§ Many premier Indian research institutes

§ + two international parterres

India’s first dedicated satellite

AstrosatØ 650 km, low inclination (6 deg) orbitØ Launched by Indian PSLV on 28th September 2015Ø Observatory class, Proposal driven mission from 2nd year

§ Simultaneous multi-wavelength monitoring of broad range of cosmic sources.

§ Sky surveys in the hard X-ray and UV bands.

§ Broadband spectroscopic studies of X-ray binaries, AGN, SNRs, clusters of galaxies and stellar coronae.

§ Studies of periodic and non-periodic variability of X-ray sources.

§ Monitoring the X-ray sky for new transients.

Main scientific focus

AstrosatØ LAXPC: Large Area X-ray Proportional Counters

§ Aeff ≈ 6000 cm2; FOV =1° X 1°; 3-80 keV; Res: 9% @ 22 keV

Ø CZTI: Cadmium-Zinc-Telluride Imager with Coded Aperture Mask§ Aeff ≈ 500 cm2; FOV = 6° X 6°;10 – 100 keV; Res: 6% @ 60 keV

Ø SXT : Soft X-ray Telescope using conical-foil mirrors§ Aeff ≈ 200 cm2 ; FOV = 0.5°; (~3‘ res); 0.3-8 keV; Res: 2% @ 6 keV

Ø SSM : Scanning Sky Monitor with 3 PSPCs and CAM§ Aeff ≈ 30 cm2 (each); 2-20 keV. Res: 20 % @ 6 keV

Ø UVIT : Ultraviolet Imaging Telescope § Two telescopes each with 38 cm aperture; N-UV, F-UV and visible bands.

Five Instruments

Astrosat CZTIHard X-ray Spectroscopy and Imaging

Core Science AreaØ Galactic X-ray binaries:

§ Long term Spectro-temporal monitoring

§ Understanding disk-jet connetion with simultaneous multi-wavelength obsservations GMRT

Ø Pulsars:§ Searching hard X-ray

counterparts for Fermi-LAT pulsars.

Ø Gamma-ray Bursts:§ Hard X-ray spectro-

polarimetric properties of very bright GRBs

Coded Mask imaging with pixilated CZT detectors

CAM assem bly

Colima tor hs g. a s s embly

Calibration hsg. assem bly

Mother board assembly

Details of a Quadrant

Detector Box Assembly

Size - 484 x 484 x 600 mm3

Weight - 50 kg Power- 50 Watts

Two FOVs: 4.67°× 4.67°; ~80°×80°

Shield Material – Tantalum + Aluminum(Passive shielding up to 100 keV)

Four Independent QuadrantsCZT-Imager

CZT-Detector planeSpecifications

§ Total area 976 cm2

§ Total pixels 16384

§ Pixel size 2.46 x 2.46 mm2

§ Field of view 4.67° x 4.67°

§ Angular resolution 8'

§ Energy resolution 8 % (@60 keV)

§ 4 x 4 cm2 (16 x 16 pixel)

§ 5 mm thick

§ ASIC based readout (2 x 128 ch. ASIC)

§ Total 64 modules (16 in 4 quadrants)

Orbotech CZT modules

Crab ResultsFirst Image Later image (less artifacts)

Pulse profilePI = 2.09

Spectrum

11

Mayukh Pahari / J S Yadav / Mithun

Powerlaw Index: 2.1

Powerlaw Norm free; (yet to be finalized).

Good χ2

Ø Relative normalizations are yet to be optimized§ Some issues in cross-calibration of all X-ray instruments

è Expected to be resolved soon

Crab: simultaneous fitting with LAXPC

Ø CZTI mask, collimators support structure designed up to 150 keV

Ø CZTI has good sensitivity to GRBs in 150 to 400 keV energy range

Ø First GRB on first day / first observation

Ø Extremely useful for optimizing noise reduction and data cleaning

GRBs with CZTI

Earth

Crab Crab

GRB151006A

CZTI Effective Area

GRB 160131AGRB 160119A

GRB 160623A

GRB in CZTI FOV!!

GRB 160325A

Alerts on dedicated web page: http://astrosat.iucaa.in/czti/?q=grb

Many more GRBs

X-ray polarimetry with CZTIØ Pixilated detectors in principle can measure polarization

§ Many attempts to measure polarization from instruments not optimized for polarimetry (RHESSI, Integral-IBIS, Integral-SPI)

Ø Can Astrosat CZTI measure X-ray polarization?§ May be above 100 keV (due to scattering probability / threshold)

(Capability in modules? Preserved in data? Meaningful sensitivity? )

For scattering polarimetry with Pixilated detectors è azimuthal bins are unequal

minmax

minmax

CCCC

+-

=µ100µµPP =

( )TRR

RMDP bkgsrc

src

+=

100

29.4µ

Compton polarimetry

Electric field vector of the incident photon

Low Z scatterer

Surrounding detector

Probability

jqjqnn

nn

nns ddrd e ÷

øö

çèæ -

¢+¢

÷øö

çèæ ¢

= 2222

cossin22

Proof-of-ConceptØ Verify multi-pixel capability of CZTI

modules

§ ~70 % Polarized beam produced by 90° Compton scattering of 122 keV X-rays from Co57

§ Energy of polarized X-rays ~100 keV

§ Time tagging à 20 µs

Time between two events Adjacent double pixel events, E2/E1>2

Co57

SourceScatterer

CZT

Polarized X-rays

CZTI Modules can detect simultaneous multi-pixel eventsE

Chattopadhyay et al. (2015)

Modulation Curve

Ø Modulation curve has only 8 bins, which have non uniform angular coverage

Ø Can be normalized with modulation curve for unpolarized beam

obsiunpoli

unpolnori N

MMN ,

,, =

100 % polarized

Unpolarized

(Lei et al., 1997)

8 bin azimuthal histogram with low energy pixel as scattering pixel

X-ray polarimetry with CZT-ImagerØ Significant Compton scattering

probability in 100 – 300 keV

Ø CZTI does have significant polarization capability

Ø Critical steps è event selection, background subtraction, geometry correction

Ø Experimentally verified simulations:~10% MDP for 1 Crab source in 500 ks

Vadawale et al. (2015)

CZTI Polarimetric observations

Simulations for CZTI show that for 1 Crab source 20 % to 60 % polarization should be detected in ~100 to 20 ks

Ø Many GRBs detected so far by CZTI

Ø Total 14 observations of Crab in first year § Exposure ranging from 10 ks to ~140 ks

Ø Almost bi-weekly monitoring of Cygnus X-1 in hard state§ Exposures ranging from 15 ks to ~120 ks

à Accurate background subtraction is critical§ Background modulation due to Earth albedo is

declination dependentà Need blank sky observations with similar DEC as source,

results in relatively small exposure ~60 ks to 80 ks

X-ray Polarimetry StatusØ Only confirmed measurements: Crab, Cygnus X-1(?), few GRBsØ Any additional contribution is highly valuable

A

CZTI Polarization AnalysisEvent selection:Adjacent double pixel events satisfying Compton criteria

Ø Validation of event selection

§ GRB Compton events light curve

§ Crab pulse profile

Background analysisØ Background pattern changes with DECØ Need to select blank sky observations with

same DEC as CrabØ Both Crab and Cygnus X-1 should be more

then 80° away Ø Small region of sky suitable for Crab

backgroundØ Background rate varies with ground trackØ Selecting only similar tracks result in

significant reduction in exposureØ Manually optimize back ground regions and

correct for exposure

Ø Rate difference does not affect modulation or polarization angle

Ø It may only affect the polarization fraction à systematic uncertainty ~10%

Crab PolarizationAll data added à Total exposure ~550 ks

Polarization Fraction è33.8±5.8

Polarization angle è143.0±2.8

Polarization Fraction è50.4±12.0

Polarization Angle è139.5±3.2

Off-pulseTotal Crab Polarization AngleIn general direction of the rotation axis

Crab PolarizationAll data added à Total exposure ~550 ks

Ø Consistent measurements over time

Ø Monthly averaging to get typical exposure à 100 ks

GRB Polarization Ø CZTI had detected more then 50 GRBs

during first year§ 11 are suitable for polarization analysis § Selected based on number of Compton events

(> 350)

Ø Modulation detection in GRB is relatively easy§ Accurate background available pre / post GRB§ High signal-to-noise§ Optimize signal by ignoring shadowed regions.

Ø Estimation of accurate polarization fraction is difficult as GRBs are highly off-axis§ Requires modulation curves for both 100% and

completely unpolarised X-rays from the location of GRB

§ Obtained by extensive Geant4 simulations with the mass model of full spacecraft

§ Require extensive computing effort

PF

9054703895-

19-

725781

GRB Polarization Selected GRBs with initial values for polarization fraction

Ø Extensive stastical analysis is under progressØ Will be reported in near future

Cygnus X-1 Polarization Ø Cygnus X-1 is in hard state through out 2016

Ø Many monitoring observations, typically of duration 15 – 20 ks

Ø Few longer exposure ~120 ks (accidental)

Ø Initial polarization analysis of few observations

Ø Astrosat-CZTI is proven to have good polarimetric capabilities in extended energy range of 100 – 300 keV

Ø Most accurate measurement of Crab hard X-ray polarization§ First time possibility of phase resolved polarimetry of Crab§ Show rich variations§ New challenges for the pulsar emission models

Ø CZTI is also a prolific GRB detector and polarimeter§ Detected more then 50 GRBs in first year§ 11 of them suitable for polarization analysis§ Show variety of polarization signatures§ Potential to constrain GRB models with bigger sample

Ø Many observations of Cygnus X-1§ Few of them suitable for polarization analysis§ Work under progress… but initial analysis show promising

results

Summary

Thanks…