Overview of CZTI and results of the PV phase A R Rao Tata Institute of Fundamental Research, India (on behalf of CZTI team)
§ TIFR: Lead institute, overall instrument design & development § VSSC: Electronic design, assembly and testing § ISAC: Mechanical design, quality consultation & project management, science team § IUCAA: Coded Mask design, analysis algorithm, calibration & Payload Operations § PRL: Polarimetry algorithm & demonstration, calibration, response matrix § SAC: Analysis software § NCRA: Radio observations
AstroSat
Detector CdZnTe DetectorOptics 2-D coded Mask
Bandwidth 15 - 100 keV
Energy Resolution 6% @100 keV
Time resolution 20 microsec
Detector Proportional counter
Optics CollimatorBandwidth 3 - 80 keV
Energy Resolution 12% @ 22 keV
Time resolution 10 microsec
Effective area 8000 cm2
Detector X-Ray CCD at the focal planeOptics Conical foil ( Wolter-I) Mirrors
Bandwidth 0.3 - 8 keVEnergy
Resolution 2.34% @ 5.9 keV
Angular Resolution 2 arc min (HPD)
Detector Position sensitive Proportional Counter(3)
Optics 1-D coded MaskBandwidth 2.5 - 10 keV
Energy Resolution 25% @ 6 keV
DetectorPhoton-counting (Intensified)
CMOS imagers
Optics Twin Ritchie Chretian 2 mirror system
Bandwidth130-180 nm200-300 nm 320-550 nm
Angular Resolution 1.8 arc sec
2
3
Swift Integral
Above 80 keV: HEAO A4 : 22 sources (2 years) Swift/BAT : 86 sources (6 years) Integral/IBIS : 132 sources (11 years)
Salient Features Ø Indirect imaging with coded aperture mask
§ Sources cast shadow on detector plane
§ Cross correlation of mask pattern with the shadow recovers source locations and flux
§ Simultaneous background measurement
Ø Mask and shielding designed up to 100 keV
§ Hard X-ray monitoring above ~100 keV
CZT-Imager
Ø Time tagged photon data with 20 �s accuracy
§ Allows Compton spectroscopy and polarimetry
Ø Alpha-tagged detector for onboard calibration
Ø Veto detector for efficient background rejection
Ø Low inclination orbit
Ø Absolute time correlation with onboard SPS
CAM assem bly
C olimatorhs g.as s embly
Calibration hsg. assem bly
Mother board assembly
Details of a Quadrant
Detector Box Assembly
Size - 484 x 484 x 600 mm Weight - 50 kg FOV – 4.8 x 4.8 Degree; 17 x 17 Degree; ~80 Degree Shield Material – Tantalum + Aluminum (Passive shielding)
Power- 50 Watts
Four Independent Quadrants
Radiator Alpha tag
CAM
6
Continuous time-tagged individual photon data (20 micro-sec)
Hard X-ray monitoring (above ~ 80 keV)
CZT-Imager Weight - 50 kg Size: 60 cm
Integral (2000 kg; 500 cm; 25.6°) Swift (1500 kg; 560 cm; 20°)
Low Inclination 6°
• Individual photon data • CZT : Energy (pulse height), pixel address,
Time of incidence (20 µs). • Veto : Energy (pulse height), time of incidence. • Alpha : time of incidence. • Simultaneity: Interaction : nano-sec. Detectors : 1-2 µ s. Electronics : 4-5 µ s. Total : 20 µ s. Chance : < 0.2 % (calculation).
Diffuse Cosmic background Calibration source
<10 <100
Crab Nebula 100 Veto triggered events ~ 250
Maximum expected event rate 1000
CZTI Ground Calibration Individual pixel calibration
Ø Spectral calibration:
§ At multiple temperatures using radio-active sources :
§ Gain, off-set; relative quantum efficiency.
Ø Imaging test with sources at known distance (a few meters).
Ø Individual photon data: § time calibration, alpha / veto
calibration: by analysis
Ø CZT response to mono-energetic photon is not Gaussian
Ø CZTI consists of 16384 czt detector pixels each having different response
Ø Need to characterize all pixels at different operating conditions
Ø Model to predict energy dependant line shape
CZTI Line profile model Ø Physics based model – effects of
§ Charge carriers trapping § Charge sharing § Escape peaks § Compton scattering
Ø Model implemented in ISIS § Simultaneous fit to gain calibrated
spectra at three energies for all pixels at five temperature
Ø Total ~80000 spectral fits § Using PRL Vikram-100 HPC cluster
Ø Proper Fitting ~90% of the pixels to obtain model parameters
§ Rest of the pixels flagged spectroscopicallly bad
Ø Group pixels with similar parameters
Ø Compute redistribution matrix for each group
§ All parameters and matrices stored in CALDB
CZTI Response Matrix
Ø Final multi-pixel response matrix § Weighted addition of response matrices for each pixel
Ø Keeping track of § Mask open fraction and effective area of pixels § Module wise energy thresholds
Ø Two different versions of response matrices § Mask weighted à for simultaneous background subtracted spectrum § non mask weighted à for independent background spectrum
No.
of P
ixel
s
No.
of P
ixel
s N
o. o
f Pix
els
No.
of P
ixel
s
μτe μτh
σ r0
Key parameter for all pixels Simulated spectra
Ø Launched on 28 September 2015 by PSLV C30
Ø Day 5: CZTI PE switched on
Ø Day 6: CZTI Quadrants switched on
Ø Day 6-9: Suppressing noisy pixels, Adjusting module thresholds
Ø Day 8: Veto HV on
Ø Day 9: First target of Astrosat- Crab
Ø Hot pixel check à continuous
Ø Six months of Performance Verification (PV) phase
CZTI Inflight operations
12
Field Light Curve Field spectrum FFT Image
First look at the data
Ø Observed count rate ~10 times more then expected Ø Level-2 pipeline failed to process voluminous on-board data Ø Level-0 to Level-1 processing chain failures Ø Auxillary information was not proper
However Ø Instrument header data was flawless. On-board electronics
working perfectly. HK parameters well behaved Ø Event data (though at higher rate) present without any issues
Ø Variance is much higher than expected
§ All events are not “independant”
§ There are “bunch” of events seem to be produced by single actual event
Ø Large number of secondary photons or particles generated due to
§ interaction of one high energy particle anywhere in instrument / satellite
§ Many of these secondaries are recorded by independent detector pixels simultaneously.
Ø Possible to identify by clever algorithm
è Advantage of having time tagged event information, unlike RT-2 or HEX experiments !!
Closer look at the data Ø Genuine X-ray events (source or background) must follow Poisson
distribution, both temporally and spatially
Bunch cleaning Ø Ignore Bunches à more
then 3 events within 20 µs Ø Some post-bunch cleaning
Ø Algorithm tracks the lost time and effective exposure for each pixel
Results § Events reduced to ~20%
§ Event file size reduced from 10 Gb to less than 2 Gb!
§ Effective loss in exposure is 1-2%
§ 300 particles are recorded as 4000 events!
§ Further filtering of flickering / noisy pixels
Bunch cleaning implemented onboard in Feb 16 by a software patch
Bunch Clean: Results
Crab Results First Image Later image (less artifacts)
Pulse profile
PI = 2.09
Spectrum
17
Mayukh Pahari / J S Yadav / Mithun
Powerlaw Index: 2.1
Powerlaw Norm free; (yet to be finalized).
Good �2
Ø CZTI normalization is about half of what is expected § Some issues in determining relative alignment
à both w.r.t other instruments and between four quadrants è About to be resolved
Crab: simultaneous fitting with LAXPC
Crab: Absolute Timing Ø Crab spin down (36 ns/day)
detected clearly in one day observation
Ø No measurable relative delay between quadrants
Ø X-ray pulse known to lead radio pulse by ~300 µs (Integral)
Ø CZTI pulse leads radio by ~490±150 µs
Ø Absolute time accuracy: ~200 µs
Integral (Oct. 2015)
CZTI (folded with radio ephemeris)
Core Science Areas
� Galactic Black Hole Binaries: Spectro-temporal long term monitoring and understanding disk-jet connection with simultaneous multi-wavelength data like GMRT etc.
� Gamma-ray bursts: Investigate hard X-ray spectro-polarimetric properties of very bright GRBs and make follow up studies using GMRT etc.
� Pulsars: Search for hard X-ray counterparts for Fermi-LAT pulsars.
� Bright nearby AGN: Make spectro-temporal long term monitoring to understand the emission mechanism.
Astrosat science: Cygnus X-1: black hole binary
Zdziarski+
GMRT
UVIT SXT
LAXPC CZTI Mt Abu
HCT TIRSPEC
HAGAR
CZTI Observations Cygnus X-1
Ø Very good coverage of Cygnus X-1 state transition from soft to hard during last six months
Ø First CZTI observation à peculiar ultra soft state, hard X-ray intensity ~150 mCrab
Ø Simultaneous NuStar observations for Oct. / Nov. observations
Ø Contemporaneous IR and GMRT observations available
Ø CZTI data very valuable, once spectrum normalization issue is resolved
GRS 1915+105
Get Actual image
CZTI Observations GRS 1915+105
Cygnus X-3
Cygnus X-3
24
Astrosat
Fermi
Swift
Gamma-ray Bursts Astrosat: 650 km; 6° Fermi: 565 km 25°.6 Swift: 600 km 20°
Ø CZTI has best sensitivity to GRBs in 150 to 400 keV energy range è Has been �lucky� with GRBs
Ø First GRB on first day / first observation
Ø Extremely useful for optimizing noise reduction and data cleaning
Ø Next two GRBs after stabilization of data cleaning, pipeline etc.
Ø Fourth GRB in direct FOV!!!
GRBs with CZTI
GRB 151006A
Earth Crab Crab
GRB151006A
Ø GRB detected on first day of CZTI observation!!
Ø Extremely helpful to stream line CZTI data analysis pipeline as well
Ø Extensively studied combining data from Fermi-GBM and Swift-BAT
8 – 25 keV, GBM, BAT
25 – 50 keV, GBM, BAT
50 – 100 keV, GBM, BAT, CZTI
100 – 200 keV, GBM, CZTI Veto
200 – 500 keV, GBM-BGO, Veto
GRB 151006A
Ø Peculiar GRB à peak energy ~ 2 MeV Ø Joint spectral fitting with GBM, BAT,
CZTI and CZTI-Veto Ø CZTI can also provide course localization
Other GRBs GRB 160131A GRB 160119A
GRB 160325A
GRB in CZTI FOV!!
GRB 160325A
X-ray polarimetry with CZT-Imager Ø Significant Compton scattering
probability in 100 – 300 keV
Ø CZTI can detect multi-pixel events
Ø Based on Geant4 simulations
Ø CZTI does have significant polarization capability Ø beyond its spectroscopic
energy range Ø Clear azimuthal modulation
Ø Better then 10% MDP for 1 Crab source in 500 ks
X-ray polarimetry with CZTI: Experimental Confirmation
(Vadawale et al. 2015)
GRBs: Compton events light curve
GRB 151006A GRB 160119A
GRB 160131A Ø Validation of the Compton events selection criteria
Ø Firm demonstration of detection of Compton events from astrophysical source
Ø Enables • Compton spectroscopy • Compton polarimetry
CZTI Polarimetry
Simulations for CZTI show that for 1 Crab source 20 % to 60 % polarization should be detected in ~100 to 20 ks
Ø Four GRBs detected so far by CZTI
Ø Total 14 observations of Crab
§ Exposure ranging from 7 ks to 100 ks
Ø Total 8 observations of Cygnus X-1
§ Exposures ranging from 15 ks to ~80 ks
à Accurate background subtraction is critical
à Background modulation due to Earth albedo is declenation dependent
à Need blank sky observations with similar DEC as source, results in relatively small exposure ~60 ks to 80 ks
GRBs: Polarization
GRB 151006A § ~1.5 sigma detection of
azimuthal modulation § Chance modulation probability
due to à unpolarized X-rays à 0.4 %
(worst case à 50 %)
GRB 160131A § ~2.5 sigma detection of
azimuthal modulation § Chance modulation probability
due to à § unpolarized X-rays à
0.0013 % (worst case à 1.7 %) § Communicated through a GCN
Degree of polarization require full mass model of S/C è under progress
GRB 151006A
GRB 160131A
ID@316, 55ks bkg1_42
ID@316, 55ks bkg2_276
ID@016, 19ks bkg1_42
I D @ 0 1 6 , 19ks
bkg2_276
Crab Polarization
Crab Polarization
Cygnus X-1 Polarization Very promising results… coming up soon
Results from 8 out 14 observations
Ø Rest of observations under processing Ø Next steps: Co-adding all observations, phase resolved polarimetry
Summary Ø Astrosat-CZTI is working flawlessly
§ Excellent data for hard X-ray imaging, spectroscopy and polarimetry
§ Perfectly complimenting rest of the Astrosat instruments
Ø Supplementary objectives è hard X-ray monitoring and hard X-ray polarimetry § Better than expectations! § Number of GRBs detected
§ Search EM counter part for LIGO events! § X-ray polarimetric capability confirmed with Crab
observations § Promising results on GRB polarimetry, with
sensitivity better then order of magnitude § More results expected soon
Acknowledgements
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
Ø VSSC: S. Sreekumar, P. Vinod, Essy Samuel, Priya
Ø SAC: Arvind Singh, Tanul
Astrosat Project Team
Astrosat Mission Team