AstroSat CZTI&Future Indian Missions
Varun Bhalerao
IIT Bombay

IIT Bombay

AstroSat CZTI and future Indian missions

Varun Bhalerao

2Astrosat
SXT:
Soft X-ray Telescope

UVIT: Ultra Violet
Imaging Telescope
LAXPC: Large Area
X-ray Proportional
Counter

CZTI:
Cadmium Zinc
Telluride Imager
SSM:
Scanning Sky Monitor
AstroSat CZTI and future Indian missions

http://astrosat.iucaa.in
Varun Bhalerao

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AstroSat results• 5 years since launch• 100+ papers published, several theses• AO-11: October 2021 to September 2022
» Last date – 25th March 2021
» 20% time open for international observers

AstroSat CZTI and future Indian missions

Varun Bhalerao

4What’s next?
• LAXPC: Continuing legacy of Ginga, RXTE/PCA» Direct successor is not being planned in India» Community will be happy to get involved with eXTP• UVIT: Highly successful UV telescope» Proposed successor: INSIST (separate UV mission)• SXT, SSM, CZTI: Amalgamated into two proposedmissions» Daksha and Broad-band X-ray polarimetry mission
AstroSat CZTI and future Indian missions

Varun Bhalerao

5Cadmium Zinc Telluride Imager
AstroSat CZTICZTI at a glance• 5 mm thick CZT detectors• 976 cm2 area
• 2.46 ✕ 2.46 mm pixels• 64 modules, 16384 pixels• Nominal energy range:
20-200 keV

AstroSat CZTI and future Indian missions

Varun Bhalerao7• Off-axis transients• Polarisation
AstroSat CZTI and future Indian missions

60 keV

75 cm268 cm2• Detectors sensitive to~400 keV

120 keV» “Open” at higher
energies!

180 keV• Collimators opaqueup to ~100 keV

33 cm2CZTI for off-axis transients

Varun Bhalerao

8AstroSat’s first science result!
θy = 58°Zθx = 34°

YX

GRB 151006A
Detected 60.7° off axis
Bhalerao et al., 2015, GCN 18422
AstroSat CZTI and future Indian missions

Varun Bhalerao

9Transient search

Credit: CIFT Team (AstroSat CZTI, IIT Bombay)
AstroSat CZTI and future Indian missions

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GRB detections

AstroSat CZTI and future Indian missions

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CZTI + Polarisation

Polarisation & Scattering

AstroSat CZTI and future Indian missions

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Polarisation & Scattering

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Polarisation & Scattering

AstroSat CZTI and future Indian missions

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CZTI and polarisation• Experimentally tested before launch• 100 – 380 keV range• Limited to very bright sources

Vadawale et al 2015
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Crab polarisation• 6σ result (800 ks), 8σ with 1.8 Ms• Significant off-pulse variation challenges models!

Vadawale et al, Nature Astronomy, 2017
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POGO+ Results
Reanalysis (18 – 160 keV)
(Chauvin et al. 2018)

Suggests strong energy dependence
More data is needed
Finer independent phase bins
Validation of dynamic phase resolved analysis
Working on both, but delayed due to pandemic
Expect paper in few months

AstroSat CZTI and future Indian missions

Varun Bhalerao

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Crab New Observations
1 Ms new data, total 1.8 Ms analysed (500 ks more observed in Sep.20)
PF: 33.3±4.1Total

PA: 143.5±1.7

PF: 35.3±7.4
PA: 144.2±2.9

Off-pulse

X-ray polarimetry is challenging!
Must have more data, better instruments
Preliminary results
Total

AstroSat CZTI and future Indian missions

100 – 150 keV

150 – 200 keV

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GRB 160821A• Violation of synchrotron line of death by the highlypolarised GRB 160821A• Chand et al., 2018,ApJ, 862, 154

AstroSat CZTI and future Indian missions

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GRB171010A
100 – 300 keVCompton event lightcurve↓ Total GRB:

100 – 200 keV: unpolarised | 200 – 300 keV: polarised
AstroSat CZTI and future Indian missions

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GRB171010A

100 – 200 keV
AstroSat CZTI and future Indian missions

200 – 300 keV
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GRB Polarisation: sample study• Prompt emission polarimetry for 11 GRBs
» Chattopadhyay et al., 2019, ApJ, 884, 123• 5-year, 22 GRB
paper in prep

AstroSat CZTI and future Indian missions

Varun Bhalerao

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Other transients with CZTIFast Radio Bursts
Continuous event mode data
• Strong limits on FRB X-ray emission» Anumarlapudi et al., 2020, ApJ, 888, 40• Detection of bursts from SGR 1935+2154» Raman et al., in prep

• “Deeper, Wider, Faster” survey» Coordinated observations with ~20 telescopes
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GW170817: AstroSat

Kasliwal et al., including VB, 2017, Science, 358, 1559
AstroSat CZTI and future Indian missions

Varun Bhalerao26https://gracedb.ligo.org/superevents/public/O3/O3a candidates

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O3 NS candidates
NameTypeDistance(Mpc)

90% area(sq deg)Counterpart

S190425z

99% BNS

156 ± 417461

NoS190426c49% BNS, 13% NSBH,24% Gap, 14% Ter

377 ± 1001131

NoS190510g

42% BNS, 58% Ter

227 ± 921166

NoS190718y

2% BNS, 98% Ter

227 ± 165

7246

No100% NSBH

267 ± 52

23NoS190901ap 86% BNS, 14% Ter

242 ± 8114753

NoS190910d

98% NSBH, 2% Ter

606 ± 197

2482

NoS190910h

61% BNS, 39% Ter

241 ± 8924264

NoS190923y

68% NSBH, 32% Ter

438 ± 133

2107

NoS190930t

74% NSBH, 26% Ter

108 ± 38

24220

No4131Yes

S190814bv

GW170817 100% BNS

AstroSat CZTI and future Indian missions

Varun Bhalerao

28GW170817-like scaling
Name

TypeDistance(Mpc)

X-ray flux

S190425z

99% BNS

156 ± 41

5e-8S190426c

49% BNS, 13% NSBH,
24% Gap, 14% Ter

377 ± 100

8e-9S190510g

42% BNS, 58% Ter

227 ± 92

2e-8

S190718y

2% BNS, 98% Ter

227 ± 165

2e-8

100% NSBH

267 ± 52

2e-8

S190901ap 86% BNS, 14% Ter

242 ± 812e-8

S190910d

98% NSBH, 2% Ter

606 ± 197

3e-9S190910h

61% BNS, 39% Ter

241 ± 892e-8S190923y

68% NSBH, 32% Ter

438 ± 133

6e-9S190930t

74% NSBH, 26% Ter

108 ± 38

1e-7

41

7e-7

S190814bv

GW170817 100% BNS

AstroSat CZTI and future Indian missions

Varun Bhalerao

TypicalX-ray /Gamma raysensitivity:~ few e-7
erg/cm2/s

29Requirements
Order of magnitude higher sensitivity(Large area, lower noise, background rejection)
Wide spectral band(1 keV to >1 MeV)
Continuous all-sky coverage(Two satellites)
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DakshaOn alert for high energy transients
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DakshaLow Energy: SDDs
1-25 keV

Medium Energy: CZT
20-200 keVHigh Energy: Scintillator
100-1000 keV

Two satellites
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Advantage Daksha• Effective area (2 satellites): 1700 cm2» Fermi: ~100 cm2 individual, ~300 cm2 total• Sky coverage:» 71% individual, ~100% two satellites» BAT: ~11%, GECAM ~100%• Energy range: 1 keV to > 1 MeV» BAT 15 – 150 keV, Fermi GBM > 8 keV
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Daksha results• Detect dozens of BNS mergers per year
» Also ~1000 on-axis GRBs per year

• Localisation:» ~10º on board, ~5º ground processing• Broadband prompt spectra» Only mission to give prompt soft spectra• Alert GW network with time, location» Lower FAR, 2✕ – 3✕ more detections!

• Huge discovery space!
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Daksha: Polarisation• 15 – 20 times CZTI effective area
• No collimators» Simpler modelling of polarisation signal• Each burst sees 6-8 faces» Different angles» Eliminate systematics• High significance GRB polarisation studies!

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Polarimetry Beyond Daksha
On behalf of Santosh Vadawale

What is the requirement?
• Hard X-ray focusing required for matching SXRsensitivity• Many international proposals !
» XRISM, eXTP, Athena, Lynx…

• Community consensus:
Broadband X-ray Polarimetry• Development lead: SAG-URSC (ISRO), RRI, PRL» Support several institutes (IUCAA, TIFR, IIT-B)

Possibility for collaboration with international groups
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Proposed configuration
Soft Xray
Concentrator +Polarimeter• Optics energy: 1-10keV• 400 cm2 @ 2keV(100 cm2 @ 6 keV)
• Focal length: 3m• Detector: GEM-TPC• Energy: 2-10 keV• Energy resolution:
15% @ 6keV
• MDP: 1% in 1 Ms
for 5 mCrab
AstroSat CZTI and future Indian missions

Hard X-ray
Concentrator +Polarimeter

Medium Energy
Polarimeter• CollimatedThomsonPolarimeter• Polix++• 3-50 keV• Effective area
~2000 cm2

• Optics energy: 1- 80keV• 400 cm2
• Focal length: 10m• Detector: 20-80 keV• Energy resolution:
10% @ 60 keV• MDP: 1% in 100ks
for 1 Crab

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Focal Plane Compton X-ray Polarimeter➢ Fully built and tested prototype▪ Active scatterer: Plastic scint.
▪ Absorber: CsI+SiPM absorber

Lead Institute:
Physical Research Laboratory,
Ahmedabad

60 keVCont.

Unpolarized

AstroSat CZTI and future Indian missions

Varun
Bhalerao(Chattopadhyay etal. 2016)

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GEM based TPC
Lead Institute: Space Astronomy Group,
URSC (ISRO), Bangalore• GEM: 70 micron hole size, 120 micron pitch• Detailed simulation of detector performance- Using
ANSYS and GARFIELD - Insight into working and input todesign and optimization of operating parameter.
• Hardware realization in progress– Discrete electronics
based, multiple channels daisy chained.

Looking for better GEM

The figure shows the 2D binned image of a photoelectron
track of a 9keV photon in Ne/DME 50/50 gas mixture at
200 torr pressure.

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Hard X-ray Optics
Lead Institute:
PRL , Ahmedabad➢ At hard X-rays, the critical angle ~ O(0.1) degree➔ long focal length➢ Depth graded multi-layer coating▪

Thickness: 20 Å to 200 Å➢ RF Magnetron sputtering technique▪

Facility setup at PRL➢ Multi-layer fabrication▪ Optimization under progress➢ Developed a software package▪ Publicly available as open source

DarpanX:▪ Python package formodelling X-ray
Multilayer mirrors

AstroSat CZTI and future Indian missions▪ Usable as PyXSPEC
model to fit XRR
measurements
Mondal et al. 2021

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Hard X-ray Optics➢ Mirror substrate is the key issue➢ Present option !
Optics
Focal length (mm)10150

Number of Shells

100

Shell length (mm)225

Outer diameter (mm)400

Shell thickness (mm)0.3

Gap between shell (mm)0.3

Slumped Glass▪

Same as used in NuSTAR▪

Very low yield▪

Not scalable for larger area /multiple modules➢ Need to explore alternatives▪

Ni-forming, Si-pore, Si-shell➢ Precession assembly is another major challenge (currently at RRI)
➢ Deployment mast ! Assuming ISRO support, commercial options available
Overall Significantly more work needed to realize hard X-ray optics
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Thank you!

AstroSat CZTI and future Indian missions

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