GRI: the Gamma-Ray Imager Mission

daughterduckUrban and Civil

Nov 15, 2013 (3 years and 6 months ago)

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GRI: the Gamma
-
Ray Imager Mission

Jürgen KNÖDLSEDER (CESR), on behalf of the GRI consortium

From INTEGRAL to GRI

INTEGRAL …

reveals a large variety of gamma
-
ray source classes


we want to zoom in to identify the sources and to study their emission mechanisms


discovers surprisingly hard emission from AXPs/SGRs


we want to understand the nature of this emission


uncovers absorbed AGN


we want to determine their high
-
energy spectra to measure their contribution


to the cosmic diffuse background


unveils a challenging positron annihilation sky map


we want to identify its origin and measure e
+

annihilation in individual sources



provides a unique view on nucleosynthesis (
26
Al,
60
Fe,
44
Ti)


we want to understand the nature of supernovae explosions and their


nucleosynthesis

The time is ripe for …

a focused, high
-
sensitivity gamma
-
ray mission

Science question:

How do thermonuclear supernova (Type Ia) explosions work?

Method:



Observe gamma
-
ray line lightcurve in a sizeable sample of Type Ia supernovae



Measure line profiles and line ratios in nearby Type Ia supernovae

Expected results:



Identify the primary thermonuclear supernova explosion mechanism



Determine the source of the intrinsic variety of SN Ia (sub
-
/super
-
luminous)



Calibrate the Type Ia supernovae standard candle

GRI science

Example: SN 1998bu 847keV line



CGRO upper limits




no discrimination between models



GRI sensitivity would easily allow to


determine the valid explosion model

Georgii et al. (2002)

Science question:

How do galactic compact objects accelerate matter?

Method:



Measure the shape/amplitude of the high
-
energy emission tail as function of state



Search for 511 keV annihilation features



QPOs @ high
-
energy

Expected results:



Physical nature of hard component
-

relation to relativistic jets



Conditions for 511 keV line: sudden ejection events?



Fundamental GR frequencies: black
-
hole spin and jet ejection

GRI science

Grove et al. (1998)

Goldwurm et al. (1992)

Morgan et al. (1997)

Science question:

What is the nature of the soft gamma
-
ray emission of pulsars?

Method:



Measure spectra and pulse morphology changes (normal pulsars, AXPs, SGRs)



Measure the high
-
energy tails and cut
-
offs (AXPs, SGRs)

Expected results:



Establish the gamma
-
ray production site and emission mechanism in pulsars



Identify the nature of hard tails in AXPs and SGRs

GRI science

Kuiper et al. (2006)

Example: AXP 1E 1841
-
045



INTEGRAL detects hard tails in several AXPs and SGRs



COMPTEL upper limits suggest spectral break < 700 keV



GRI will measure the AXPs and SGRs high
-
energy


spectra precisely (slope, cut
-
off energy)

Science question:

What is the nature of the AGN soft gamma
-
ray emission?

Method:



Measure the AGN high
-
energy spectra and their variety in a sizeable sample



Determine the AGN cut
-
off energy (as function of AGN type)

Expected results:



AGN spectral cut
-
off energy and variety



AGN contribution to cosmic gamma
-
ray background radiation



Origin of the high
-
energy emission (disk or jet)

GRI science

Risaliti (2002)

Weidenspointner (1998)

?

Science question:

What is the source of galactic positrons?

Method:



Search for 511 keV annihilation features in potential candidate sources



Image the central bulge region

Expected results:



Identification of primary galactic positron source



Determine positron escape fractions and yields



Positron source distribution in the central bulge

GRI science

Knödlseder et al. (2005)

GRB/Hypernovae

Novae, SN Ia

XRB

µQSO

Science question:

How do massive stars explode?

Expected results:



Information about dynamics, mixing and symmetry during core collapse



Nucleosynthetic stellar yield calibrations



Core
-
collapse SN contribution to galactic positron budget

GRI science

Leising & Share (1990)

no mixing

mixing

SN 1987A

Method:



Measure gamma
-
ray line intensities and profiles in core
-
collapse supernovae



Measure line and continuum emission in galactic supernova remnants



Search for gamma
-
ray line emission from massive star associations

Diehl & Timmes (1998)

GRI science

Science question:

How do Solar flares accelerate particles to very high energies?

Method:



Spectro
-
imaging of the prompt
g
-
ray emission from flaring active regions



Measure the development of the radioactive patch after the flares

Expected results:



Determine the composition and energy spectrum of the accelerated particles



Identify the acceleration mechanism



Observe the mixing processes in the Solar convection zone

Hurford et al. (2005)

Tatischeff et al. (2006)

GRI science

Gamma
-
Ray Polarization
-

the ultimate dimension:

Probing the nature of high energy emission

The combined measurement of polarization angle and degree of linear polarization

provides vital information about the emission mechanisms

Pulsars & Supernova Remnants:



understand the relation between gamma
-
ray and multi
-
l

emissions



discriminate between pulsars models (polar gap vs. outer cap)

~40% polarization has been measured by INTEGRAL from the Crab pulsar

Soft Gamma
-
Ray Repeaters and AXP:



probe magnetic photon splitting

~25% polarization expected

Compact objects:



probe the geometry of the accretion disk

~30
-
60% polarization possible for optiocally thin disk (~10% for thick disks)

GRI science requirements

Requirements for a future gamma
-
ray mission:

Access to non
-
thermal Universe and gamma
-
ray lines


cover soft gamma
-
ray energy range (~150 keV
-

1 MeV)


Sensitivity leap in soft gamma
-
rays


reach 50 µCrab


Contemporaneous observation down to hard X
-
rays


monitoring capability in the 20
-

200 keV band


Angular resolution for counterpart identification


arcmin


Polarimetry for identification of emission processes

Taking the sensitivity leap

Courtesy: Peter von Ballmoos

GRI sketch

60
-

80 m

detector spacecraft

DSC

3.8 m

optics spacecraft

face
-
on

edge
-
on

crystal lens

structure / spacecraft

mask

detector

sketch not to scale

Lens perfomances

Lens summary (optimisation studies still ongoing):



13772 Ge crystals, 20697 Cu crystals



Lens effective area:


500
-

900 cm
2

@ 160
-

520 keV & 700 cm
2

@ 800
-

900 keV



Mosaicity (= angular resolution): 30” (high
-
energy)
-

60” (low
-
energy)

60"

30"

Formation requirements

Formation Flight:



R/F metrology for coarse formation control



Optical metrology for fine formation control

Lens crystals

SiGe crystal boule

(courtesy: IKZ)

Cu crystal with mosaicities of

30
-

60 arcsec are available

(courtesy: ILL)

Lens crystals:



Ge crystals have already been used for CLAIRE balloon project



Cu crystals with the required properties are available



Massive (several 10 000) crystal cutting / characterisation needs to be developed

Crystal developments

Silver and gold crystals

Better diffraction efficiency for the

same mass

Composite crystals

Build a “gradient” crystals from individual crystal wafers (e.g. Si)

(courtesy: N. Barrière)

Gradient crystals

Reduce the backreflection

Efficiency gain of factor ~2 measured

(courtesy: B. Smither)

Goal: Improve the efficiency of crystals

Lens requirements

Lens requirements:



Mounting and control of several 10 000 crystal is a technological challenge



R&D work underway

Cu crystal monochromator

(courtesy: ILL)

CLAIRE Ge crystal lens (integration)

(courtesy: P. von Ballmoos)

Lens angular response

3 point source lens response

(event density on detector plane)

on axis, 2’ off axis, 4’ off axis

GRI imaging of 4 point sources

model

reconstruction

GRI imaging:



FOV defined by detector area (20 x 20 cm


10 arcmin diameter)



Dithering allows imaging with ~ arcmin resolution (precision set by mosaicity)

Lens detector

Lens detector basic elements

Detector stack


High QE requires detector thickness of several cm

(feasible with stack)


Stack can be used as Compton camera for background reduction


Compton camera measures gamma
-
ray polarization


Design considerations


stack built of CZT
(modest energy resolution)

or Ge
(cooling, annealing)


additional Si layers on top of stack may improve Compton camera?


BGO shield as collimator and active background reduction?


LaBr
3

scintillator on bottom may improve QE at high energies?

CZT or Ge stack

Si tracker (optional)

LaBr
3

absorber (optional)

BGO shield (optional)

sketch not to scale



Detector design under study

Hard X
-
ray monitor

Use coded mask for low
-
energy (20
-
200 keV) coverage

Collimation


considerably reduces extragalactic background (< 100 keV)


Double CZT layer


top layer used for photoelectric absorption


bottom layer used as shield (low E) and as Compton absorber (high E)

lens detector

passive shield

CZT layer(s)

collimator

sketch not to scale

Detector technology

Cross
-
strip GeDs

Vol: 81 cm
3
, Res: 1.6 mm
3


(courtesy: S. Boggs)

Current developments

Development model for large area

hybridised CZT detectors

(courtesy: B. Swinyard)

Low
-
noise GeD pre
-
amps

1.6 keV FWHM

(courtesy: S. Boggs)

256 pixel CZT detector

Thickness 5 mm

(courtesy: E. Caroli)

Wide
-
band ASIC (20
-

2000 keV)

2x8 channels; < 1 mW/channel

(courtesy: E. Caroli)

Hard X
-
ray monitor

Use single reflection multilayer mirror instead of a coded mask

Advantages w/r mask


better hard X
-
ray sensitivity


small/light detector


Disadvantages w/r mask


no imaging (small or no FOV)


technical feasibility needs still to be demonstrated

Material combination: W/Si

Substrate thickness: 0.2 mm

Substrate: Si

Mirror length: 60 cm

Mirror radii: 9
-

44 cm

Example: HEFT mirror

Estimated effective area

Allsky monitor

The use of a Compton stack as lens detector offers an interesting possibility

Use the lens detector as allsky monitor


GRI can find its own ToO


Additional survey science (monitoring of source variability , diffuse emission)

Use of Compton kinematics for allsky monitoring (courtesy: A. Zoglauer)

GRI performance summary

Parameter

Requirement

Goal

Lens Energy range (keV)

150
-

600 & 800
-

900

100
-

1300

Hand X
-
ray monitor energy range (keV)

50
-

200

20
-

200

Continuum sensitivity (ph cm
-
2

s
-
1

keV
-
1
) (3
s
, 100 ks)

10
-
7

3 x 10
-
8

Line sensitivity (ph cm
-
2

s
-
1
) (3%, 3
s
, 100 ks)

3 x 10
-
6

10
-
6

Energy resolution

3%

0.5%

FOV (arcmin)

5 diameter

10 diameter

Angular resolution (arcsec)

60

30

Time resolution (µs)

100

100

Polarimetry (MDP, 3
s
) (for 10 mCrab in 100 ks)

5%

1%

Observing constraints

ToO response time

< 1 day

50% sky coverage

ToO response time

< few hours

Allsky coverage

Requirements: baseline to fulfill GRI science objectives

Goals: possible evolutions (to be studied)

GRI summary

GRI adresses:

The physics of supernova explosions

Particle acceleration in compacts objects and SNRs

The nature of pulsar high
-
energy emission

The high
-
energy emission of AGN

GRI implies:

A new technology to observe the gamma
-
ray sky

Formation flying

A moderate launch mass (~ 2 tonnes total in L2)

GRI offers:

An unprecedented sensitivity leap in soft gamma
-
rays


(observation of hundreds of XRB and AGN in gamma
-
rays)

Simultaneous soft gamma
-
ray and hard X
-
ray coverage

Arcmin angular resolution

Polarimetry

The GRI consortium

DNSC (Copenhagen)

APC (Paris)

CESR (Toulouse)

CSNSM (Orsay)

IAP (Paris)

ILL (Grenoble)

LAM (Marseille)

INAF Brera

INAF
-
IASF Bologna

INAF
-
IASF Milano

INAF
-
IASF Palermo

INAF
-
IASF Roma

Observatory of Roma

University of Ferrara

IOFFE (St. Petersburg)

SINP, MSU (Moskow)

MPE (Garching)

University of Coïmbra

SRON (Utrecht)

University of Utrecht

CNM (Barcelona)

IEEC/CSIC (Barcelona)

IFAE (Barcelona)

Mullard Space Science Laboratory (London)

Rutherford Appleton Laboratory

University of Southampton

Argonne National Laboratory (Chicago)

Space Science Laboratory (Berkeley)