Scientific prospects in soft gamma-ray

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Nov 26, 2013 (3 years and 10 months ago)

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Filippo

Frontera


University of
Ferrara

a
nd

INAF
-
IASF, Bologna

on
behalf of
the “LAUE”
Collaboration

Presented by

John B. Stephen

Scientific prospects in soft gamma
-
ray
astronomy thanks
to the
LAUE project

SPIE Conference on “
Optics
for EUV, X
-
Ray, and

Gamma
-
Ray
Astronomy
VI”


25
-
29
August
2013

Introduction

1/2


Hundreds of hard X
-
ray sources
discovered with INTEGRAL and
Swift surveys.


Polarized photons above 400
keV

discovered;


Asymmetric distribution of the
511
keV

line in the GC.



Requirement of much more
sensitive instruments for deep
studies of the discovered sources
and new phenomena.

Introduction

2/2


The only viable way is the
use of hard X

ray
focusing telescopes.


NuSTAR

is the first
mission with focusing hard
X
-
ray telescopes, with
sensitivity two orders of
magnitude better.
However:


Hard X

ray
passband

limited (<79
keV
) with maximum sensitivity
around 30
keV
.


Extension to higher
energies is crucial to settle
many open issues (see
later)



Requirements for soft gamma
-
ray
telescopes
(>80/100
keV
)
:


C
ontinuum
sensitivity
at least two
orders of
magnitude better than
that of INTEGRAL
at the
same
energies:


G
oal
: a few x10
-
8

ph
/(cm
2

s
keV
)
in 10
5

s
,

10
-
15

erg/(cm
2

s
keV
);


Much higher line sensitivity (Goal: 10
-
6

ph
/cm
2

s in 10
5

s in the case of a narrow line);


much
better (<
1
arcmin
) imaging
capability.





First experience: the HAXTEL project

Virgilli

et al.
2011

Frontera

et al. 2008


Multistep building approach;


6 m focal length;


Flat mosaic crystals of Cu (111);


Mosaic spread of a few
arcmin
.


HAXTEL results

1
st

prototype

2
nd

prototype

1
st

prototype vs. 2
nd

prototype

A PSF improvement obtained,
but not sufficient.


A new assembling technology was needed for long focal lengths.

Laue

Project



Main goals:


More accurate
assembly technology for long focal
lengths. Required cumulative error budget <10
arcsec
;


Better reflection
efficency

and better focusing;



Development of a 20 m FL lens petal;


Feasibility and accommodation study
of a space
lens
made of petals.


Laue Consortium:


Scientific Institutions:


UNIFE, INAF/IASF
-
Bologna, CNR/IMEM
-
Parma;



and Industry
:



DTM
-
Modena, TAS I
-
Milan and Turi
n
.


Approach


For a more accurate assembly technology:


Development of
an apparatus that would allow to
correctly orient and
fix each
crystal to the lens frame
under the control of a gamma

ray beam.


Fixed lens petal;


M
ovable gamma

ray beam remaining parallel
to the
lens axis
.



For a higher reflection efficiency and better
focusing:


development
of bent
crystals.

L
ocation for the apparatus development

The
LARIX tunnel of the University of
Ferrara at
the starting time of the LAUE project

Developed Apparatus


S
ee talk
by
Virgilli


Main apparatus components 1/2


A
collimated (20
arcmin
) and movable (both tilt and
translation) gamma

ray
source
(
Emax
= 300
keV
).


A
beam
-
line
21
m long, 60
cm inner
diameter, under
vacuum. Initial design 70 m.


3. At the exit of the
beamline
, a final shield of the
gamma

ray beam with a square hole in the center,
that
hosts a W slit with variable aperture.


Both source
and slit can be translated
leaving the
gamma
-
ray beam parallel to the lens axis.


Lens

petal frame (see slide).


Petal Frame

Main apparatus components 2/2


A six

axes
motorized robot (hexapod)
for the fine
positioning of each crystal tile on the
lens frame
under
control
of

gamma

ray pencil beam.


Focal
plane detectors (a gamma

ray imager and a
spectrometer).

Details in the talk by
Virgilli
.


C
lean
room with humidity and temperature control,
where both final
slit and
petal
frame are
located.


All

movements
are
motorized and
controlled
by the
console
room located outside the
tunnel. See talk by
Caroli

et
al.

Once
the diffracted photons are focused on the lens
focus, the crystal tile is fixed to the lens frame.



Bent samples of perfect
Ge
(111) developed at
UNIFE;

(talk by
Guidi

et al.)




Bent samples of mosaic
GaAs

(220) 25
arcsec

spread, developed at
IMEM
-

Parma

(talk by C. Ferrari et al.)


Developed

crystals

Buffagni
+

2012

Guidi
+ 2011

P
roduction of bent crystals for LAUE


Massive production (300 in total) of
Ge

(111) and
GaAs

(220) bent crystals 2 mm thick
.


Curvature of the produced crystals tested in the LARIX
facility at 59.2
keV

monochromatic line (K
α
1

fluorescente
line of the W
anode

of X

ray

tube) with
satisfacory

results
.


Apparatus alignment


Three steps of
increasing accuracy
:


Mechanical alignment;


Optical alignment;


Gamma
-
ray alignment



Final Alignment Test


Image
of
two 100 µm W crosses,
located in correspondence of the
final slit and lens frame, as
detected
by the
focal plane
gamma
-
ray

Optical equipment

Gamma
-
ray monitor of incident radiation


Positioned at the end of the
beamline

to
monitor gamma
-
ray beam intensity and
stability.

Petal focusing
-
capability test: spectrum
1/2


At 20 m focal length in the expected focus:


Test of both diffracted spectrum and image.



S
pectral results:



In addition to the diffracted line also a background spectrum
due to X
-
ray polychromatic source.

Diffracted

Diffracted

Petal focusing
-
capability test:
spectrum

2/2


BKG rejected by means of a
Pb

shield just on
the back of the lens frame.


Diffracted spectrum by
Ge
(111) crystal tile after
the cure:

Petal focusing
-
capability test:
imaging


Imaging of the diffracted beam is more critical,
also due to the beam divergence, even if small.


Diffracted
image from
GaAs

crystal
tile

Possible configuration of a space lens
made of petals

From the feasibility study performed by Thales
-
Alenia

Space
-

Italy


Branch of Turin

Expected performance of a lens made of
petals


Assumptions:


Bent crystal tiles 2mm thick


Material:
Ge

(111) or
GaAs

(220)


Passband
: 90
-
600
keV


Focal length: 20 m.


Crystal
-
tile cross
-
section: 30x10 mm
2



Inter
-
distance between crystal tiles: 0.1 mm (that of
the assembling petal).

Expected on
-
axis Point Spread Function
and its dependence on radial distortion

On
-
axis PSF for
Ge
(111) with
no radial distortion.

D
ependence of the
fwhm

PSF
on radial distortion for
Ge

and
GaAs


Expected on
-
axis continuum sensitivity
(3
σ
)
in

10
5

s

Comparison between
Ge

and
GaAs

lenses in 10
logarthimic

energy bins

Comparison between
Ge

and
GaAs

lenses for
Δ
E=E/2

(
Ge
) Flux sensitivity:
3.6x10
-
13

erg/cm
2

s

(@ 300keV)

(
Ge
) Flux
sensitivity:
1.5 x10
-
13

erg/cm
2

s


(@
300keV)

Expected on
-
axis sensitivity to narrow
emission lines
in 10
5

s

Examples of issues that can be faced with
the proposed
Laue lens




High energy emission physics
in the presence of super
-
strong magnetic fields (
magnetars
);


Non thermal processes

in cosmic sources (e.g
.,
AGN);


Origin and distribution of high energy cut
-
offs in AGNs
spectra;


Origin of
Cosmic
X
-
ray background (
CXB) at E>100
keV


Precise role of non
-
thermal mechanisms in extended
objects (e.g., Galaxy Clusters);


Gamma
-
ray source polarization.



Determination
of the antimatter production processes and
its origin
.


Dark matter probe


M
agnetars

Goetz et al. 2006



Which is the origin of the high energy
component?


E.g., Thompson
&
Beloborodov

(2005
)
model:

synchrotron originated by pair
production.



Crucial to know the cutoff of the high energy
spectrum
.







4
U
0142
+
61
(Kuiper et al.
2006
)


Lens sensitivity

In 90
-
600
keV

Physics of accretion onto Galactic
compact objects in binary systems 1/2


Spectra of compact objects extend beyond 100
keV
.


However the spectra beyond 100
keV

or less are scarcely
known, even of strongest sources.


Even
NuSTAR
, due its limited
passband

(<80
keV
), will
not be capable to do that.

Her

X
-
1
low

ON

Cyg

X
-
1 soft state

Physics of accretion onto Galactic
compact objects in binary systems
2
/
3


With the proposed Laue lens:


In
the case of X

ray pulsars
, new discoveries of
high energy cyclotron features and/or
harmonics of
lower energy
features, thus higher magnetic field
strengths
and its properties
investigated
.


In the case of low

magnetic field NS (like atoll

sources, bursting sources, transient sources),

a
broad energy spectrum that extends to high energies
gives key information about
the geometry
of the
emission and the origin of the emission in different
states.

Physics of accretion onto Galactic
compact objects in binary systems 3
/3


In the case of BH
sources,


power
-
law
tails (
E
cutoff
) and
their behavior in different
spectral states vs.
corresponding photon
index (see slide).


crucial for establishing
production
of non
-
thermal
emission (e.g.,
Titarchuk

+2010, Laurent+2012),

E
mission
physics of RQ AGNs




Basic emission scheme is
known: Compton up
-
scattering of seed photons


But:


W
hich
is
the electron
temperature?


Is there a non
-
thermal
component?


Current direct
-
viewing
telescopes and
NuSTAR

cannot study
high energy
tails.


P
hoton
index and high
energy cut
-
off measurements
are crucial

for AGN physics.


Proposed Laue lens can
discover new science.


Perola

+
2002

Emission physics of
RL AGN
(
Blazars
)


Two humps in the
SED:


one
interpreted as
synchrotron
emission,


the
other as
IC
(SSC
and/or EC
).



But dip between
humps never observed.


The sensitivity of our
lens can do that.


Ghisellini

2011

400
keV


































Ghisellini

2009

400
keV

Lens sensitivity in 10
5

s



I
n current CXB
synthesis models (
Gilli
+
2007) of assumption of
RQ
-
AGN
populations
with


a distribution of
photon indices,


fixed
E
cut

(=200
keV
)





Is it right to assume a
fixed E
F

?



CXB (<100
keV
)


Likely due to
Blazars
.


But:


The most recent results on
Blazars


are in
15
-
55
keV

(
Ajello+2009).



O
nly
assumptions
about
high energy
spectrum




Gamma

ray
observations
are
crucial


CXB (>
100
keV
)

Ajello

et al.
2009

Comastri

et al. 2006

RL
-
AGNs



Diffuse annihilation line
emission with INTEGRAL
(integrated flux: 1.7x10
-
3

ph
/cm
2

s).



Origin still unknown.


Several models proposed:


D
ark matter;


Antimatter


Source of radioactive elements
like
26
Al,
56
Co,
44
Ti


Gamma Source (e.g., Pulsar)


BH
Binaries



More
sensitivity

and
imaging

capability

P
ositron annihilation from GC


Weidenspointner
+
2008

Gamma
-
ray polarization



A very strong polarization
signal found from Cygnus X
-
1
with
INTEGRAL above 400
keV
;



The proposed lens can extend
this
search to
weaker
sources.


Laurent at al. 2011

250
-
400
keV

400
-
2000
keV

Conclusions


A new apparatus has been developed for building
Laue
lenses

with ling focal length (20 m)



For the first time bent crystals have been
developed and used for a lens petal.


An industrial study shows the feasibility of a lens
made of petals.


The energy band beyond 100
keV

is crucial for
settling many key
-
importance open
issues.



Concrete prospect
for
proposing a
broad band
(e.g., 1
-
600
keV
) satellite mission
based on Laue
lenses and multilayer optics.