Filippo Frontera University of Ferrara

andINAF-IASF, Bologna

on behalf of the “LAUE” CollaborationPresented 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):

– Continuum sensitivity at least two orders of magnitude better than that of INTEGRAL at the same energies: – Goal: a few x10-8 ph/(cm2 s keV) in 105 s,

10-15 erg/(cm2 s keV);– Much higher line sensitivity (Goal: 10-6 ph/cm2 s

in 105 s in the case of a narrow line);– much better (< 1 arcmin) imaging capability.

First experience: the HAXTEL project

Virgilli et al. 2011Frontera et al. 2008

• Multistep building approach;• 6 m focal length;• Flat mosaic crystals of Cu (111);• Mosaic spread of a few arcmin.

HAXTEL results

1st prototype 2nd prototype

1st prototype vs. 2nd 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 Turin.

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;– Movable gamma–ray beam remaining parallel to the

lens axis.

• For a higher reflection efficiency and better focusing:– development of bent crystals.

Location for the apparatus development

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

Developed Apparatus

See 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.

• Clean 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

Production 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 (Kα1 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.

• Spectral results:

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

Diffracted Diffracted

Petal focusing-capability test: spectrum2/2

• 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 mm2 – 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.

Dependence of the fwhm PSF on radial distortion for Ge and GaAs

Expected on-axis continuum sensitivity (3σ) in 105 s

Comparison between Ge and GaAs lenses in 10 logarthimic energy bins

Comparison between Ge and GaAs lenses for ΔE=E/2

(Ge) Flux sensitivity: 3.6x10-13 erg/cm2 s (@ 300keV)

(Ge) Flux sensitivity: 1.5 x10-13 erg/cm2 s (@ 300keV)

Expected on-axis sensitivity to narrow emission lines in 105 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

Magnetars

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.

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

Lens sensitivityIn 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 (Ecutoff)

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),

Emission physics of RQ AGNs

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

• But:– Which is the electron

temperature? – Is there a non-thermal

component?• Current direct-viewing

telescopes and NuSTAR cannot study high energy tails.

• Photon 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 105 s

• In current CXB synthesis models (Gilli+ 2007) of assumption of RQ-AGN populations with • a distribution of

photon indices,• fixed Ecut (=200 keV)

• Is it right to assume a fixed EF ?

CXB (<100 keV)

• Likely due to Blazars.• But:

• The most recent results on Blazars are in 15-55 keV (Ajello+2009).

• Only 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/cm2 s).

• Origin still unknown. • Several models proposed:

• Dark matter;• Antimatter• Source of radioactive elements

like 26Al, 56Co, 44Ti• Gamma Source (e.g., Pulsar)• BH Binaries

• More sensitivity and imaging capability

Positron 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.