GRI: The Gamma-Ray Imager Mission
M. Hernanz?
Institut de Ciències de l’Espai (CSIC-IEEC), Facultat de Ciències, Campus UAB,
08193, Bellaterra, Spain. hernanz@ieec.uab.es
Summary. Gamma-rays reveal the most powerful sources and the most violent
phenomena in the Universe. Cosmic explosions and cosmic accelerators are the major science themes addressed in the gamma-ray regime. Europe is now playing a
leading role in the field of gamma-ray astronomy, thanks to ESA’s INTEGRAL observatory, launched in 2002. INTEGRAL is providing a global overview of the soft
gamma-ray sky, but deeper and more focused observations of gamma-ray sources
are needed. In soft X-rays a comparable step was taken going from the Einstein
satellite to the XMM/Newton observatory. Technological advances in the past years
in the domain of gamma-ray focusing using Laue diffraction have paved the way
towards a gamma-ray mission, providing major improvements regarding sensitivity
and angular resolution. Such a future Gamma-Ray Imager will allow to study particle acceleration processes and explosion physics in unprecedented detail, providing
essential clues on the innermost nature of the most violent and most energetic processes in the Universe. The main aspects of the mission, together with details about
the Spanish expected contribution will be presented to inform the community and
encourage additional contributions.
1 Gamma-ray astronomy and the INTEGRAL
observatory
Cosmic gamma-rays are the most energetic photons from the Universe, and
they trace the most energetic phenomena, such as cosmic acceleration and
cosmic explosions. Gamma-rays provide a view of the non thermal Universe, in
contrast with photons of lower energies, which mainly trace thermal processes.
There is a large variety of sites of gamma-ray emission: the nearby sun,
the very distant gamma-ray bursts and many other galactic and extragalactic
sources, such as binaries hosting a compact star (black hole, neutron star
or white dwarf), pulsars, supernova remnants, stellar explosions (novae and
supernovae), active galactic nuclei (AGN). Acceleration of electrons up to
?
on behalf of the GRI consortium, composed of members from the countries Denmark, France, Germany, Italy, Ireland, Poland, Portugal, Russia, Spain, The
Netherlands, United Kingdom and the United States. A complete list of GRI
consortium members can be found on http://gri.iasf-roma.inaf.it/.
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relativistic energies is at work in the vicinity of compact stars, i.e. in compact
binaries and AGNs, and also as a consequence of supernova explosions. These
electrons are responsible for the emission of gamma-rays when interacting
with electric and magnetic fields, as well as with photons. Protons can also
be accelerated and produce very energetic gamma-ray photons.
Another way to produce gamma-rays -in the MeV range- is through nuclear deexcitation of nuclei previously excited through collisions with energetic
cosmic rays; radiaoctive decay (β + or β − ) and electron capture processes also
produce nuclei in excited states, which emit gamma-ray photons when they
de-excite. It is worth reminding that β + decays produce positrons which annihilate with electrons giving a signature at 511 keV, that allows tracing
antimatter in the Universe.
The highly penetrating nature of gamma-rays unveils regions obscured at
longer wavelengths, thus revealing new classes of sources and regions where
crucial emission processes take place. In summary, a unique view of the Universe is provided by gamma-ray astronomy, allowing to probe non thermal
processes, to penetrate deep into compact objects, to trace radioactive nuclei
produced in cosmic explosions and so, to explore the most energetic phenomena of the evolving violent Universe.
However, this intrinsic penetrating nature of gamma-rays is a handicap
for their detection, specially in the MeV energy range. It is well known that
Compton scattering is the dominant process of matter radiation interaction
at those energies (whereas photoelectric absorption and pair production cross
sections are larger at lower and higher energies, respectively [7]). Compton
scattering is complicated to handle, and also its cross sections are pretty low,
as compared to their neighbor processes. All in all, detection of gamma-rays
in the MeV regime (where the positron annihilation line and most of the
radioactive lines lie) is really very challenging. In the 1990s, the COMPTON
Gamma-Ray Observatory, launched from the Space Shuttle, included in its
payload the Comptel instrument (a COMPton TELescope), which provided
a wealth of information during its period of activity (1991-2000) about the
radioactivity in the Galaxy, e.g. the first map of the radioactive nucleus 26 Al,
and the first detection of 44 Ti in a supernova remnant (Cas A). However, the
energetic resolution of the scintillation detectors of the Comptel instrument
was modest, as well as its spatial resolution. An important step forward in
both aspects came with ESA’s INTEGRAL observatory, launched in October
20021 .
The two main instruments of the INTEGRAL satellite are SPI and IBIS,
both with imaging capabilities thanks to their coded masks and to their
position-sensitive detectors, and covering the energy range 15 keV–10 MeV.
The fields of view of SPI and IBIS are 16◦ × 16◦ and 9◦ × 9◦ , respectively.
The spectral resolution of the spectrometer SPI (which has 19 high purity
Ge detectors in its focal plane) is excellent, ∼ 0.2% at 1 MeV. SPI has also
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see http://integral.esac.esa.int/ for details
GRI: The Gamma-Ray Imager Mission
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imaging capability (spatial resolution 2.5◦ ), which has allowed it to do the first
image of the 511 keV line emission from electron-positron annihilation all over
the sky. The precise line shape measurements provide important insights into
the various phases of the interstellar medium where annihilations occur; the
spatial distribution shows a dominant contribution from the galactic bulge,
a very puzzling discovery indeed. Regarding IBIS, its detector area includes
2 layers – a 2600 cm2 CdTe array (ISGRI) and a 3100 cm2 CsI array (PICSIT) – giving it an excellent spatial resolution of 12’ and a moderate spectral
resolution, ∼ 10% at 1 MeV. IBIS has discovered a large number of new
sources, enlarging very significantly the census of known gamma-ray emitters
(see [3] and references therein, for the third IBIS/ISGRI soft gamma-ray survey catalog, which comprises more than 400 high-energy sources detected in
the 18–100 keV range, during the first 3.5 years of INTEGRAL’s operation).
The good health of the instruments will enable INTEGRAL to provide the
most complete survey ever done in the energy range it covers.
2 A step forward: GRI
INTEGRAL has significantly improved our knowledge of the gamma-ray sky,
thanks to its very good imaging capability, spectral resolution and sensitivity (specially for narrow lines). Its wide field of view allows extensive surveys
of the hard X-ray and soft gamma-ray sky, providing the first all-sky maps
in those energy ranges and discovering many new sources, often of puzzling
nature. But there is a growing need to perform more focused studies of the
observed phenomena and reach phenomena not observable yet. A sensitivity
leap in the MeV regime, where most gamma-ray lines are emitted, is needed to
detect the nucleosynthetic products of individual supernovae and novae and
thus give a direct insight into the explosion mechanism. Point sources like
compact binaries, pulsars and AGNs, require higher sensitivity observations,
to uncover their yet poorly understood emission mechanisms. High angular
resolution observations of the galactic bulge, where enhanced positron annihilation has been unambiguously revealed by INTEGRAL/SPI, are crucial to
find the source of the positrons.
A major improvement in sensitivity in the MeV range, by a factor around
100, is required to detect the lines from supernova explosions, taking into account that those lines are very broad because of the large expansion velocities
of the ejecta. In addition, a significative improvement in angular resolution
is required to detect positrons in individual sources and thus locate the origin of antimatter in the Galaxy. Such a step forward is not easy to make,
since detecting photons in the MeV range is really challenging, as explained
in previous section.
A brief review of gamma-ray imaging techniques helps to clarify the situation and the available options. Current telescopes in this energy domain make
use of geometrical optics (shadow casting by a modulating aperture, as for
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M. Hernanz
instance a coded mask) or quantum optics (Compton scattering kinematics).
These techniques are reaching and impasse, since “bigger does not necessarily
mean better” [8]: an increase in collecting area leads to an increase in detector volume (and thus background), so the signal/background ratio does not
increase linearly with the collecting area. A solution to this impasse exists,
if an instrument concept based on coherent scattering is designed: inside a
crystal gamma-rays can interact coherently via Bragg diffraction. In a crystal, an incident photon is deviated an angle θ which depends on its energy E
and on the crystal plane spacing d, through the Bragg diffraction condition
2dsinθ = nλ = nhc/E. In a Laue diffraction lens, the crystals are arranged
in concentric rings, and oriented in such a way that incident photons of a
particular energy are focused on a small focal spot (and thus a small detector
volume). Therefore, a large collecting area (and thus signal) is perfectly compatible with a low background noise, and unprecedented sensitivities can be
attained [2, 9].
A prototype of a Laue diffraction lens, CLAIRE, has been developed in
CESR (Toulouse) and tested successfully in the lab, in a long distance test
performed near Figueres (Spain) and in a CNES balloon flight, (from the
french Alps to Bordeaux), with the collaboration of our institute (IEEC and
CSIC) [1, 4]. The gamma-ray lens concept has thus been proved and a space
mission based on it can be faced.
A gamma-ray lens with a complementary hard X-ray telescope, either
based on a coded mask or on a multilayer mirror, would achieve a broad
band energy coverage (from some tens of keVs to at least a MeV), allowing
detailed studies of particular sources with unprecedented sensitivity and angular resolution. A collaborative effort between many european countries and
the US (see http://gri.iasf-roma.inaf.it/), has started a couple of years ago
to define the scenario for a future gamma-ray mission (named the “GammaRay Imager”, GRI), which fits well into the framework of ESA’s Comic Vision
2015-2025 planning; this mission would be a perfect successor for INTEGRAL,
deepening the study of the phenomena this observatory has unveiled [5].
2.1 Science objectives
The scientific objectives of GRI encompass two major themes: cosmic explosions and cosmic accelerators [6]. GRI is expected to address and clarify
various important scientific topics:
Understand how thermonuclear (i.e, type Ia) supernova explosions work,
through the study of the gamma-ray lines of their radiaoctive explosion
products (profiles and light curves). The same applies to the less luminous
(in gamma-rays) core collapse supernovae.
• Unveil the source of galactic positrons, by high resolution imaging of the
galactic bulge at 511 keV.
• Discover nucleosynthesis products during nova outbursts to better understand their physics and explosion conditions.
•
GRI: The Gamma-Ray Imager Mission
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Learn how galactic compact objects accelerate matter.
Identify the nature of the hard emission tails dicovered by INTEGRAL in
Anomalous X-ray Pulsars and Soft Gamma-Ray Repeaters.
• Settle the question of the origin of the extragalactic soft gamma-ray background, by precise measurements of the spectral energy distributions of
AGNs, and by resolving the diffuse emission into individual sources.
• Analysis of particle acceleration in solar flares, by performing spectroimaging of active regions.
• Perform multi-wavelength studies of galactic sources revealed in adjacent
energy bands, in particular at GeV-TeV energies, by GLAST, HESS and
MAGIC, to constrain the nature of the accelerated particles.
•
•
2.2 The GRI mission
The GRI mission requirements derived from our scientific goals are summarized in table 1 (see [5] for details).
Table 1. Mission requirements for a future European gamma-ray mission (sensitivities are for 100 ks and a detection significance of 3σ)
Parameter
Energy band (keV)
Continuum sensitivity (ph cm−2 s−1 )
Narrow line sensitivity (ph cm−2 s−1 keV−1 )
Energy resolution
Field of view (arcmin)
Angular resolution (arcsec)
Time resolution (µs)
Polarization (for 10 mCrab)
Requirement
50-900
10−7
3 × 10−6
3%
5
60
100
5%
Goal
50-1300
3 × 10−8
10−6
0.5%
10
30
100
1%
GRI concept is based on a broad band Laue diffraction gamma-ray lens,
made of mosaic crystals. This type of crystals allows for a non-negligible energy bandpass, whereas with perfect crystals only very narrow lines could be
detected (this is an ideal case not found in supernova (or nova) explosions, a
major topic for GRI). One of the lines of interest is the 847 keV line, from
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Co, produced in type Ia supernovae. The associated focal length is very
large, typically ∼ 100 m (depending on the type of crystals used, Ge or Cu
according to recent studies performed at IKZ in Berlin and ILL in Grenoble). Therefore, it is clear that the lens and the detector should be separated
a large distance, which means that formation flying is required. Feasibility
studies performed by CNES have shown that the instrumental requirements
(less than ±10 cm in focal distance and ±1 cm in lateral displacement of the
detector spacecraft with respect to the lens axis) are perfectly affordable.
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M. Hernanz
A preliminary concept of GRI includes a Laue lens with two sets of crystals
(∼ 14000 Ge crystals and ∼ 21000 Cu crystals, covering two broad energy
bands around the crucial energies 511 keV and 847 keV, i.e. 160-520 keV and
800-900 keV, TBD), plus a complementary hard X-ray telescope for energies
lower than 200 keV. The way to cover larger energies around 1 MeV with
additional crystal rings is still under discussion.
The detector design is still under study; a detector stack either of CZT or
Ge, is the preferred option, and its use as an all-sky monitor is envisioned.
The Spanish technological contribution is still to be decided; but we already have started a collaborative effort with the CNM (CSIC) and the IFAE,
to build a detector prototype; we hope to enlarge this collaboration with other
Spanish centers, with known expertise in the field, as for instance GACE in
Universidad de Valencia. An important participation in GRI (both in the scientific and the instrumental part) would be a perfect follow-up of the large
and successful participation of Spain in INTEGRAL.
Acknowledgements
The GRI project is funded by CNES, ASI and Spanish MEC through AYA200406290-C02-01
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