Article

ORIGIN: Metal Creation and Evolution from the Cosmic Dawn

Experimental Astronomy (Impact Factor: 1.99). 04/2011; 34(2):519.
Source: arXiv

ABSTRACT

ORIGIN is a proposal for the M3 mission call of ESA aimed at the study of
metal creation from the epoch of cosmic dawn. Using high-spectral resolution in
the soft X-ray band, ORIGIN will be able to identify the physical conditions of
all abundant elements between C and Ni to red-shifts of z=10, and beyond. The
mission will answer questions such as: When were the first metals created? How
does the cosmic metal content evolve? Where do most of the metals reside in the
Universe? What is the role of metals in structure formation and evolution? To
reach out to the early Universe ORIGIN will use Gamma-Ray Bursts (GRBs) to
study their local environments in their host galaxies. This requires the
capability to slew the satellite in less than a minute to the GRB location. By
studying the chemical composition and properties of clusters of galaxies we can
extend the range of exploration to lower redshifts (z ~ 0.2). For this task we
need a high-resolution spectral imaging instrument with a large field of view.
Using the same instrument, we can also study the so far only partially detected
baryons in the Warm-Hot Intergalactic Medium (WHIM). The less dense part of the
WHIM will be studied using absorption lines at low redshift in the spectra for
GRBs.

Full-text

Available from: Daniele Spiga
Exp Astron
DOI 10.1007/s10686-011-9224-7
ORIGINAL ARTICLE
ORIGIN: metal creation and evolution
from the cosmic dawn
Jan-Willem den Herder · Luigi Piro · Takaya Ohashi · Chryssa Kouveliotou ·
Dieter H. Hartmann · Jelle S. Kaastra · L. Amati · M. I. Andersen ·
M. Arnaud · J.-L. Attéia · S. Bandler · M. Barbera · X. Barcons ·
S. Barthelmy · S. Basa · S. Basso · M. Boer · E. Branchini ·
G. Branduardi-Raymont · S. Borgani · A. Boyarsky · G. Brunetti ·
C. Budtz-Jorgensen · D. Burrows · N. Butler · S. Campana · E. Caroli ·
M. Ceballos · F. Christensen · E. Churazov · A. Comastri · L. Colasanti ·
R. Cole · R. Content · A. Corsi · E. Costantini · P. Conconi · G. Cusumano ·
J. de Plaa · A. De Rosa · M. Del Santo · S. Di Cosimo · M. De Pasquale ·
R. Doriese · S. Ettori · P. Evans · Y. Ezoe · L. Ferrari · H. Finger ·
T. Figueroa-Feliciano · P. Friedrich · R. Fujimoto · A. Furuzawa · J. Fynbo ·
F. Gatti · M. Galeazzi · N. Gehrels · B. Gendre · G. Ghirlanda · G. Ghisellini ·
M. Gilfanov · P. Giommi · M. Girardi · J. Grindlay · M. Cocchi · O. Godet ·
M. Guedel · F. Haardt · R. den Hartog · I. Hepburn · W. Hermsen · J. Hjorth ·
H. Hoekstra · A. Holland · A. Hornstrup · A. van der Horst · A. Hoshino ·
J. in ’t Zand · K. Irwin · Y. Ishisaki · P. Jonker · T. Kitayama · H. Kawahara ·
N. Kawai · R. Kelley · C. Kilbourne · P. de Korte · A. Kusenko · I. Kuvvetli ·
M. Labanti · C. Macculi · R. Maiolino · M. Mas Hesse · K. Matsushita ·
P. Mazzotta · D. McCammon · M. Méndez · R. Mignani · T. Mineo ·
K. Mitsuda · R. Mushotzky · S. Molendi · L. Moscardini · L. Natalucci ·
F. Nicastro · P. O’Brien · J. Osborne · F. Paerels · M. Page · S. Paltani ·
K. Pedersen · E. Perinati · T. Ponman · E. Pointecouteau · P. Predehl ·
S. Porter · A. Rasmussen · G. Rauw · H. Röttgering · M. Roncarelli ·
P. Rosati · E. Quadrini · O. Ruchayskiy · R. Salvaterra · S. Sasaki · K. Sato ·
S. Savaglio · J. Schaye · S. Sciortino · M. Shaposhnikov · R. Sharples ·
K. Shinozaki · D. Spiga · R. Sunyaev · Y. Suto · Y. Takei · N. Tanvir ·
M. Tashiro · T. Tamura · Y. Tawara · E. Troja · M. Tsujimoto · T. Tsuru ·
P. Ubertini · J. Ullom · E. Ursino · F. Verbunt · F. van de Voort · M. Viel ·
S. Wachter · D. Watson · M. Weisskopf
· N. Werner · N. White ·
R. Willingale · R. Wijers · N. Yamasaki · K. Yoshikawa · S. Zane
Received: 1 April 2011 / Accepted: 11 April 2011
©SpringerScience+BusinessMediaB.V.2011
J.-W. den Herder (
B
) · J. S. Kaastra · E. Costantini · J. de Plaa ·
R. den Hartog · W. Hermsen · J. in ’t Zand · P. Jonker · P. de Korte
SRON Netherlands Institute for Space Research,
Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
e-mail: J.W.A.den.Herder@sron.nl
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Abstract ORIGIN is a proposal for the M3 mission call of ESA aimed at the
study of metal creation from the epoch of cosmic dawn. Using high-spectral
resolution in the soft X-ray band, ORIGIN will be able to identify the physical
conditions of all abundant elements between C and Ni to red-shifts of z = 10,
and beyond. The missio n will answer questions such as: When were the first
metals created? How does th e cosmic metal content evolve? Where do most
of the metals reside in the Universe? What is the role of metals in structure
formation and evolution? To reach out to the early Universe ORIGIN will use
Gamma-Ray Bursts (GRBs) to study their local environments in their host
galaxies. This requires the capability to slew the satellite in less than a minute
to the GRB location. By studying the chemical composition and properties of
clusters of galaxies we can extend the range of explo ration to lower redshifts
(z 0.2). For this task we need a high-resolution spectral imaging instrument
with a large field of view. Using the same instrument, we can also study the
so far only partially detected baryons in the Warm-Hot Intergalactic Medium
(WHIM). The less dense part of the WHIM will be studied using absorption
lines at low redshift in the spectra for GRBs. The ORIGIN mission includes a
Transient Event Detector (coded mask with a sensitivity of 0.4 photon/cm
2
/s
in 10 s in the 5–150 keV band) to identify and localize 2000 GRBs over a five
year mission, of which 65 GRBs have a redshift >7. The Cryogenic Imaging
Spectrometer, with a spectral resolution of 2.5 eV, a field of view of 30 arcmin
L. Piro · L. Colasanti · A. Corsi · A. De Rosa · M. Del Santo · S. Di Cosimo ·
B. Gendre · M. Cocchi · C. Macculi · L. Natalucci · P. Ubertini
INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica, Rome, Italy
T. Ohashi · Y. Ezoe · Y. Ishisaki · H. Kawahara · S. Sasaki
Tokyo Metropolitan University, Tokyo, Japan
C. Kouveliotou · A. van der Horst · M. Weisskopf
Marshall Space Flight Center, Huntsville, AL, USA
D. H. Hartmann
Department of Physics and Astronomy, Clemson University, Clemson, SC, USA
L. Amati · E. Caroli · M. Labanti
INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica, Bologna, Italy
M. I. Andersen · J. Fynbo · J. Hjorth · K. Pedersen · D. Watson
Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen,
Copenhagen, Denmark
M. Arnaud
Service d’Astrophysique, CEA Saclay, Gif-sur-Yvette, France
J.-L. Attéia
Observatoire Midi-Pyrénées, LAT, Toulouse, France
S. Bandler · S. Barthelmy · N. Gehrels · R. Kelley · C. Kilbourne · S. Porter ·
E. Troja · N. White
NASA Goddard Space Flight Center, Greenbelt, MD, USA
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and large effective area below 1 keV has the sensitivity to study clusters up
to a significant fraction of the virial radius and to map the denser parts of
the WHIM (factor 30 higher than achievable with current instruments). The
payload is complemented by a Burst InfraRed Telescope to enable onboard
red-shift determination of GRBs (hence securing proper follow up of high-z
bursts) and also probes the mildly ionized state of the gas. Fast repointing is
achieved by a dedicated Controlled Momentum Gyro and a low background is
achieved by the selected low Earth orbit.
Keywords X-ray · Mission · Gamma-ray bursts · Clusters of galaxies ·
Warm-hot intergalactic medium · Chemical evolution
1 Introduction
Metals play a very important role in star formation and stellar evolution,
and ultimately lay the foundation of planet formation and the d evelo pment
of life. Beginning with metal free (Population III) stars, the cycle of metal
enrichment started when their final explosive stages injected the first elements
beyond Hydrogen and Helium into their pristine surroundings [17]. These
ejecta created the seeds for the next generation of stars (Population II). So
the cycle of cosmic chemical evolution began. Baryons trapped in dark matter
M. Barbera · G. Cusumano · T. Mineo · E. Perinati · S. Sciortino
INAF-Istituto di Astrofisica Spaziale, Palermo, Italy
X. Barcons · M. Ceballos
IFCA, Santander, Spain
S. Basa
Observatoire de Marseille, Marseille, France
S. Basso · S. Campana · P. Conconi · G. Ghirlanda · G. Ghisellini · D. Spiga
INAF, Osservatorio Astronomico Brera, Milan, Italy
M. Boer
Observatoire de Haute Provence, Haute Provence, France
E. Branchini · E. Ursino
Università Roma III, Rome, Italy
G. Branduardi-Raymont · R. Cole · M. De Pasquale · I. Hepburn · R. Mignani ·
M. Page · S. Zane
Mullard Space Science Laboratory, University College of London, London, UK
S. Borgani · M. Girardi · M. Viel
INAF-Osservatorio Astronomico, Trieste, Italy
A. Boyarsky
CERN, Geneva, Switzerland
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potential wells started to form stars, which then became the building blocks of
proto-galaxies. Galaxies merged, building up massive structures, and provided
the energetic radiation that re-ionized and lit up the dark Universe, starting the
cosmic dawn [21]. Finding out when these first stars were created and how the
metal abundances evolved is the core quest of ORIGIN. Models of hierarchical
structure formation suggest that these early proto-galaxies had low masses,
small luminosities , and were metal poor. Simulations of star formation in these
environments indicate that the typical mass of the earliest stars exceeded a
hundred solar masses [29]. These processes started a few hundred million years
after the Big Bang. Identifying objects at these look back times is a frontier of
observational cosmology. Ultra-deep surveys in the optical and the infrared
have resulted in a few detections already out to redshifts of 10 (about 500
million years after the Big Bang), but it is clear that star formation started
even earlier. The only natural phenomena that can directly probe the baryonic
environments of these first stars are Gamma Ray Bursts (GRBs) [6, 14].
These ultra-luminous explosions are believed to be embedded in star-forming
G. Brunetti
INAF-IRA, Bologna, Italy
C. Budtz-Jorgensen · F. Christensen · A. Hornstrup · I. Kuvvetli
DNSC/Technical University of Denmark, Copenhagen, Denmark
D. Burrows
Penn State University, University Park, Philadelphia, PA, USA
N. Butler
University of California, Berkeley, CA, USA
E. Churazov · M. Gilfanov · R. Sunyaev
Max-Planck-Insitut für Astrophysik, Müchen, Federal Republic of Germany
A. Comastri · S. Ettori · L. Moscardini
INAF-Osservatorio Astronomico, Bologna, Italy
R. Content · R. Sharples
Durham University, Durham, UK
R. Doriese · K. Irwin · J. Ullom
NIST, Boulder, CO, USA
P. Evans · P. O’Brien · J. Osborne · N. Tanvir · R. Willingale
Leicester University, Leicester, UK
L. Ferrari · F. Gatti
Istituto Nazionale di Fisica Nucleare, Genova, Italy
H. Finger
University Space Research Association, Huntsvile, AL, USA
T. Figueroa-Feliciano
MIT, Cambridge, MA, USA
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regions, effectively acting as beacons that illuminate and pinpoint the unique
cradles of early nucleosynthesis [9].
Following these early phases, gravity leads from the first population of
stars in small galaxies to large-scale clusters of galaxies. The production and
distribution of elements within these evolving dynamic structures needs to be
mapped in greater detail than has been achieved to date. Simulations of large-
scale structure by Springel et al. [32], and others, beautifully demonstrate how
the power spectrum of matter evolves, and how voids and filaments emerge as
natural features of the dynamic Universe. Simulations of Dark Matter (DM)
on the scale of the Milky Way’s halo [13]showthattheDMdominatedlocal
environment is highly structured as well. Baryons, making up only about 4%
of the cosmic budget, trace some of these structures, and are essential to our
ability to gather knowledge of how and when the Universe produces stars and
gaseous flows. Only a fraction of the baryons end up in stars, most remain
in diffuse structures; and some baryons that did end up in stars, re-emerge in
the diffuse component after nuclear processing in the explosive final stages
of massive stars. Transport of gas within galaxies, i.e., infall into and outflow
from galaxies, and similar processes on the scale of clusters, creat ed a rich
distribution of metal-enriched gas on small and large scales. We need an
accounting of the whereabouts and the conditions of the cosmic baryons to
understand the feedback processes responsible for the density-temperature-
P. Friedrich · P. Predehl · S. Savaglio
Max-Planck-Institut für Extraterrestrische Physik, Garching, Federal Republic of Germany
R. Fujimoto · A. Hoshino
Kanazawa University, Kanazawa, Japan
A. Furuzawa · Y. Tawara
Nagoya University, Aichi, Japan
M. Galeazzi
University of Miami, Coral Gables, FL, USA
P. Giommi
ASI Data Center, Frascati, Italy
J. Grindlay
CfA, Harvard University, Cambridge, MA, USA
O. Godet · E. Pointecouteau
CESR Centre d’Etude Spatiale des Rayonnements, Toulouse, France
M. Guedel
University of Vienna, Vienna, Austria
F. Haardt · R. Salvaterra
University of Insubria, Como, Italy
H. Hoekstra · H. Röttgering · J. Schaye · F. van de Voort
Leiden University, Leiden, The Netherlands
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abundance p atter ns that are observed in galaxies, clusters of galaxies, and the
filamentary bridges between clusters predicted in simulations. Extending our
knowledge about the metal enrichment processes on large scales is the second
quest of ORIGIN. The study of chemical composition in clusters as a function
of redshift will help us understand the conditions under which these structur es
were formed. In the local Universe the metal content of the WHIM can be
used to distinguish between different metal diffusion models.
The study of how metal enrichment proceeded from the initial (primordial)
conditions emerging from Big Bang Nucleosynthesis to those of present stars
and galaxies is a formidable challenge, but is now possible for the first time
due to advances in cryogenic X-ray detectors. X-ray spectroscopy has the
unique capability of simultaneously probing all the elements (C through Ni),
in all their ionization stages and all binding states (atomic, molecular, and
solid), and thus provides a model-independent s urvey of the metals. ORIGIN
employs high-resolution X-ray spectroscopy and imaging to detect most of
these elements to very high redsh ifts. The most distant star forming regions
can be probed with rapid response spectroscopy of bright GRBs and local
cluster structures can be studied with a wide field of view imaging survey. The
filamentary WHIM is probed along the sight lines of the GRBs in absorption
and can also be mapped using the wide field of view imaging capability of
ORIGIN. Infrared spectroscopy will provide independent and complementary
A. Holland
Open University, Milton Keynes, UK
T. Kitayama
Toho University, Chiba, Japan
N. Kawai
Tokyo Institute of Technology, Tokyo, Japan
A. Kusenko
University of California at Los Angeles, Los Angeles, CA, USA
R. Maiolino · F. Nicastro
INAF-Osservatorio Astronomico di Roma, Rome, Italy
M. Mas Hesse
Centro de Astrobiología (CSIC-INTA), Madrid, Spain
K. Matsushita · K. Sato
Tokyo University of Science, Tokyo, Japan
P. Mazzotta
Universitá de Roma Tor Vergata, Rome, Italy
D. McCammon
University of Wisconsin, Madison, WI, USA
M. Méndez
Groningen University, Groningen, The Netherlands
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information on chemical abundances by detecting low ionization absorption
lines. Thus a combination of near-field cluster surveys with far-field GRB
response-spectroscopy provides an optimal strategy to map cosmic chemical
evolution from re-ionization to the present.
Figure 1 demonstrates the observational space uniquely address by ORI-
GIN. The combination of fast re-pointing response with the high spectral
resolution (R) and the large grasp (Area × Field of View), make ORIGIN
a powerful tool for transient and wide f ield high-resolution spectroscopy.In
contrast to previous GRB-dedicated satellites (e.g. Swift), ORIGIN will be
totally autonomous in determ ining not only the location, but also the redshift,
physics and chemistry of the ISM surrounding the GRB. Compared to the
ASTRO-H mission, a Japanese mission to be launched in 2014, ORIGIN has,
in addition to the fast response, a larger field of view (factor of 100) and a larger
effective area below 2 keV (factor of 7), combined with a factor of 2 better
spectral resolution. ORIGIN is highly compl ementary to the capabilities of the
International X-ray Observatory (IXO), which has higher angular resolution
and effective area.
K. Mitsuda · Y. Takei · T. Tamura · M. Tsujimoto · N. Yamasaki
Institute of Space and Astronautical Science, JAXA, Kanagawa, Japan
R. Mushotzky
University of Maryland, College Park, MD, USA
S. Molendi · E. Quadrini
INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica, Milano, Italy
F. Paerels
Columbia University, Columbia, NY, USA
S. Paltani
ISDC, University of Geneva, Versoix, Switzerland
T. Ponman
University of Birmingham, Birmingham, UK
A. Rasmussen · N. Werner
KIPAC/Stanford, Palo Alto, CA, USA
G. Rauw
Liege University, Liege, Belgium
M. Roncarelli
University of Bologna, Bologna, Italy
P. Rosati
ESO, Garching, Federal Republic of Germany
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Fig. 1 The key characteristics
of ORIGIN (grasp, reaction
time, resolution) compared to
existing and proposed
missions
2 Science
With its unique capability of high spectral resolution and imaging, ORIGIN
will advance many fields of astrophysics. Its continuous m onitoring of a large
part of the sky will further increase the scientific return, as ORIGIN is sensitive
to all types of transient phenomena in the hard X-ray band. In this section,
we limit ourselves, however, to the main quests of ORIGIN: the study of the
cosmic metal enrichment history.
O. Ruchayskiy · M. Shaposhnikov
Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland
K. Shinozaki
Aerospace, Research and Development Directorate, JAXA, Ibaraki, Japan
Y. Suto
University of Tokyo, Tokyo, Japan
M. Tashiro
Saitama University, Saitama, Japan
T. Tsuru
Kyoto University, Kyoto, Japan
F. Verbunt
Utrecht University, Utrecht, The Netherlands
S. Wachter
Caltech, Pasadena, CA, USA
R. Wijers
University of Amsterdam, Amsterdam, The Netherlands
K. Yoshikawa
Tsukuba University, Ibaraki, Japan
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2.1 The first stars
ORIGIN is uniquely designed to answer several key questions about star
forming processes in the early Universe. By measuring GRB redshifts and
Fig. 2 Top panel Simulated
CRIS X-ray spectrum of a
medium bright afterglow
(S
X
= 4 × 10
6
erg/cm
2
integrated between
50 s–50 ks) at z = 7
characterized by deep narrow
resonant lines of Fe, Si, S, Ar,
Mg, from the gas in the
environment of the GRB. An
effective column density of
2 × 10
22
cm
2
has been
adopted, consistent with the
values observed in GRB
afterglows. Middle top panel
IR spectrum of GRB050904
at z = 6.3 as observed with
Subaru 3.4 days after the
burst [19]. Middle bottom and
bottom panel Simulation of
the same spectrum with
BIRT, starting 280 s after the
GRB trigger and lasting for
1,000 s. Only a part of the
BIRT wavelength range is
shown. The Lyman break is
very well identified in the
spectrum as well as most of
the key absorption lines
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abundances in the circumburst medium deep into the era of re-ionization
(z > 6), we will discover when star formation started and how it evolved into
the present day structures. Most importantly, ORIGIN is the only Cosmology
mission that can chart the abundance patterns that prevailed in these early
dark matter—baryon systems.
GRB explosions taking place in these proto-galaxies would be easily de-
tected with ORIGIN. Figure 2 shows a simulated X-ray afterglow of an
explosion at z = 7, as measured with the ORIGIN/CRIS. Multiple n arrow
spectral lines are identified with ionized metals in the burst environment,
allowing measurement of relative abundances and the determination of the
redshift. To secure redshift measurements even in the case of exceedingly low
metallicities, the mission also carries an IR telescope to measure the Lyman
break in GRB spectra. This IR telescope provides also column densities for
low ionization states of elements.
ORIGIN will collect about 400 GRBs/year covering the full redshift distri-
bution. About twice per mon th a GRB from the re-ionization era will trigger
the instruments. The resulting multi-element abundance patterns will map the
evolving chemical composition of the early Universe, “fingerprint” the elusive
Pop III stars, and constrain the sha pe of the Initial Mass Function (IMF) of the
first stars [29].
2.2 The history of metal production in clusters of galaxies
The cosmic history of metal production and the circulation of these metals
throughout the Universe is a fundamental astrophysical question. Clusters
of galaxies are excellent laboratories to study these processes since 85%
of the baryons are in the hot X-ray emitting gas and, due to their deep
gravitational potential, clusters retain all the metals that were produced inside
them. High-resolution X-ray spectroscopy of this gas will unveil the history of
nucleosynthesis (Fig. 3).
Almost all metals heavier than oxygen are produced by supernova (SN)
progenitors and most of the atoms heavier than silicon originate from the
SNIa sub-class. It is still unknown which progenitor systems produce all,
or the bulk of Type Ia explosions. Analysis of the chemical abundance of
clusters with CCD detectors on XMM-Newton and Suzaku have shown an
abundance pattern of Si, S, Ca, Ar, Fe and Ni which is not consistent with
the theoretical predictions of classical SNIa. This indicates either that the
theory needs modification, as detailed analysis of the brightest SNIa remnant
(Tycho) indicates, or that clusters are predominantly enriched by different
types of SNIa . However, these data have only been obtained for the very
brightest, local clusters and even then only in their central regions. It is thus
not clear how general these results are. ORIGIN measures a much wider
range of clusters over a large redshift range, and determines the precise rati os
of elemental abundances such as Ca to Ar and Ni to Fe which are sensitive
to the details of the explosion mechanism. Thus the ORIGIN X-ray spectra
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Fig. 3 Top Expected
abundance ratios relative to
iron for a 100 ks exposure
with ORIGIN for a typical
cluster, showing the
contributions from AGB
stars, SN Ia an SNcc [12, 36 ].
The relative weight of low
and high Z elements varies
considerably for the different
contributions. ORIGIN will
measure 20 elements from C
to Ni (XMM-Newton can
only measure 7elements).
Middle Total metal yields
different SNIa models
compared to a standard
WDD2 model. Bottom Total
ccSNe yields for a top-heavy
IMF compared to the yields
for a Salpeter IMF [11, 25].
ORIGIN will discriminate
these SN Ia models and the
IMF by accurately measuring
the relative metal content
will constrain the nucleosynthesis models for SNIa (see Fig. 3 top panel for
typical abundance ratio’s). In addition to the abundant elements ORIGIN will
measure trace elements like Na, Al, Ti, Cr, Mn and Co, integrated over the
cluster core. The Mn/Cr ratio is a sensitive tracer of the metallicity of the
progenitor, while the Na abundance is a sensitive measure of the slope of
the IMF. Thus, measurements of the abundances of these elements reveal the
epoch when these systems were formed and their IMFs. The bulk of the lighter
metals (O, Ne, Mg) are formed by core-collapse supernovae (ccSNe or SNII),
although these systems also produce heavier elements. To disentangle both
contributions, the full range of elements from O to Ni ne eds to be measured.
SNII have massive, short-lived progenitors, so the bulk of the metals created
by them is produced and redistributed rapidly [27] after star formation, starting
at the epoch of re-ionization and peaking around z = 2.However,details
about the IMF and what these stars exactly produced are not well known [2].
Measuring the abundances of N-Mg will constrain these parameters (Fig. 3).
Finally, C and N have a different origin in intermediate-mass AGB stars, and
are returned to the ISM by stellar mass loss. When and where these metals
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were produced is uncertain, and ORIGIN will be able for the first time to map
them both in space and time.
2.3 Evolution of clusters of galaxies
While the integrated time history of metal enrichment can be studied in detail
in a sample of relatively nearby bright clusters and groups of galaxies, their
evolution can also be studied directly by observing clusters as a function of
redshift. Until now, it has only b een possible to measure evolution of the Fe
content (see Fig. 4). With ORIGIN, we will obtain accurate abundances of
many key elements, including iron (mostly from SNIa), oxygen (mostly from
ccSNe) and nitrogen (predominantly AGB stars) out to cluster redshifts of
1.3, 1.0, and 0.8, respectively. We will also obtain abundances of several other
elements and will address the following questions: How do the abundances in
clusters evolve over time? Did ccSNe dominate at early stages, or was there a
more complex evolving population? Is the AGB star population co-evolving
with the SNIa population?
2.4 Cosmic filaments
Cosmological hydrodynamic simulations suggest that the missing baryons at
z < 2,contributing40% of the cosmic baryon budget, can be accounted
for by a diffuse, highly ionized WHIM, preferentially distributed in large-
scale filaments connecting the nodes of the Cosmic Web [10]. This gas is
extremely hard to detect: its bulk resides in structures with T > 10
6
Kbut
the thermal continuum emission is much too faint to be detectable against
the overwhelming fore - and back-ground emission. The only characteristic
radiation from this medium will be from discrete transitions of highly ionized
Fig. 4 Time evolution of the
iron abundance in a sample of
high-z clusters as measured
with Chandra. Each point is
the average for 10 clusters
[1]. ORIGIN will do similar
measurements out to z > 1.2
for O (as shown), Ne, Mg, Si,
S and Fe and for five other
elements up to z 0.5
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C, N, O, Ne, and possibly Fe. At T 10
6
K, the primary tracers [8](OVII,
OVIII)cannotbeprobedwithUVobservationsandareonlydetectableby
soft X-ray spectroscopy [26]. Indications of the WHIM in this temperature
range were obtained with Chandra and XMM-Newton observations of the
21.6 Å resonance absorption line of OVII [24] in the sight line of the Sculptor
Wall. Evidence for the warm tail of the WHIM, where 10–15% of the missing
baryons reside, has been obtained via UV-absorption line studies with FUSE
and HST-COS. ORIGIN will have sufficient line sensitivity and energy res-
olution to measure gas de nsitie s down to 10
5
cm
3
, 30 times smaller than
currently probed in clusters. ORIGIN can detect these WHIM lines both in
emission, and in absorption against early-stage GRB afterglows, which are the
only sufficiently bright, numerous, and distant (z > 1) sources to guarantee a
statistically significant sample size of WHIM detections. An example of what
GRB X-ray spectroscopy would yield is shown in Fig. 5. We modeled the
properties of the WHIM with large scale DM + hydrodynamic simulations,
with a parameterized treatment of stellar feedback, u sing the simulations of
Borgani et al. [5]andVieletal.[35] to investigate various WHIM models.
While mere detections of X-ray absorption and emission from ionized
metals will reveal the prese nce of the ‘missing baryons’, our more am bitio us
goal is to detect and characterize the physical state of the WHIM: its temper-
ature, density, spatial distribution, and trace the metal enrichment of the IGM
and its interplay with the history of star formation and feedback processes.
Figure 6 illustrates how well ORIGIN will discriminate among different IGM
metal enrichment models through absorption spectroscopy of the WHIM. In
absorption we expect to measure about 300 filaments in 5 years from a sample
of 500 bright afterglows [4]. These observations will allow us to estimate the
temperature and the density of each absorption system with 15% accuracy.
In addition, for about 30% of these systems we will also detect the associated
X-ray line emission, once the afterglow has faded. A deep field of several
Fig. 5 Emission spectrum of
a4arcmin
2
area (top), and
absorption spectrum (bottom)
of the same region of the sky,
as measured by ORIGIN. The
top panel shows the emission
of two red-shifted
components in black,while
the emission of the Galactic
foreground is displayed in
purple.Thebottom panel
shows the spectrum of the
same systems, but now in
absorption using a bright
GRB afterglow as a beacon
Page 13
Exp Astron
Fig. 6 Top filaments in absorption. Solid lines show predicted number of OVII absorption lines
per unit redshift as a function of line EW. Black bars show the OVII line statistics built up in
1yearandmagentain5yearswithORIGIN.Thecurves are for different metal diffusion models:
highly localized to the region of metal synthesis (red), and diffusion into the IGM via SN feedback-
related processes (black). These two models represent the spread in current theoretical predictions
and can, already after a year, be clearly distinguished. Bottom filaments in emission, showing the
expected number of O VII+O VIII lines per unit redshift above a given O VII surface brightness.
The cyan area gives the observational uncertainties assuming the high-metal diffusion model. It
illustrates that the measurements are very distinctive. The case refers to a 50
%%
× 50
%%
field, observed
for 1 Ms with ORIGIN
square degrees will secure faint emission structures over 16 Mpc at z = 0.3.
Intensities of emission line (scales as n
2
L) and absorption lines (nL) from the
same WHIM cloud will yield the density n and the line of sight depth L in a
very effective way.
A1MsexposurewithORIGINwillrevealabout1200emittingsystemsper
deg
2
with joint 5σ detection of O VII and O VIII line, and will measure the
temperature with 30% error [33, 34]. Figure 6 shows the expected dN/dz
of these emitters. The observation of so many different WHIM properties
will allow discrimination among different stellar feedback and metal diffusion
mechanisms. Combining constraints from different observations lifts possible
model degeneracy.
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Exp Astron
3 Mission profile
To enable the science described above we use a Transient Event Detector
(TED) to locate GRBs and fast repointing to observe the GRB afterglow with
a wide field X-ray telescope equipped with a Cryogenic Imaging Spectrometer
(CRIS) alongside a capable Burst InfraRed Telescope (BIRT). The wide field
of the X-ray instrument also provides sensitive complimentary observations of
the local Universe. This powerful combination gives us three different tracers
of chemical evolution covering three different epochs.
Absorption lines from the environment of GRBs exhibit Equivalent Widths
of 1eV,whileevenweakerabsorptionlinesatEW0.2 eV are imprinted
by the cosmic filaments. Detection of such weak lines requires a fluence S >
500 photons eV
1
,correspondingto10
6
erg cm
2
in the 0.3–10 keV band.
This requirement naturally implies an effective area A > 1000 cm
2
and a
spectral resolution !E 2.5 eV, which is provided by the Cryogenic Imaging
Spectrometer (CRIS). Because GRB afterglows fade quickly, one needs rapid
localization and repointing capability, with a spectrometer pointing at the
source within 60 s after the trigger. A Transient Event Detector (TED) with
FoV of 4srandasensitivityof0.4photoncm
2
s
1
between 5–150 keV,
integrated over 10 s, is required to detect at 12σ,andthuslocalizewithin3
%
,
2000 GRBs in 5 years. From the prompt and afterglow fluence distributions
observed by Swift we expect 500 afterglows with fluence >10
6
erg cm
2
,
sufficient to carry out high-resolution spectroscopy. Out of the 2000 GRB,
TED will provide 125 GRBs at z > 6and65atz>7over the mi ssion life time.
This sample allows us to derive quantitative conclusions. For the brightest
afterglows (fluence >10
6
erg cm
2
in the 0.3–10 keV band), we can measure
metal column densities as low as H equivalent 10
21
cm
2
. This will allow us to
access gas at metallicities as low as 1% of solar for the de nser regions expected
in early stars; in even denser regions (N
H
> 10
23
cm
2
) the accuracy will be
further improved. The redshift of these afterglows will be measured with a
precision of 0.1%. For regions of very low metallicity, the redshift will be
secured by measuring the Lyman break. Therefore aBurstInfra-RedTelescope
(BIRT) complements the payload. With a resolution of 20 over the range of
0.5–1.7 µm, it allows the determination of the redshift of all observed bursts
between 5.5 and 12 using the Lyman break in the spectra within 1%. With an
additional resolution of 1000, this instrument can also measure low ionization
lines, complementing the characterization of metallicity.
ORIGIN will complement its study of the high-z Universe with studies of
the metal content at lower redshifts (clusters of galaxies, WHIM). This requires
alargefieldofviewandequallygoodenergyresolution.Here,spectral
resolution is set by the required contrast of the extraordinarily faint emis-
sion against the background (in strum ent background, unresolved extragalactic
point sources, galactic foreground emission and, in the case of clusters, thermal
continuum emission). The low background, crucial for these measurements, is
achievable with a Low Earth Orbit (LEO), a small focal ratio of the telescope,
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Exp Astron
and optimized detector shielding. The instrumental background will be as low
as 2 · 10
5
counts s
1
arcmin
2
keV
1
,whichisaboutanorderofmagnitude
lower than the level of the cosmic X-ray background [22, 31]. The angular
resolution of the CRIS is set by the typical size of gas concentrations in WHIM
filaments of about 30
%%
(100 kpc at z = 0.2 ), the field of view and the capability
to spatially resolve the emission (e.g. of groups of galaxies, turbulent velocit y
in clusters, etc.). With a 30
%%
HEW and a 30
%
field of view we will be able to
observe these objects in a single or a few adjacent observations. The expected
characteristic emission line intensity of the WHIM is 0.1 photon cm
2
s
1
sr
1
in the strongest O K-shell line (out to redshift 0.3). CRIS will detect such an
emission from a typical filament at the 5σ level easily in 1 Ms.
Finally, the fast repointing capability will allow ORIGIN to measure the
WHIM f ilaments in absorption using GRBs as backlights (for 250 line of
sights). Combined with measurements of these same filaments in emission, the
density of the filament can be uniquely determined.
The prime goal of the IR telescope (BIRT) is to enable the determination
of the redshift for GRBs at z > 5.5 irrespective of the metallicity. This is
achieved with low-resolution spectra fo r GRBs over the range 0.7–1.7 µm
corresponding to the redshifted Lyman break. The collecting area should be
such that a large fraction of all GRBs will provide a good detection. BIRT, with
alimitingmagnitudeofH
AB
= 22 in imaging mode, covers the known decay
of GRBs observed by Swift and previous missions. Obscured bursts, which will
escape the redshift determination by BIRT, will have large column densities
and therefore their redshifts can be determined by CRIS.
3.1 Detection of high redshift GRBs
GRBs are amongst the best sources to study the high redshift Universe, due to
their existence at high redshift, combined with their exceptional brilliance (cf.
galaxies and QSOs) and lack of proximity effects (cf. QSOs). The afterglow
flux of high-z events is comparable to those of closer bursts, due to the effect
of spectral K-correction and because time dilation leads to sampling of the
earlier, brighter part of the afterglow.
The TED sensitivity, low energy threshold and field of view were optimized
in order to localize high-z bursts. The low energy threshold, compared to Swift,
has the double advantage of increa sing the sensitivity and bringing into the
instrument bandpass more high-z events, whose peak energy is redshifted. It
also makes the instrument sensitive for X-ray flashes. A larger solid angle
increases the number of events. In the trade-off between the sensitivity and
solid angle we have favored the latter. This choice results in a larger number of
(high-z) events characterized by afterglows bright enough to derive a redshift
on the fly. Thus the four modules of the TED are orie nted in different direc-
tions, providing a total field of view of 4 sr. The expected number of high-z
bursts has been calculated based on the independent models described in
Salvaterra et al. [28]andButleretal.[7]. Both models reproduce the observed
logN–logS relation. They also reproduce the number of high-z bursts (z > 5)
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Exp Astron
Fig. 7 The expected
distribution in redshift of
events localized by TED in
one year. The red histogram
and corresponding curve are
for Swift [7]. Using the
same model the ORIGIN
expectations are shown
in blue
observed by Swift, taking into account that this number is a lower limit, given
the observational bias against the identification of high redshift bursts. Most
models indicate that the rate of GRBs increases faster than the SFR at high
redshift.
By folding into Butler’s model the TED performance we expect to detect
about 400 bursts per year, 25 at z > 6 and 13 with z > 7. From the model of
Salvaterra et al. [28]wederiverangeswhichareconsistentwiththemodelof
Butler et al. [7]. If we take the most pessimistic estimate we derive a lower limit
of 12 GRB at z > 7 in 5 years. These numbers are consistent with the fraction
of high redshift (z > 6) bursts estimated from samples of optical-IR follow up
of Swift bursts [16, 18]. TED will deliver a factor 4 more bursts than Swift due
to its decreased low energy threshold and increase in solid angle (see Fig. 7).
3.2 Observation program
The observation program covers 4 key topics: (a) when a GRB is detected the
satellite will slew to this position and determine the redshift in <2 ks using
BIRT. Bright or high redshift GRBs will be observed for a total of 50 ks.
For the bright GRBs this is sufficient to collect typically 10
6
counts in the
X-ray spectrum. For these sources BIRT will provide photometry and a high
resolution spectrum (R = 1000). These events will be used to study GRB host
galaxies. At the same time bright GRBs are used as backlight to detect the
filaments of the cosmic web in absorption. A total of 2000 GRBs over 5 year
are expected, of which we observe 500 bright GRBs for 5 0 ks; (b) to study
metal abundances in the nearby Universe observations of different clusters
is planned. These observations cover abundance patterns inside clusters, the
metal content up to a significant fraction of their virial radii and the evolution
of clusters with redshift. A total of 20 Ms is needed for this; (c) the present
Universe will be studied by a deep map of 2.5 × 2 deg
2
and we expect with the
given sensitivity to characterize the denser part of filaments (ρ
gas
/'ρ
gas
( > 80)
in the O VII and O VIII lines. In addition there will be a guest observer
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Exp Astron
Fig. 8 The three ORIGIN instruments including their main components. One TED unit at the
back is visible (orange). The lower section of BIRT contains the primary and secondary mirror
program (30% of the time) which allows for the community to exploit the
unique capabilities of ORIGIN (Fig. 8).
4Instruments
The mission requirements correspond to a payload with three instruments: The
Cryogenic Imaging Spectrometer (CRIS) is the prime instrument. It allows for
wide field imaging of a 30
%
area on the sky with a spectral resolution <2.5 eV
(inner array) and <5 eV (outer array). Its effective area is >1,500 cm
2
at
1keVand>150 cm
2
at 6 keV. A separate section of the detector has been
optimized for the detection of the very high count rates expected from GRB
afterglows. Its confusion limit is 10
15
erg cm
2
s
1
for 0.5–2 keV and its point
source line detection sensitivity at 0.5 keV (5σ) is typical 2 · 10
7
photons
cm
2
s
1
for a 100 ks observation. The Transient Event Detector (TED) will
detect GRBs in the 5–150 keV energy range, similar to the Swift/BAT. Its
solid angle is >4 sr and its sensitivity is >0.4 photons/cm
2
/s in the 5–150 keV
range. The Burst InfraRed Telescope (BIRT) has the prime goal to determine
Page 18
Exp Astron
the redshift for all bursts beyond z > 5 .5 ,irrespectiveofthemetallicityof
the host galaxy. It has a bandpass of 0.5–1.7 µm, an H-band sensitivity limit
H
AB
= 20.8, and a field of view of 6
%
× 6
%
with a low-resolution spectrometer
(prism, mode LOW-RES). We will implement two additional BIRT modes:
aphotometricmode(IMAGE)withasensitivitylimitofH
AB
= 22.2,which
allows an accurate determination of positions in four different bands; and a
high resolution mode (HIGH-RES) with R 1000 to derive column densities.
The relevant absorption lines yield redshifts for GRBs at z < 5.5.Thethree
instruments work together for GRBs as illustrated in Fig. 9.
Fig. 9 Timeline of ORIGIN observations following a GRB detection. Within 200 s the redshift of
the burst is determined by BIRT
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Exp Astron
Bright GRBs (>10
6
erg/cm
2
in the 0.3–10 keV band) will be localized
coarsely (<6
%
) by TED. The spacecraft will slew rapidly and observations by
CRIS and BIRT commence. CRIS will use its GRB spectrometer section to
obtain a sub-arcmin position, while BIRT will use its LOW-RES spectrometer
to obtain a rapid redshift. A shorter slew will then allow use of the BIRT
IMAGE mode to identify a sub-arcsec position, while the wide field of the
CRIS spectrometer will allow X-ray data collection to continue. BIRT will
also obtain spectra for bright GRBs or fainter GRBs at low redshifts: position
information from the IMAGE mode of BIRT is fed to a small tip/tilt mirror
(<1
%
) which redirects the beam onto the BIRT spectrometer. The same mirror,
in combination with a dedicated gyro attached to BIRT, is also used to correct
for high frequency jitter in the BIRT pointing. BIRT will alternate between the
IMAGE and HIGH-RES mode. If the source fades below a certain brightness
(set from the ground), the GRB observation will end and the normal observing
program is resumed. During this process TED continues to monitor the sky for
transient events, however, depending on user criteria, a GRB observation can
be prematurely ended to follow up a more exciting event.
4.1 The cryogenic imaging spectrometer
For CRIS we are leveraging some of the most significant technology devel-
opments carried out for the International X-ray Observatory (IXO). We will
use the same detector technology developed for the X-ray microcalorimeter
spectrometer on IXO; for the optics we rely partially on the IXO silicon pore
optics technology (SPO) and partially on classical Wolter I optics; and for
the detector cooling we use a system which is functionally equivalent to the
one proposed for IXO. The instrument has been optimized to give a high
grasp (effective area × solid angle) by having a short focal length (2.5 m).
Table 1 Key characteristics of the cryogenic imaging spectrometer (CRIS)
Parameter Inner Outer GRB
Required (eV) 2.5 5.0 2.5
Goal (eV) 1.5 3.0 1.5
FoV (arcmin
2
) 10 × 10 30 × 30 6 × 6
Full detector
Energy range 0.2–8 keV
Angular resolution 30
%%
(HPD)
Effective area Required (goal)
0.5 keV 1,000 (1,500) cm
2
1.5 keV 700 (1,000) cm
2
6.0 keV 100 (200) cm
2
Point source line detection 2 · 10
7
photons cm
2
s
1
sensitivity (5σ at 0.5 keV)
Confusion limit for 0.5–2 keV 10
15
erg c m
2
s
1
E-scale stability 1 eV/hr
Good grade events >80% at 50 counts/s/pix (!Enominal)
Non-X-ray background 2 · 10
2
counts/cm
2
/s/keV
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Exp Astron
Considering the maximum size of the cooled detector this corresponds to a
30
%%
field of view. With this configuration we maximize the effective area at
lower energies (<1keV)whileretainingareasonableeffectiveareaat6keV
(>150 cm
2
). The short focal length also has a low moment of inertia, necessary
for fast re-pointing. The key characteristics of CRIS are listed in Table 1.
Optics To meet the ORIGIN requirements of high effective area at 1 keV
and a working range of 0.2–8 keV, we propose a telescope design based on a
hybrid mirror technology: the outer part of the mirror will be built with Silicon
Pore Optics (SPO) and the inner part with a Wolter type I telescope using
standard Ni electroforming technology (Fig. 10). This choice reduces the mass
(the SPO is very light). For the high-energy response the inner mirror is needed
(as SPO optics has only been demonstrated at radii down to 0.3 m). SPO relies
on using the flat surface of coated Si wafers as reflectors and stacking a set
of these wafers in elements that, placed behind each other, can approximate
the required geometry of a two-re flection focusing optical element. A fully
automated stacking robot is operating to assemble stacks of plates according
Fig. 10 a, b Top Design of
the CRIS hybrid mirror. Inner
part classical Ni formed shells
mounted on a spider. Outer
part SPO modules (yellow)
mounted in four petals (grey).
The green part is the
structural support. Bottom
engineering view of the CRIS
focal plane assembly
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Exp Astron
to the IXO optics design with 20 m focal length and a bending radius of 0.74 m.
For our application we have to stack shorter wafers with an inner bending
radius of 0.3 m. The smaller bending radius has been studied by finite element
modeling and has been demonstrated by bending and bonding of two mirror
plates with this radius. A Half Equivalent Width (HEW) angular resolution of
<20
%%
has been repeatedly demonstrated by X-ray pencil beam measurements
performed at PTB in Berlin for a set up to 30 plates of an IXO mirror module,
mounted in representative flight configuration. These results indicate that in
the case of ORIGIN, the 30
%%
HEW requirement, being dominated by the
conical approximation to the Wolter type I optics, is achievable. To increase
the mirror reflectivity, the SPO will be coated with a three layer reflecting
surface: C (25 Å)–Ni (25 Å)–Pt (300 Å). To estimate the effective area we
assumed the proven thickness of the reflectors (170 µm) but FEM calculations
suggest that we can reduce this to 120 µmresultinginagainof10%ofthe
area. For the Wolter type I telescope the baseline is Au coating, but with using
aPt/Cmulti-layercoating(15to50layersrangingbetween40100Åusingan
extra Pt layer on the outside), we can boost the area at 6 keV by about 70%.
The HEW increases from close to 20
%%
in the center to about 40
%%
near the edge
of the field of view. For the higher energies the angular resolution can be as
good as 15–20
%%
in the center. The vignetting is modest at lower energies (less
than 20% between the center and edge of the FoV) but increases for higher
energies.
Detector For the detector we h ave selected an array of calorimeters for
which the temperature rise after absorbing the photon, is measured by a
very sensitive thermometer. Using a Transition Edge Sensor (TES) operated
at 100 mK a spectral resolution of <2.5 eV at 6 keV is feasible [15, 20].
The detector assembly consists of the detector, its electronics and the cooling
system. The detector has a number of different sections including an inner
array of 26 × 26 pixels with energy resolution of 2.5 eV (optimized for E
max
=
10 keV), and an outer array of 72 × 72 pixels. In the outer array four pixels are
read out by a single TES connected by four different strong thermal links. This
type of detector allows identification of the X-ray absorbing pixel from the
pulse shape before the TES and four absorbers come into thermal equilibrium
[30]. The resolution is <5eV(optimizedforE
max
= 5 keV). There is also a
third array overlaying the outer array for detecting GRBs. This array consists
of 20 × 26 pixels and is placed 8 mm out of focus. The intense X-ray beam of
GRBs is spread over a sufficiently large number of pixels that the maximum
count-rate capability of each pixel is not exceeded. The detector area of the
outer array behind this GRB section, will of course, not be populated. The total
number of channels to be read out is 2201. The focal plane assembly needs to
be compact, and thermally and magnetically isolated from the environment
(i.e. the temperature stability of the cold stage needs to better than 1 µK
rms). The detector signals are multiplexed near the detector, which reduces
the wiring between the cryogenic detector and room temperature. The SQUID
read-out amplifiers are required to be in close proximity to the detectors, and
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Exp Astron
Fig. 11 Estimated on-axis
effective area for CRIS, as a
function of energy
these SQUIDs must also be magnetically well shielded. In Fig. 10 we show an
engineering view of the focal plane assembly.
The cooling system for ORIGIN includes a combination of a last stage
cooler (providing the 45 mK heat sink temperature for the detector), two
Joule Thomson coolers (J-T) and four 2-stage Stirling Coolers. The JT coolers
provide the 4K stage (each JT cooler is pre-cooled by 2-stage Stirling coolers)
and the other 2-s tage Stirling cooler cool the thermal shields. We have chosen
averyconservativeapproachassumingthatthedewarwillbelaunchedunder
vacuum (reducing the acoustic loads on the blocking filters). Tests on these
filters are planned and, if successful, may eliminate the need for the vacuum
enclosure, resulting in a mass saving (50 kg). We have selected a combination
of three adiabatic demagnetization refrigerators (ADR) to cool the detector
assembly from 4 K down to 45 mK because of the high TRL level of this
technology. Following magnetization of the salt pills, cooling is provided by
the relaxation of the spins in the magnetized material. To reach the 45 mK
level from 4 K the three ADRs (with two GLF stages and a single CPA salt
pill for the last stage) need to be in series. The recycling time of the last stage
cooler is less than 2 h, with a hold time of 31 h.
Figure 11 shows the effective area of CRIS. The drop below 0.5 keV is
mainly due to the optical filters in the cryostat. The edge around 2 keV is due
to the mirror reflectivity and the drop at higher energies is a combination of
the detector (absorber quantum efficiency for a given detector thickness) and
a drop in the mirror effective area as a function of energy.
4.2 The transient event detector
We will use a coded mask detector [23] to monitor a large fraction of the sky for
transient events. The energy band between 5 and 200 keV is optimal to identify
GRBs: the currently operational mission Swift works in this regime but with a
higher threshold of 15 keV. For ORIGIN we have a modular approach with
4 units, tilted outward by 30
on average. This yields a field of view as wide
as 4 sr, with extension up to 4.6 sr for brighter bursts. The detector and mask
size as well as the pixel size have been tuned to the instrument requirements
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Exp Astron
(field of view, source location accuracy and sensitivity). The key characteristics
of TED are listed in Table 2.
The design of TED is largely modular and based on the heritage of
INTEGRAL and Swift. The 4 U are identical and the detection planes and
masks have rectangular shapes. The detection plane of one instrument is an
assembly of 12 CdZnTe (CZT) modules mounted on a spider structure. Each
module in turn is formed by an array of 16 × 8 square crystals of size 1 cm
2
and
thickness of 2 mm. An array of 4 × 4 pixels is then formed in each crystal by
electrode segmentation. The detector pixel size is 2.5 mm and there are 2048
pixels/module. The active detection area is 1,536 cm
2
for 1 U. The coded mask
has a pixe l size of 2.8 mm, for a random pattern of holes and an open fraction of
40%. The mask is 0.8 mm thick and is fixed to a rectangular support gri d on top
of the shield. It will be built by etching or laser cutting single pieces of tungsten
of 26.4 × 26.4 cm
2
each. On top of the mask we put a thin opt ical blocking filter
to suppress the thermal load from partial exposure to sunlight. The angular
resolution of this system is 23.8
%
, which allows for a location accuracy of 3
%
(90% confidence) at the detection limit of 12σ.
Each single module is equipped with a bias unit and a digital electronics
board providing AD conversion of the signals. For each module specific bias
voltages can be set. These bias boxes, providing power to the modules, are
mounted on the unit close to each detector module and connected to it by
flat cables as successfully implemented in the INTEGRAL/IBIS detector. For
each unit, the 12 digital FEE board s are controlled by the Unit Electronics
Box (UEB) mounted on the short side of the unit. This electronics provides
configuration control (noisy pixels, low thresholds) and event processing.
The TED Instrument Control Unit (ICU) receives data from the four UEBs
and performs the TM/TC and S/C I/F functions, and the GRB trigger and
positioning. An identical ICU is implemented as a cold redundant unit that
remains switch-ed off in nominal conditions. The four TED units are tilted
by 30
with respect to a plane that is inclined in turn by 15
from the satellite
platform. The sensitivity of the instrument has been optimized and is consistent
with the 4 sr coverage. The effective area is >200 cm
2
over a field of view of
4 sr, corresponding to a fluence limit prompt emission of 10
6
erg/cm
2
(for a
GRB of 20 s and a trigger integration time of 10 s). This is shown in Fig. 12.
For bursts as bright as 2 × 10
6
erg cm
2
the field of view is even 4.6 sr.
Table 2 Key characteristics
of TED
Parameter Value
Field of view 4sr
Energy range (keV) 5–200 (3–200)
Angular resolution 23.8
%
Source location accuracy (12σ) <3
%
(goal: 2
%
)
Energy res. (100 keV, FWHM) 3%
Count rate/unit (min/max) 4–15 k
Sensitivity (ph cm
2
s
1
in 10 s, 5–150 keV) 0.4 (12σ)
Software processing time (s) 20 (10)
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Exp Astron
Fig. 12 a, b Left a TED unit. The main parts are the detector modules, visible in red, the shield
support and mask grid support. On the external side are the module bias boxes and the electronics.
Right the flux limit as function of solid angle covered by the TED. For reference we also provide
the corresponding effective area (vertical axis). GRBs with a fluence of the prompt emission as
low as 10
6
erg/cm
2
occurring within 4 sr will be detected
During normal operations the TED units will monitor the sky waiting for
a GRB or a transient to occur. In the meanwhile, TED will produce spectral-
imaging data (normal mode)inwhichdetectorimagesareprovidedinapre-
defined set of energy bands. The on-board trigger will be based on sampling
count rates at unit and module levels with different integration times and
energy ranges, and includes an imaging trigger based on an on-board catalog
of known sources, to discriminate non-GRB events. This procedure is standard
in satellites like Swift and AGILE. When the trigger signal is produced, the
ICU performs the imaging part of the analysis (trigger mode) and the GRB
position is generated and transmitted to the S/C within 20 s. It is not required
to collect data during slews. For safety TED units will be put autonomously in
a safe mode when the Sun is in their field o f view. While this happens for one
(or two) units the other TEDs continue to operate nominally.
4.3 The burst Infrared telescope
In order to ensure the discovery of the optical counterpart of a GRB, pin-
pointing its host galaxy and determining the redshift, an optical/near-IR
telescope is planned. To date, all GRB redshifts have been measured with
optical or near-IR instruments. Deter mination of GRB redshifts from 0 to 12 is
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Exp Astron
achieved with a wavelength range of 0.5 to 1.7 µm. BIRT will detect the GRB
counterpart and measure the redshift by using a combination of multi-band
imaging (IMAGE), R = 1,000 integral-field dispersed spectroscopy (HIGH-
RES) and R = 20 low dispersion slitless spectroscopy (LOW-RES).
Initially, in parallel with CRIS observations, the counterpart will be ob-
served in the LOW-RES mode, which will identify high-redshift GRBs and
provide an approximate redshift from the Lyman-alpha break, while they are
in their brightest phase. Once CRIS has identified the X-ray counterpart,
the precise position will be determined in the IMAGE mode. For brighter
counterparts, HIGH-RES spectroscopy will measure a precise redshift from
absorption lines.
BIRT is an optical to IR Cassegrain telescope with a 0.7 m primary mirror
of Silicon Carbide with a Hawaii-2RG 2K × 2K HgCdTe detector. Below
the primary mirror the image plane is split into three independent optical
paths, splitt ing the beam over the three BIRT operating modes. In the LOW-
RES mode the dispersing element is a prism. A 4 × 4 spatialpixel image slicer
directs the light onto a grating for the HIGH-RES mode. The IMAGING
mode uses dichroics to separate the four different bands, which are imaged
simultaneously. Focus is maintained by heaters on metering rods between the
primary and secondary mirrors. The telescope utilizes a baffle tube which
extends 3.8 m forward of the primary mirror, together with a secondary mirror
baffle, an inner-primary mirror baffle; baffling of the field and pupil stops
within the instrument box for straylight suppression. With this design the
observing constraints of GRBs will be dictated by the CRIS instrument. The
key characteristics are given in Table 3.Thetelescopewillbepassivelycooled
to 270 K. The detector and the camera optical baffle must be cooled actively
to minimize the background signal. This requires a miniature pulse-tube cooler
(MPTC) connected to a radiator.
The instrument will gather data from three distinct sky regions on different
parts of the detector. A 6
%
× 6
%
field of view is used for low-resolution slitless
spectroscopy with R 20. A section of 1
%
× 1
%
, slightly offset with respect to the
LOW-RES field of view, is used for imaging in 4 bands. A small region of 2
%%
×
2
%%
is used for high-resolution integral field spectroscopy with a resolution of
R = 1000. These distinct regions are mapped onto a single detector. Targets are
switched between LOW-RES and IMAGE by re-pointing the satellite using
the GRB position as provided by CRIS. For the HIGH-RES mode the GRB
position inside the BIRT field of view mu st be known with an accuracy of
Table 3 Prime characteristics of BIRT
Imaging Low-res High-res
Wavelength range (µm) 0.5–0.7, 0.7–1.0, 1.0–1.3, 1.3–1.7 0.7–1.7 0.5–1.7
Field of View 1
%
× 1
%
6
%
× 6
%
2.1
%%
× 2.1
%%
Limiting mag. (AB) H = 22.2 H = 20.8 H = 19.3
Spectral resolution R 3–4 20 1000
Spatial resolution 0.2
%%
,0.3
%%
,0.4
%%
,0.5
%%
0.3–0.5
%%
0.2–0.5
%%
Page 26
Exp Astron
0.1
%%
. This is obtained from the data in the IMAGE mode. Placement of the
GRB on the HIGH-RES section and corrections for drift and high frequency
perturbations (e.g. due to the compressors of the cryo-cooler) is achieved by a
tip/tilt fold mirror internally to the BIRT. Slow drifts of the spacecraft will be
monitored using stars in the IMAGE section and high frequency perturbations
will be monitored using a gyro directly attached to BIRT.
5Spacecraft
For the spacecraft design we take a very conservative approach. In practice
this implies that we seek to decouple spacecraft systems as much as possible,
neglecting any potential benefits from a closer integration. This approach
minimizes system complex ity, taking advantage of the large mass capability
for a satellite in LEO with a Soyuz launcher. This increases the weight, but the
simpler design, based on off-the-shelf units, will reduce the cost. In Fig. 13 we
show an engineering view of the satellite. For most subsystems (power, thermal
subsystem, data handling, propulsion and deorbiting, mission operations) we
selected off- the-shelve systems. For the communications we require the X-
band. In addition, satellite-to-relay satellite communication is included for fast
transfer of GRB positions to the ground to enable rapid follow up. The AOCS
system (fast repointing) and the long baffles to reduce straylight have been
studied in detail.
Attitude and orbit control The attitude and orbit control has to meet the
requirements for a fast responding satellite: (a) three ‘slow’ re-pointings per
orbit (to allow for a 70% observing efficiency in a LEO); (b) one fast,
Fig. 13 Accommodation of
the payload and satellite
systems in ORIGIN and
accommodation in the fairing
(top left). The launch mass,
including adaptor and
margins is 2904 kg and the
total power including margin
is 2790 W
Page 27
Exp Astron
autonomous, repointing per orbit; (c) each o rientation fixed with respect to a
celestial object and (d) illumination due to Sun and Earth to be avoided along
the telescope bore sights The mastering of fast repointing AOCS solutions
within Europe in the last few years (e.g. Pleiades) ensures that th ese challenges
can be met. To reduce the required capabil ity of the actuators the Moment of
Inertia (MoI) can be minimized by symmetric mounting of equipment near
the Center of Mass (CoM). A major driver is the focal length of CRIS (i.e.
the distance to the heavy mirror). This is a trade-off between the actuator
sizing and the hig her energy response of the telescop e. For this we rely o n
existing and proven technology although mass savings could be achieved if the
bending radius of th e SPO can be reduced from 0.3 to 0.2 m. The CoM and MoI
for the satellite are 0, 0, 1.2 m with respect to the launch adaptor and 5,500,
5,300, 2,400 kg m
2
,respectively.Toachievefastrepointingwehavebaselined
Control Momentum Gyros (CMGs) which give an overall lower system mass
and power than a Reaction Wheel solution and allow the 1
/s agility to be met
using the of-the-shelf Honeywell M-50 in a 4 U pyramid configuration. For the
unloading of the CMG momentum we will use Magnetic Torquers during every
orbit (noon time). The AOCS system is complemented by two Sun sensors, two
GPS systems, one IMU (Inertial Measurement Unit) and two magnetometers.
As star tracker we have selected the SODERN Hydra system. All units chosen
are available off-the-shelf.
Straylight In a Low Earth Orbit not only the repointing is important but
also the sky visibility, as the instruments (CRIS and BIRT) are sensitive to
straylight (<2.5 10
9
photon/cm
2
/s at the dewar entrance for CRIS and <10
5
photons/cm
2
/s on the BIRT detector). Based on a preliminary analysis we need
areductionofthereflectedSun/Earthlightof10
6
for CRIS, corresponding
to at least two reflections inside the baffle. An estimate of the self-baffling
properties of the X-ray optics including integrated baffles indicates that a
reduction of 10
3
from the optical straylight baffle is sufficient. With a 3.5 m
long baffle for CRIS (yellow tube Fig. 13) this is feasible with an exclusion
cone of half angle 24
. The baffle length can be traded against the fraction
of sky available but the baseline length fits comfortably within the fairing
whilst giving access to >53% of the sky during the whole orbit. For BIRT
attenuation by 10
10
is required. This is achieved by a long baffle tube together
with baffles on the secondary, around the M1 Cassegrain aperture, and within
the instrument enclosure. The primary baffle tube is lined w ith conical vanes
such that the majority of the incident light not absorbed is directed back into
space. The baffle tube has a sloping entrance so that it is longer on the CRIS
side to prevent light scattering from the outside of the CRIS ba ffle into the
BIRT optics. The BIRT baffle length (on the shortest edge) to aperture ratio
is larger than that used on Swift UVOT (5.4 for BIRT, compared with 5.0 for
UVOT), and BIRT incorporates field and pupil stops within its optical trains
so that the BIRT straylight rejection will be superior to that of UVOT, which
has demonstrated near-zodiacal-limited performance down to an Earth limb
angle of 25
[3].
Page 28
Exp Astron
6Scienceoperations
ORIGIN will follow th e typical ESA share of responsibilities: ESA is respon-
sible for the Mission Operation Control (MOC) and the Science Operations
Center (SOC). The SOC sets the observing schedule and take advantage of
the capability to autonomously repoint the satellite. A list of pre-planned
targets will be uploaded, allowing the independent execution of the schedule
for several days. For the guest observer program the target selection will be
carried out through an open call to the community and subsequent peer review.
Key programs, requiring long observations, will be selected in consultation
with the community through the Science Working Team (representatives of
instrument teams and the science community).
A Science Data Center (SDC) will be established to routinely process flight
data and to provide the analysis software to the community. Responsib ilities of
the SDC include data processing, integration of instrument specific software in
the analysis package (including testing), delivery of generic tools for the analy-
sis of the science data and the distribution of this software to the community
including user support for the use of these tools. The instrument teams will be
responsible for the health and calibration of their instruments, for defining the
trigger c riteria and providing the instrument specific software. Data from valid
GRB triggers and consequent follow up measurements by the other ORIGIN
instruments (positions, spectra, light curves) will be transmitted to the ground
in real time and distributed via the internet to the worldwide community .
The data will be ins pecte d on a daily basi s by instrument teams a nd the SDC
to ensure quality. This reduces the load on the SOC. This working scheme
operates successfully for Swift.
7 Conclusion
ORIGIN is an exciting missi on to study the metal enrichment from redshifts
>7 up to the present. We selected some of the prime science which demon-
strates the power of high spectral resolution observations in the soft X-ray
band to study cosmic chemical evolution. With a five year mission duration
we expect around 65 GRBs at redshifts >7. This requires a satellite which
identifies and localizes bursts and can autonomously slew to the position of the
GRB. For bursts with low metallicity, the redshift cannot be determined from
the absorption lines in the X-ray spectra and therefore the payload includes
the capability to determine the redshift in the IR. This payload is well feasible
within the envelope of an M3 mission. Detailed studies have demonstrated that
fast re-pointing with a large sky visibility (>53%) is feasible in a Low Earth
Orbit using available technology. Also the other satellite systems are not over
demanding. For the instruments we use available technology (th e coded mask
instrument to detect GRBs and t he IR telescope to determine independently
the redshift) or exploit technology which has been developed for IXO (the
cryogenic instrument and the optics).
Page 29
Exp Astron
Acknowledgements The team likes to express its appreciation for the support of Astrium UK
for the present study. Earlier studies, which also confirmed the feasibility of this concept were
carried out by Thales/Alenia and NASA/MSFC.
References
1. Balestra, I., Tozzi, P., Ettori, S., Rosati, P., Borgani, S., Mainieri, V., Norman, C.: Evolution in
the iron abundance of the ICM. P Th PS 169,49(2007)
2. Bastian, N., Covey, K.R., Meyer, M.R.: A universal stellar initial mass function? A critical look
at variations. ARA&A 48,339(2010)
3. Breeveld, A.A., Curran, P.A., Hoversten, E.A., et al.: Further calibration of the Swift ultravi-
olet/optical telescope. MNRAS 406,1687(2010)
4. Branchini, E., Ursino, E., Corsi, A., et al.: Studying the warm hot intergalactic medium with
gamma-ray bursts. ApJ 697,328(2009)
5. Borgani, S., Murante, G., Springel, V., et al.: X-ray properties of galaxy clusters and groups
from a cosmological hydrodynamical simulation. MNRAS 348,1078(2004)
6. Bromm, V., Loeb, A.: GRB cosmology: probing the early universe. AIPC 937,532(2007)
7. Butler, N.R., Bloom, J.S., Poznanski, D.: The cosmic rate, luminosity function, and intrinsic
correlations of long gamma-ray bursts. ApJ 711,495(2010)
8. Bykov, A.M., Paerels, F. B. S., Petrosian, V.: Equilibration processes in the warm-hot inter-
galactic medium. SSRv 134,141(2008)
9. Campana, S., Romano, P., Covino, S., et al.: Evidence for intrinsic absorption in the Swift
X-ray afterglows. A&A 449,61(2006)
10. Cen, R., Ostriker, J.P.: Where are the baryons? II. Feedback effects. ApJ 650,560(2006)
11. Chieffi, A., Limongi, M.: Explosive yields of massive stars from Z = 0 to Z = Z
solar
.ApJ608,
405 (2004)
12. de Plaa, J., Werner, N., Bleeker, J.A.M., Vink, J., Kaastra, J.S., Méndez, M.: Constraining
supernova models using the hot gas in clusters of galaxies. A&A 465,345(2007)
13. Diemand, J., Kuhlen, M., Madau, P.: Clumps and streams in the local dark matter distribution.
Nature 454,735(2008)
14. Gallerani, S., Salvaterra, R., Ferrara, A., Choudhury, T.R.: Testing reionization with gamma-
ray burst absorption spectra. MNRAS 388,L84(2008)
15. Gottardi, L., et al.: Proc. ASC (2010, in press)
16. Greiner, J., Krühler, T., Klose, S., et al.: The nature of “dark” gamma-ray bursts. A&A 526,
A30 (2011)
17. Heger, A., Woosley, S.E.: Nucleosynthesis and evolution of massive metal-free stars. ApJ 724,
341 (2010)
18. Jakobsson, P., Levan, A., Fynbo, J.P.U.: A mean redshift of 2.8 for Swift gamma-ray bursts.
A&A 447,897(2006)
19. Kawai, N., Yamada, T., Kosugi, G., Hattori, T., Aoki, K.: GRB 050904: subaru optical spec-
troscopy. GCN 3937,1(2005)
20. Kilbourne, C.A., Doriese, W.B., Bandler, S.R.: Multiplexed readout of uniform arrays of TES
X-ray microcalorimeters suitable for constellation-X. Proc. SPIE. 7011,701104(2008)
21. Loeb, A., Ferrara, A., Ellis, R.S.: First Light in the Universe. Springer (2008)
22. McCammon, D., Almy, R., Apodaca, E., et al.: A high spectral resolution observation of the
soft x-ray diffuse background with thermal detectors. ApJ 576,188(2002)
23. Natalucci, L.; Feroci, M.; Quadrini, E., et al.: Design of a CZT gamma-camera for GRB and
fast transient follow-up: a wide-field-monitor for the EDGE mission. Proc. SPIE 6686,66860T
(2007)
24. Nicastro, F., Zezas, A., Drake, J., et al.: Chandra discovery of a tree in the X-ray forest toward
PKS 2155-304: the local filament? ApJ 573,157(2002)
25. Nomoto, K., Tominaga, N., Umeda, H., Kobayashi, C., Maeda, K.: Nucleosynthesis yields of
core-collapse supernovae and hypernovae, and galactic chemical evolution. Nucl. Phys. A 777,
424 (2006). doi:10.1016/j.nuclphysa.2006.05.008
Page 30
Exp Astron
26. Paerels, F.B.S., Kaastra, J., Ohashi, T., Richter, P., Bykov, A., Nevalainen, J.: Future instru-
mentation for the study of the warm-hot intergalactic medium. SSRv 134,405(2008)
27. Schindler, S., Diaferio, A.: Metal enrichment processes. SSRv 134,363(2008)
28. Salvaterra, R., Della Valle, M., Campana, S., et al.: GRB090423 at a redshift of z 8.1. Nature
461,1258(2009)
29. Schneider, R., Ferrara, A., Salvaterra, R., Omukai, K., Bromm, V.: Low-mass relics of early
star formation. Nature 422,869(2003)
30. Smith, S.J., Bandler, S.R., Brekosky, R.P., et al.: Development of position-sensitive transition-
edge sensor x-ray detectors. IEEE Trans. Appl. Supercond. (2009). doi:10.1109/TASC.2009.
2019557
31. Snowden, S.L., Egger, R., Freyberg, M.J.: ROSAT survey diffuse x-ray background maps. II.
ApJ 485,125(1997)
32. Springel, V., Frenk, C.S., White, S.D.M.: The large-scale structure of the universe. Nature 440,
1137 (2006)
33. Takei, Y., Ursino, E., Branchini, E., et al.: Studying the warm-hot intergalactic medium in
emission. ApJ (2010). arXiv:1011.2116
34. Ursino, E., Branchini, E., Galeazzi, M., et al.: Expected properties of the two-point autocorre-
lation function of the IGM. MNRAS 413 (2010). doi:10.1111/j.1365-2966.2011.18597.x
35. Viel, M., Haehnelt, M.G., Springel, V.: Inferring the dark matter power spectrum from the
Lyman α forest in high-resolution QSO absorption spectra. MNRAS 354,684(2004)
36. Werner, N., de Plaa, J., Kaastra, J.S.: XMM-Newton spectroscopy of the cluster of galaxies 2A
0335+096. A&A 449,475(2006)
Page 31
  • Source
    [Show abstract] [Hide abstract] ABSTRACT: We review the status of sterile neutrino dark matter and discuss astrophysical and cosmological bounds on its properties as well as future prospects for its experimental searches. We argue that if sterile neutrinos are the dominant fraction of dark matter, detecting an astrophysical signal from their decay (the so-called 'indirect detection') may be the only way to identify these particles experimentally. However, it may be possible to check the dark matter origin of the observed signal unambiguously using its characteristic properties and/or using synergy with accelerator experiments, searching for other sterile neutrinos, responsible for neutrino flavor oscillations. We argue that to fully explore this possibility a dedicated cosmic mission - an X-ray spectrometer - is needed.
    Full-text · Article · Jun 2013 · Physics of the Dark Universe