Simbol-X was an X-ray telescope project with participation from France, Italy and Germany. It would have been the first instrument at all to measure photon energies above 15 keV with the use of Wolter-focusing optics. The start of the project was originally planned for the year 2014 but cancelled due to lack of money. The main goal of Simbol-X was the construction of an instrument with a huge collecting area over a wide energy range as well as a very low internal detector background. One result could have been new discoveries of many weak unknown X-ray sources. The huge field of view and a large quantum efficiency as well as a very good resolution and a low internal background of Simbol-X could have brought a breakthrough in the field of X-ray physics and in the theories of particle acceleration.

Image: Simbol-X (CNES, Oliver Sattler, March 2006)

Start: planned was a launch for the year 2014

Energy range: about 0.5 - 80 keV

Scientific goal: New information about the behaviour of matter around compact objects, about the Cosmic X-ray Background (CXB - Cosmic X-ray Background), the acceleration of particles and the nucleosynthesis in young supernova remnants (SNRs) as well as the observation of Active Galactic Nuclei (AGNs) at cosmological distances.

Instrument:

Simbol-X would have been the first scientific instrument made of two aligned modules which are moving together in formation flight. There are two separate parts which were to be launched together, one for the mirrors and one for the main detector. By separating the detector and the focusing optics, it is possible to focus and detect photons with energies above 15 keV, which is the limit reached up to now using focusing techniques. At higher photon energies the focal length would be much to large for the usual instrument with detector and optics on one platform. The focusing optics of Simbol-X are Wolter-mirrors with a focal length of about 20 m and made out of different mirror layers. Therefore, photons with energies up to 80 keV can still be focused. The present sensitivity and angular resolution for this energy range can be improved by up to two orders of magnitude. The experience from the previous mission XMM-Newton was a good help for the engineering of the mirrors and the idea of nested Wolter-I mirrors evolved from this mission. The main difference to XMM-Newton is the material of the mirrors. The Simbol-X mirror shells are built of platin and carbon to provide the requirements for a huge field of view and a high sensitivity to focus much higher photon energies. The multi-layers consists of alternating layers of high and low z (atomic number) material to be able to focus photons with energies three times higher as reached for single layers. The diameter of the mirrors has values from 26 cm up to 65 cm while the weight is reduced in comparison to XMM-Newton by reducing the thickness of the mirror layers. Additionally, a so-called spider wheel is present to provide the required stability of the mirrors on both sides.

Technical parameters of Simbol-X:

Detector:

Simbol-X has a Low Energy Detector (LED) as well as a High Energy Detector (HED) which are described in the following.

LED

The LED was developed at the semiconductor lab of the Max-Planck Institutes in Neuperlach and consists of a matrix of 128 x 128 pixels. Those pixels are partitioned in four quadrants which are integrated in a monolithical silicon waver. All quadrants have their own front-end-electronics (CAMEX - Charge Amplifier and MultiplEXer) and are read out simultaneously in a time of 128 μs. The detector type is a DePFET (DEpleted P-channel Field Effect Transistor).

Image: prototype of a waver made out of 128 x 128 pixels (MPI Semiconductor Laboratory)

The DEPFET consists of a huge (8 cm x 8 cm x 450 μm) and fully depleted block of silicon with a p-MOSFET, which is placed at the centre of the front side of each pixel. The task of the DEPFET is the storage, measurement and deletion of electrons produced by the incident radiation. Around the p-MOSFETs are driftrings to transport the charges to the read-out area. Due to the high flexibility of those rings, the size of the pixels can be adjusted according to the different requirements. The size variation is from 1cm x 1cm up to 50 μm x 50 μm without any effect on the energy resolution. The pixels used for the Simbol-X mission (Macro Pixel) have a size of 625 μm x 625 μm.

Images: DEPFET model (MPI Semiconductor Laboratory)

The read-out module of the MOSFET can collect up to 10^5 electrons. The signal of those electrons is measured by signal sampling as a voltage while connecting an external gate. After the measurement of a signal, the accumulated charges can be erased by feeding with a positive charge. Afterwards, the voltage at the source-connection can be measured with an empty internal gate which is called baseline sampling. The difference is equivalent to the number of collected electrons during the time of integration. A thin layer of aluminium is put on the entrance window in order to shield optical light. Just as well, the temperature around the waver has to be constant on -40°C. That's necessary to reduce thermal noise and to reach the intended energy resolution of < 150 eV FWHM at 5,9 keV.

HED

The HED consists of 64 high quality CdZnTe crystals, each with 16 x 16 pixels and a pixel size of 625 μm. Each crystal has its own read-out electronis. The energy range goes from 5 - 80 keV and is slightly overlapping with the one of the LED. The focal area is split in eight independent sectors.

Image: Simbol-X HED with 64 CALISTE modules (CEA/DAPNIA)

The HED is a self-triggering detector and has no frame time contrary to the LED. As soon as the signal in a pixel reaches a certain value, an event package including time, energy and pixel information is generated.

AC

The LED as well as the HED are surrounded by an anticoincidence shield. Thereby, the cosmic background is minimized. The active anticoincidence (AC) shield consists of a plastic scintillator where the incoming radiation is generating optical photons. Those photons are detected by Photo-Multiplier Tubes (PMTs). If a signal is measured by the anticoincidence shield as well as by the detector in a certain time span, it can be neglected. Those signals are thought to be made by background events.

Image: Simbol-X: left: cross section of the focal plane, right: cooling box of aluminium for the HED (CEA, Saclay)

Technical properties of the LED and HED:

Scientific goals:

• Movement of matter around black holes: By assignment of the different regions of a spectrum to the associated components (for example synchrotron emission in a jet, inverse Compton effect in the corona or emission of the accretion disk), one wants to get new insights in the geometry of accreting black holes. The spin of the black hole may be measured with the structure of the relativistic iron line. Simbol-X was to measure the high energetic radiation from supermassive black holes in the centre of our galaxy. This could contribute to a further understanding of the interactions in and around those objects.
• Strongly absorbing AGN: Simbol-X could discover many unobserved AGN up to distances of about z∼0,5-1,5. Up to now, only very few Compton Thick AGN were observed by XMM-Newton and Chandra. Additionally, Simbol-X should analyse weak X-ray sources with a hard spectrum in the soft energy range. The variability and therefore the form of emission of short and long living sources as well as the high-energetic part of the spectrum of X-ray binaries should be analysed.
• Cosmic X-ray background: Simbol-X should resolve about half of the Cosmic X-ray Background (CXB) at 10 - 40 keV. Analysis of ROSAT data helped to explore the biggest part of the CXB at about 1 keV. Unobscured quasars at a high red shift (z∼1,5-2) are responsible for this part of the CXB. Observations with XMM and Chandra showed that most of the CXB up to energies of 5-6 keV originates from relatively low luminosity sources at distances of about z∼1. One idea is also, that the appearance of the sky in the X-ray regime for energies above 10 keV is different from the predictions and expectations. Until now, we don't know much about the CXB at those energies and about the origin of the peak in the spectrum at about 30-50 keV. Due to its extraordinary high sensitivity, Simbol-X could find many other Compton thick sources and analyse another big part of the CXB. The analysis of the origin of the CXB could also contribute to the analysis of the formation and development of structures in our universe.
• Particle acceleration: Simbol-X should observe sources which accelerate particles up to very high energies. Supernova remnants (SNRs) and broad X-ray jets as in Centaurus A or Pictor A are examples of those sources. One can determine the maximum energy for the acceleration of electrons by measuring the synchrotron spectrum of the source in hard X-rays. Probably there would evolve new ideas for mechanisms which lead to this energy limit.
• Nucleosynthesis in young SNRs: For a better understanding of the nucleosynthesis one has to make observations of hard X-ray and gamma ray lines. Those lines are related directly to the mass of the synthesizing elements. The high sensitivity of Simbol-X could lead to a localisation of emission regions and to a measurement of the velocity. This could lead to new insights in the dynamics of the explosion.

Image: CXB spectrum: measured and predicted contributions from the different populations of AGN. The lines show the different calculated contributions from unobscured AGN (red), obscured Compton-thin AGN (blue) and Compton-thick AGN (black). The total contribution of AGNs and galaxy clusters is shown in the magenta line. (Gilli et al, 2007)

Contribution from Tübingen to Simbol-X:

--> Monte-Carlo simulations for the detector background

--> Construction of a Science Verification module (SVM): in a cooled vacuum laboratory both detectors shall be run together

National and international cooperation:

The project started out of a cooperation of the french space agency CNES and the italian space agnecy ASI. German contribution consisted of the Max-Planck Institute for extraterrestrial physics (MPE, Garching), the semiconductor laboratory of the Max-Planck Institute (MPI-HLL), the TU Darmstadt, the university of Erlangen-Nürnberg and the IAAT.

Sources:

• Ferrando et al., 2005, 2008, Gilli et al., 2007, Della Ceca et al., 2007, Campana, 2007, Comastri et al., 2007, Lehmann et al., 2001, Zhang et al., 2006, Dirks et al., 2006, Meuris et al., 2008, Laurent et al., 2008, Ferrando and Giommi, 2007
• Images from: CNES, Oliver Sattler, März 2006, MPI Halbleiter Labor, CEA, Saclay