SVOM (Spaceborne multiband astronomical Variable Objects Monitor) mission
SVOM is a joint Chinese-French satellite observatory dedicated to the study of GRBs (Gamma-Ray Bursts) in the next decade. The goal of the mission is to study the most powerful explosions in the universe out to the era of the first generation of stars, SVOM will locate hundreds of GRBs signifying the deaths of massive stars or with the merging of two compact stars. In both cases the end product of the explosion is a black hole or a young magnetar surrounded by a torus of matter that is quickly accreted onto the compact object (in seconds), releasing huge amounts of energy in two transient relativistic jets. When one of the jets is pointed to the Earth, we see a bright high energy transient followed by a quickly fading afterglow. 1) 2) 3) 4) 5)
The study of GRBs sheds light on several key questions of modern astrophysics, like the physics at work in astrophysical relativistic jets, the end of life of massive stars and the birth of stellar mass black holes, the history of massive star formation over the ages, the history of the reionization of the universe and of the chemical enrichment of galaxies, etc. Some of these questions are directly connected with the study of GRBs, while other use them as lighthouses illuminating the remote regions of our universe, out to redshift z ≥10. The detection of GRBs will thus remain of high priority in the coming years.
The objective of the SVOM mission is to continue the exploration of the transient universe with a set of space-based multi-wavelength instruments, following the way opened by Swift. SVOM is a space mission developed in cooperation between the CNSA (Chinese National Space Agency), CAS (Chinese Academy of Science) and the French Space Agency (CNES). The mission features a medium size satellite, a set of space and ground instruments designed to detect, locate and follow-up GRBs of all kinds, a anti-Sun pointing strategy allowing the immediate follow-up of SVOM GRBs with ground based telescopes, and a fast data transmission to the ground.
The Chinese and French laboratories involved in the mission are : NAOC at Beijing, IHEP at Beijing, XIOPM at Xi'an, SECM at Shanghai, CEA-Irfu at Saclay, IRAP at Toulouse, APC at Paris and LAM at Marseille. In addition, MPE of Garching, Germany and the University of Leicester, UK, are contributing to the mission. The launch of the satellite by a Chinese Long March rocket is scheduled for 2021 and the minimum lifetime of the mission is 3 years.
The satellite carries two wide field high energy instruments: a coded-mask gamma-ray imager called ECLAIRs, and a gamma-ray spectrometer called GRM, and two narrow field telescopes that can measure the evolution of the afterglow after a slew of the satellite: an X-ray telescope called MXT and an optical telescope called VT(Visible Telescope). The ground segment includes additional instrumentation: GWAC (Ground Wide Angle optical Camera) monitoring the field of view of ECLAIRs in real time during part of the orbit, and two 1 m class robotic GFTs (Ground Follow-up Telescopes).
SVOM has some unique features: an energy threshold of ECLAIRs at 4 keV enabling the detection of faint soft GRBs (e.g. XRFs and high-z GRBs); a good match in sensitivity between the X-ray and optical space telescopes which permits the detection of most GRB afterglows with both telescopes; and a set of optical instruments on the ground dedicated to the mission. The mission has recently been confirmed by the Chinese and French space agencies for a launch in 2021, and it has entered in an active phase of construction.
The main science goal of SVOM is the study of cosmic transients detected in hard X-rays and in the optical domain. While the mission has been designed for the study of GRBs (Gamma Ray Bursts), it is also well suited for the study of other types of high-energy transients like TDEs (Tidal Disruption Events), AGNs (Active Galactic Nuclei), or galactic X-ray binaries and magnetars. For these types of sources, SVOM is both a "discovery machine", with wide field instruments that survey a significant fraction of the sky [(ECLAIRs, GRM (Gamma Ray Monitor) and GWAC (Ground Wide Angle Camera)], and a "follow-up machine", with fast pointing telescopes in space and on the ground (MXT (Microchannel X-ray Telescope), VT, and GFTs) that provide a multi-wavelength follow-up of different kinds of sources, with good sensitivity and a high duty cycle. The follow-up can be triggered by the satellite itself or from the ground, upon reception of a request for ToO (Target of Opportunity) observations.
Some Background: Forty years after its discovery, the Gamma-Ray Burst (GRB) phenomenon is not yet completely understood . The cosmological nature of these transient sources of gamma-rays has been established, and their association with explosions of massive stars (>30 Msun) is a scenario that reproduces most observations, at least for long duration GRB. They have been detected up to redshift 8.2, with the possibility to investigate the early Universe (star formation history, re-ionization era), and to derive cosmological parameters. Conversely for short duration GRB, mostly described by coalescences of two compact objects (black holes, neutron stars or white dwarfs), the situation is less consensual and lacks a good sample of afterglow observations. Open questions concern the physical processes during the prompt phase (particle acceleration, radiation), the GRB classification, the characterization of GRB host galaxies and progenitors, as well as fundamental physics issues like Lorentz invariance, the origin of cosmic rays and gravitational waves. 6)
Time-domain astrophysics: the discovery space after Swift: Astronomy is truly undergoing a revolution in terms of our ability to monitor the time-variability of the Universe in a continuous way using new facilities coupled with fast computers. The opening up of the temporal domain is transforming our knowledge of how the Universe evolves, particularly for objects which are undergoing explosive change, such as a supernova or a Gamma-ray Burst (GRB). These explosive events can release enormous amounts of power both in electromagnetic radiation and in non-electromagnetic forms such as neutrinos and gravitational-waves and test our understanding of the laws of physics under the most extreme conditions.
Observing facilities which are currently on-line enable the sky to be monitored fairly continuously in real-time over large areas and across the electromagnetic spectrum, capturing the temporal behavior of the Universe in a way previously unattainable. Examples facilities include the LOFAR radio telescope, the Pan-STARRs optical facility and the Swift and Fermi high-energy satellites. Non-electromagnetic facilities are also now observing, particularly the Advanced LIGO-VIRGO gravitational-wave observatory, which recently found its first source, and the IceCube neutrino experiment. The data from all these facilities have already opened up the temporal domain, but are just a foretaste of what is to come.
Many of the previously developed theories have come under intense strain by new observational results, such as the highly variable emission seen at late times in GRBs, the discovery of extremely luminous supernovae and the unexplained fast radio bursts. Theoretical models predict a variety of exotic explosions and stellar mergers, together with their multiple signatures across the electromagnetic spectrum. Theory also predicts that some will be accompanied by gravitational wave, neutrino and high-energy particle emission. The provision of SVOM in the next decade will coincide with the multi-messenger era and will provide a critical element of the era of time-domain astronomy both by finding transients and by following-up those from other facilities.
In the period when SVOM will fly, the number of transients found will increase by several orders of magnitude as even more powerful facilities come on-line, in particular the LSST (Large Synoptic Survey Telescope), 7) and the SKA (Square Kilometre Array). 8)The sheer grasp of the new facilities, which will produce thousands of alerts per day from variable or transient sources, mean we will require super-computers to process the data in real-time and smart algorithms to broker which transients to focus on with follow-up facilities. The importance of the temporal domain has been recognized in recent reports by the Chinese Space and Technology Roadmap, 9) the European Union ASTRONET group 10) and the USA National Research Council Decadal Survey (Committee for a Decadal Survey of Astronomy and Astrophysics, 2010). 11)
The astronomical panorama in 2020: The astronomical panorama of the next decade will be shaped by new instruments developed to address various outstanding questions raised by present day astrophysics. This panorama encompasses large radio, infrared, visible and gamma-ray telescopes, advanced gravitational wave interferometers and neutrino detectors of the km3 class (Figure 1), as well as simulations with powerful computers. These instruments will revolutionize our understanding of astrophysics in fields as diverse as the first ages and the reionization of the universe, the nature of the dark universe (dark matter and dark energy), the demography and role of black holes, exoplanets and planetary formation, and fundamental physical processes. Young fields, like Time Domain Astronomy and Multi-Messenger Astrophysics are also expected to grow very fast, bringing new discoveries (Ref. 5).
China will be responsible for the mission, launch, satellite and operations and will share responsibility with France for design and construction of the instruments and the ground segment.
Figure 2: Schematic showing the SVOM spacecraft with its multi-wavelength space payload. It consists of two wide-field instruments: ECLAIRs & the GRM (Gamma-Ray Monitor) for the observation of the prompt emission and two narrow field instruments: the MXT (Micro channel X-ray Telescope) and the VT (Visible Telescope) for the observation of the afterglow emission. Right: Space and ground instruments join to enable a unique coverage in time and wavelength (image credit: SVOM collaboration, Ref. 1)
• The Phase C kick-off of the SVOM project started in January 2017 12)
• In June 2016, the SVOM SRR (System Requirements Review) was completed.
• In October 2015, the University of Leicester has announced the signature of a contract to develop an innovative new type of X-ray mirror for a telescope to be flown on an orbiting observatory. 13)
• Phase B kick-off of the SVOM project started in September 2014.
Launch: The SVOM satellite mission (launch mass of ~930 kg) is planned to be launched in 2021 with a Long March-2C vehicle from XLSC (Xichang Satellite Launch Center), China.
Orbit: Near-circular orbit, altitude of ~625 km, inclination = 30º, period of ~86 minutes. With these parameters the satellite passes through the SAA (South Atlantic Anomaly) several times per day, inducing an overall dead-time of 13 to 17%.
Figure 3: The SVOM orbit with an inclination of 30° the satellite passes through the SAA which induces a dead time of (13-17)% (image credit: SVOM collaboration)
Figure 4: Artist's rendition of the deployed SVOM spacecraft (image credit: SVOM collaboration) 14)
In order to facilitate measuring the redshifts of GRBs detected with ECLAIRs, the instruments of SVOM will be pointed close to the anti-solar direction. Most of the year the optical axis of the SVOM instruments will be pointed at about 45º from the anti-solar direction. This pointing is interleaved with avoidance periods during which the satellite passes away from the Sco X-1 source and the galactic plane. This strategy ensures that SVOM GRBs will be in the night hemisphere and quickly observable from the ground by large telescopes.
The SVOM attitude law:
Figure 5: To detect GRB on the night side ⇒ attitude law : roughly antisolar (image credit: SVOM collaboration)
Table 1: Optimization of the SVOM attitude law
Figure 6: SVOM - CXG (Camera for X- and Gamma-rays) FOV, Attitude law B1 (image credit: SVOM collaboration)
Figure 7: The SVOM attitude law: consequences on the exposure map 1 year scenario (image credit: SVOM collaboration)
Figure 8: Distribution of the useful time of the mission (image credit: SVOM collaboration)
Table 2: Distribution of the useful time of the mission
Figure 9: Evolution of the distribution of the useful time (image credit: SVOM collaboration)
As soon as a GRB will be located, its coordinates and its main characteristics will be sent to the ground within seconds with a VHF antenna. The VHF signal will be received by one of the ~40 ground stations distributed around the Earth below the orbit. The data will then be relayed to the Operation Center, which will send SVOM alerts to the Internet via the GCN and VO Event networks , and to the ground instruments GWAC and the GFTs.
Figure 10: Prompt dissemination of GRB parameters. Goal : 65% of the alerts received within 30 seconds (image credit: SVOM collaboration)
SVOM can also perform target of opportunity observations (with MXT and VT for instance), with a delay of few hours, which depends on the availability of uplink communication with the satellite.
Figure 11: Telecommand upload link. Sanya is dedicated , the others (Kourou & HBK) are on request. Time delay related to upload the slew commands : 70% [40%] within 6  hours (image credit: SVOM collaboration)
SVOM will try to select high-z GRB candidates by analyzing MXT and VT data, and multi-band photometry data from GFTs. If a GRB is detected by MXT in the soft X-rays, but not by VT in the optical band, it will be selected as a candidate of high-z or an optically dark GRB. Ground telescopes with NIR capabilities are encouraged to follow up these candidates as soon as possible to try to measure their redshift. GFTs will be able to measure photometric redshifts by observing GRB afterglows with multiple filters from the visible to the NIR bands.
Figure 12: Common field of view : SVOM - JWST (image credit: SVOM collaboration)
Legend to Figure 13: Each GRB detected in the Southern Sky will be followed by LSST . Thanks to the antisolar choice, the transients sources detected by LSST could be observed immediately by SVOM.
In summary, SVOM will be a highly versatile astronomy satellite, with built-in multi-wavelength capabilities, autonomous re-pointing and dedicated ground followup. With its peculiar pointing strategy and the low energy detection threshold, SVOM is expected to improve the number of GRBs detected at high redshift, and hence to contribute to the use of GRBs as probes of the young Universe. — Beyond the GRB studies emphasized here, SVOM will bring new observations about all types of high energy transients, in particular those of extragalactic origin (TDEs, AGNs, etc.).
Sensor complement: (ECLAIRs, GRM, MXT, VT)
The main SVOM general objective is the survey of GRBs (Gamma Ray Bursts), in coordination with ground telescopes. The other main on-board instruments are ECLAIR (Gamma-ray Imager), GRM (Gamma Ray Monitor) and VT (Visible Telescope). The MXT (Microchannel X-ray Telescope) is based on a cooled silicon-based detector, provided by the Max-Planck-Institut für Extraterrestrische Physik (MPE) and encapsulated in a camera developed by CEA, and a set of microchannel plates manufactured by Photonis Technologies SAS (France).
Figure 14: Illustration of the SVOM spaceborne and ground-based instruments (image credit: SVOM collaboration)
ECLAIRs (Gamma-ray Imager)
The ECLAIRs project of CNES is managed by CEA Saclay (Commissariat à l'Energie Atomique), France. ECLAIRs (Figure 15) is the instrument on board the satellite that will detect and locate the GRBs. ECLAIRs is made of four parts: a pixelated detection plane (1024 cm2) with its readout electronics, a coded mask, a shield defining a field of view of 2 steradians (89º x 89º), and a processing unit in charge of detecting and locating transient sources. The detection plane is made of 200 modules of 32 CdTe detectors each, for a total of 6400 detectors of size 4 x4 x 1 mm. Each module is read by a customized ASIC connected to an electronics that encodes the position, the time and the energy of each photon. One of the requirements of ECLAIRs is to reach an energy threshold of 4 keV, in order to study soft GRBs (Gamma Ray Bursts) like X-Ray flashes and highly redshifted GRBs. 15) 16)
As shown in Figure 15, the first modules satisfy the low energy threshold requirement. 17) The coded mask is a square of side 54 cm located at a distance of 46 cm from the detection plane; it has an opening fraction of 40% and provides a localization accuracy of several arc minutes (~14' for a source at the limit of detection). The instrument features a count rate trigger and an image trigger, like Swift. These triggers are computed, from the photon data, in several energy bands and on time-scales ranging from 10 ms to several minutes. Our simulations show that ECLAIRs will detect 70-80 GRBs/yr.
Figure 15: Left: Schematics showing the different subsystems of ECLAIRs except the data processing unit in charge of the GRB detection and localization. Right: Laboratory spectral measurements performed on a detector module prototype (a 32 CdTe pixel matrix) with radioactive sources (241Am). The red vertical line corresponds to the expected low energy threshold of the ECLAIRs camera (image credit: ECLAIRs collaboration)
GRM (Gamma Ray Monitor)
GRM is a gamma-ray non-imaging spectrometer will extend the prompt emission energy coverage. GRB (Gamma Ray Burst) alerts are sent in real-time to the ground observers community.
GRM consists of a set of three detection modules. Each of them is made of a scintillating crystal (sodium iodide), a photomultiplier and its readout electronics. Each detector has a surface area of 200 cm2 and a thickness of 1.5 cm. One piece of plastic scintillator in front of NaI(Tl) is used to distinguish low energy electrons from normal X-rays. The three modules are pointed at different directions to form a total field of view of 2 sr, within which rough (of the order of 10 degrees radius) localization of transient sources can be achieved on-board.
The energy range of the GRM is 15-5000 keV, extending the energy range of ECLAIRs towards high energies to measure EPeak for a large fraction of SVOM GRBs. The project expects that GRM will detect > 90 GRBs/yr. GRM will have a good sensitivity to short/hard GRBs, like the GBM of Fermi. GRM can generate on-board GRM-only triggers, taking use of only GRM detectors. Such triggers with localization information will be transferred to ECLAIRs for trigger enhancement on the short GRBs, and to ground facilities (e.g. GWAC, GW experiments) for joint observations. A calibration detector containing one radioactive 241Am isotope is installed on the edge of each detection module, for the purpose of gain monitoring and energy calibration. In addition, a particle monitor auxiliary to GRM can generate South Atlantic Anomaly alerts and help protecting the detection modules.
MXT (Microchannel X-ray Telescope)
MXT is soft X-ray instrument on board SVOM, featuring a cooled silicon detector, provided by MPE (Max-Planck-Institut für Extraterrestrische Physik). MXT is a very light (<35 kg), and compact (<1.2 m) focusing X-ray telescope. Its large field of view (1 degree) and its sensitivity below the mCrab level make of MXT a very good instrument to identify and precisely localize (below the arc minute) X-ray transients in non-crowded fields, and to study them in detail, thanks to its excellent spectral response. It is designed to measure radiation from 0.2 to 10 Kev with a maximum sensitivity around 1 keV. MXT is designed to measure radiation from 0.2 to 10 keV. 18)
MXT is composed by five main subsystems: an optical module based on square MPO (Micro-Pore Optics), a camera, a carbon fiber structure, a data processing unit and a radiator (Figure 16). A small baffle provides a protection against direct sun illumination. The interface with the satellite is made through 3 fixation zones and a titanium ring. The nominal focal length of the instrument is F=1 m, although studies are going on to increase it to 1.15 m.
The optics of MXT is based on a "Lobster Eye" geometry and optimized for a narrow-field use (Figure 17 left). The rays hit the inner walls of the micro-pores with grazing incidence. The pores are square with d=40 µm size and a pitch of p=52 µm. The inner walls are coated with a 25 nm Ir layer to boost the reflectivity. The pores are grouped in plates of 40 x 40 mm side (600,625 pores), 21 of them are used to full the aperture (Figure 16, right). Their thickness is optimized to avoid vignetting and maximize the effective area. The central plates have L=2.4 mm thickness while the outermost ones have L=1.05 mm. They are bonded on an aluminum frame which upper face is a sphere of radius 2000 mm, with 10 µm machining precision. The MPO are covered with a 70 nm Aluminum film to avoid thermal flux and straylight from entering the instrument.
The PSF (Point Spread Function) has a peculiar form, and is composed by a central spot and two cross arms (Figure 17, right): X-rays entering in the MPOs can either be reflected twice and focused in the central PSF spot, or reflected just once and focused in the PSF arms. For MXT, about 50% of the incident X-ray flux is focused in the central spot, 2 x 22% in the arms, and the rest in a diffuse patch. Thanks to the "Lobster Eye" geometry the vignetting is very low, of the order of 10-15% at the edge of the FOV.
Camera: The PSF is imaged on a silicon (Si) based pn CCD having 256 x 256 75 µm side pixels associated with FEE (Front-End Electronics) based on two CAMEX. It is fully depleted (450 µm depth) and has excellent low-energy response (45-48 eV (FWHM) @ 277 eV), and energy resolution (123-131 eV FWHM @ 5.9 keV). The spatial sampling is good enough to avoid any degradation of the PSF. The CCD is thermally controlled by 3 TEC (Thermoelectric Cooler) at -65°C (to reduce noise) with a daily stability better than ±1ºC. The CCD is covered by a 100 nm Aluminum layer to protect against UV/Visible straylight because the CCD is still sensitive in this spectral range. This results in a slight transmission loss in the X-ray range. It is also protected against background X-rays by an aluminum shielding. A filter wheel can put various filters in front of the CCD and its entrance cone. The external box of the camera provides the interfaces with the radiator, the front end electronics and the tube (Figure 18).
With a 1.0 m focal length, each pixel corresponds to 15.4 arcsec. The FOV is in practice limited by the CCD and is a 57 x 57 arcmin square. At 20 arcmin FOV radius the vignetting factor is greater than 0.9.
The instrument effective area is a combination of the optics effective area, the transmission of the aluminum filters, the size of the CCD and its quantum efficiency.
X-rays: The PSF at the center of the FOV was extensively studied by UoL with a dedicated software. We report here a comparison of what we obtained with Zemax in a simplified manner.
Modeling in Zemax: Zemax is a general optical software widely used in the world and which benefits from a high level of validation. It can be used to model small wavelengths provided the objects are not too small (>10 λ) and the index data are available. For this last point, we used the database of CXRO (Center for X-Rays Optics). The size of the pores and of the wedges between them (few μm to tens of μm) is much larger than the criteria of 10 λ (here typically 10 nm).
The glass used for the MPO has a high density. Hence, in the soft X-rays range, even a small thickness of glass is enough to stop rays. This allows to consider only reflected rays and to use traditional ray tracing in a quite simple manner.
In NSC (Non-Sequential Sources) mode, Zemax assumes that all the objects are defined before launching the rays. The basic element we use is an "extruded object". By defining a section and stretching it along one direction, Zemax can represent this way pores or group of pores. As it is impossible to represent simultaneously the ~12 million pores involved in MXT optics, the way to proceed is to represent a small subset of pores, launch rays for this subset, measure the energy on a detector, move the subset onto the aperture and cumulate the energy deposited on the CCD. The programming of this loop can be done using the ZPL language and dedicated macros. The spherical geometry of the "lobster eye" is quite easy to implement. Various manufacturing errors can be introduced at the subset level, like an orientation error or a pore shape error. Performing this process with a subset of 1 pore is much too fine and takes a too long time to simulate. We found that using a 5 x 5 pores subset (Figure 19), with uniform manufacturing errors at this scale, was a good compromise between computation time and representativeness of the geometry. 25 of this objects forms a multi-fiber (25 x 25 pores).
The number of rays to launch plays an important role. To avoid excessive time computation ones has to use the minimum number of rays to correctly represent the PSF. We performed some tests with increasing number of rays and we found that having 10 to 100 rays for one multi-fiber (625 pores) was sufficient to have better than 1% precision results. The source object is just a rectangle, the size of a multi-fiber.
The reflectivity of the 25 nm Ir layer is modelled with tabulated data from the CXRO database (angle, wavelength) assuming a roughness of 1.3 nm rms (Figure 20, left). The 70 nm aluminum film in front of the MPO plays a role in the effective area, so its transmission as to be taken into account (Figure 20, right, CXRO database). - The detector is modelled simply as an array of 256 x 256 75 µm size pixels.
Perfect geometry and pores: Assuming perfect pores and a perfect geometry, the FWHM of the PSF central spot is limited by three aberrations:
• Spherical aberration: Δθs= 4 √2 (d/L)3
• Diffraction: Δθd= 2 (λ/d)
• Geometric pore size: Δθg= d/F
For the adopted MXT parameters, the three aberrations have approximately the same value, 10 arcsec.
MPO manufacture errors: In the end, the FWHM is not dominated by these factors but by the manufacturing errors which lead to imperfect geometry and limit the FWHM. The two most important ones are the pore shear and the pore alignment errors. The first one is a distortion of the pore shape from a square to a parallelogram (angle θh). It is introduced when the square flat MPO is slumped onto a spherical tool under pressure. It is more important in the corners of a MPO plate than in the center. The pore alignment error (angle θa) is induced by the stacking process or the slumping. The axis of the pores is no more aligned with the normal to the spherical surface, deviating the output rays with an angle twice that of the alignment error. Another important error is the pore figure error (angle θf): the inner walls of the pores are not perfectly flat but have low order WFE (Wavefront Errors) which depend mainly on the etching process. The effect of qa and qf overlaps and are not easily separated. Typical values from UoL are θh =0.3° and θaf =0.75 arcmin.
In our simulations we assumed that qh is constant over one MPO and θaf can be represented by a gaussian error with 0.75 arcmin rms standard deviation. No finer dependence of these errors with the position in one plate (corner / edge / center) has been taken into account. The typical effect of the shear error is found (Figure 21, left): the main peak is split into four smaller peaks. Cumulating all the errors (Figure 21, right), we found a 4.1 arcmin FWHM to be compared with the 4.4 arcmin of UoL. These results are in good agreement given the simplifications we made.
The effective area could also be computed at various energies and is compared with the UoL data on Figure 22 (left). The Zemax results fit reasonably well with UoL data within a 10-15% precision.
Figure 22: Left: Simulated effective area of the instrument for θh=0.3, θaf=0.75 arcmin rms. In green, total effective area, in blue, central spot only. Red squares and purple diamonds are the Zemax simulation. Right: impacts of assembly errors on the defocus curve. The reference model is with θh=0.3º, θaf=0.75 arcmin rms. sR is the standard deviation of the MPO radius, <Rm> is its averaged value. srxry is the standard deviation of the tip/tilt error of the MPO mounted on the frame (image credit: CNES, CEA)
EUV (Extreme Ultraviolet): The aluminum film on the MPO has a transmission window in the EUV spectral range (Figure 23). For the GRB localization, this could be a problem if bright sources are in the vicinity because of the induced noise. That's why we studied the optical response in this range. The previous computations performed in X-rays can be extended in the EUV range, but the diffraction has now to be taken into account as the diffraction angle is Δθd =3.4 to 8.6 arcmin when l varies from 40 to 100 nm. One way to put diffraction in Zemax NSC mode is to use a diffractive source instead of a normal one. Unfortunately this only works for simple source shapes, like a square. The previously described computation procedure can be adapted, but using a single pore as a basic object and taking advantage of MPO symmetries. This lead to quite high computation time on a standard PC (few days) but remains feasible if not too much runs are required. The scattering is not taken into account directly, but the reflection coefficients computed in the CXRO database are taken with 1.3 nm rms roughness (at 40 nm, supposed to be constant up to 80 nm).
Figure 23: Left: EUV simulation on one pore with characteristic diffraction effect of a square pore aperture. Right: PSF simulation for 21 MPO at λ=80 nm with θh=0.3º, θaf=0.75 arcmin rms, srxry=0.3º rms (image credit: CNES, CEA)
Figure 23 shows the diffraction of one pore (left) and the simulated PSF for the full aperture at 80 nm including manufacturing errors (right). We found that the FWHM is 5.1 arcmin at 40 nm and 7 arcmin at 80 nm. It is very close to a quadratic sum of the FWHM without diffraction and the Δθd value. The effective area of the central spot without the Al transmission is close to 43 cm2 (including 170 nm Al transmission, we have 28 and 6 cm2 at 40 and 80 nm).
So MXT forms focused images of EUV sources, with a significant effective area and with a FWHM enlarged by the diffraction. At higher wavelength (> 200 nm), the diffraction dominates and becomes huge so that no focused image is formed. Estimations show that a G2V star at 100 pc (EUV flux leads to 0.6 x 10-5 e-/px/0.1s. For a O9V star at 300 pc, we get 3800 e-/px/0.1s. In some cases, bright EUV sources in the FOV could be a problem for MXT if it is used to point in too crowded areas. A dedicated filter will be placed on the filter wheel.
Results: The Earth is the main optical straylight source for MXT. The important parameter to study is the guard angle between the instrument boresight and the earth limb, as this is an operational constraint for the mission. The current proposed value is 20°, derived from the SWIFT mission. As the satellite altitude is about 600 km, MXT has potentially a large portion of the Earth in its field of view. Using the PST previously computed, the orbital parameters, the Earth flux from SPOT satellites data and for UV Earth albedo, a 2D integral over the portion of Earth having a factor of view with the optic entrance pupil can be computed numerically. It is multiplied by the transmission of the 70 nm film (t70), including the 1/cosi effect on the thickness, the transmission of the 100 nm coating on the CCD (t100) and the efficiency ήq of the CCD . t70 includes a fraction u of "open" pores per MPO with unity transmission. They result from defaults in the aluminum film and depressurization during the launch. The current requirement is u=50.
With the baseline design (dark blue curve, diamond), the noise limit is reached at 12° angle. For this value, the integral is dominated by a small area of the earth with the minimal incidence angles on the optics (i ranging from 10 to 25°). At 20° angle the margin is quite good (factor of 5). For low guard angles, the dominating paths to the CCD are the specular rays out of the pores hitting indirectly the CCD after reflection on the tube walls. When increasing the guard angle, the portion of the Earth seen by the optics decreases. The incident power on the optics decreases from 8.3 to 2.3 W when varying from 10 to 40°. Also, the average incidence angle increases, hence the collecting area decreases with cosi. The integral is less dominated by a small area. The scattered rays in the pores with direct path to the CCD dominate more. Spectrally, the noise is dominated by near IR because of the increasing transmission of the Al film in this range, despite the low energy of the photons.
In summary, by using Zemax, we could model the in FOV X-rays properties of MXT optics. The obtained results are well in line with the UoL predictions using the state of the art for manufacturing errors: 0.75 arcmin for pore alignment/figure error and 0.3° for the shear error. We find that during assembly of the MPO on the frame, tip/tilt errors shall be lower than 1 arcmin. Furthermore, the standard deviation of the MPO radius of curvature shall be less than 50 mm rms. The optics also forms an image in the EUV range but it is enlarged due to diffraction. As the collecting area is not negligible, care as to be taken with bright sources in the FOV while detecting a GRB. A dedicated filter is implemented on the wheel. The straylight analysis shows that the sun is not a problem because the radiator and the other instruments are lower than the aperture. The moon has a negligible effect and could be as close as 10° from the instrument boresight. The earth will be the dominating straylight source in the visible/IR. The 20° guard angle is safe with a quite good margin. Given the many uncertainties of our computations it is not possible to lower this limit. The tube inner walls shall be covered with a diffusive black paint. No long baffle is required, only a short one to prevent the sun from hitting directly the optics in some rare cases. Further work will include a more precise model of the various errors, especially the recently measured distortions occurring at the multi-fiber interfaces, and test on samples to validate the models.
VT (Visible Telescope)
VT is a dedicated optical follow-up telescope on board the SVOM satellite. Its main purpose is to detect and observe the optical afterglows of gamma-ray bursts localized by ECLAIRs. It is a Ritchey-Chretien telescope (Figure 24) with a diameter of 40 cm and an f-ratio of 9. Its limiting magnitude is about 22.5 (MV) for an integration time of 300 seconds VT is designed to maximize the detection effciency of GRB's optical afterglows.
Instead of a filter wheel, a dichroic beam splitter is used to divide the light into two channels, in which the GRB afterglow can be observed simultaneously. Their wavelength ranges are from 0.4 µm to 0.65 µm (blue channel) and from 0.65 µm to 1 µm (red channel). Each channel is equipped with a 2k x 2k CCD detector. While the CCD for the blue channel is a normal thinned back-illuminated one, a deep-depleted one is adopted for the red channel to obtain high sensitivity at long wavelengths. The QE (Quantum Effciency) of the red-channel CCD at 0.9 µm is over 50%, which enables VT to have the capability of detecting GRBs with the redshift larger than 6.5. The field of view of VT is about 26 ' x 26', which can cover the error box of ECLAIRs in most cases. Both CCDs have a pixel size of 13.5 µm x 13.5 µm, corresponding to spatial resolutions of 0.77 arcsec. This ensures the GRB positioning accuracy to be greatly improved by VT from several arcmin (ECLAIRs) and tens of arcsec (MXT) to a level of sub-arcsec.
In order to promptly provide the GRB alerts with the sub-arcsec accuracy, VT will do some data processing on board. After a GRB has been localized by the coaligned MXT, lists of sources are extracted from the VT sub-images whose centers and sizes are determined by the GRB positions and the corresponding error boxes provided by MXT. The lists are immediately downlinked to the ground through the VHF network. Then, the ground software will make finding charts with these lists (Figure 24) and search the optical counterparts of the GRB by comparing the lists with the existing catalogs.
If a counterpart is identified, an alert will be then produced and distributed to the world-wide astronomical community, which is useful for triggering large ground-based telescopes to measure the redshifts of the GRBs by spectroscopy. VT is expected to do a good job on detecting high-redshift GRBs. The confirmed high-redshift GRBs are rare in the Swift era, in contrast to a theoretical prediction of a fraction of more than 10%. 19)
This is probably due to the fact that for most Swift GRBs the early-time optical imaging follow-up is not deep enough for a quick identification and some faint GRBs cannot be spectroscopically observed in time by the large ground-based telescopes. This passive situation will be significantly improved by SVOM, due to the high sensitivity of VT, in particular at long wavelength, and the prompt optical-counterpart alerts. Additionally, the anti-solar pointing strategy of SVOM allows GRBs to be spectroscopically observed by large ground-based telescopes at the early time of the bursts. Consequently, more high-redshift GRBs are expected to be identified in the SVOM era. VT is also used to support the platform to achieve the required high pointing stability. A FGS (Fine Guidance Sensor) is mounted on the VT focal plane to measure relative image motions. Its images are processed in real-time by a specialized data processing unit to get the centroid positions of several stars brighter than the magnitude of 15 (MV ). The results are sent at a frequency of 1 Hz to the platform to improve the pointing stability, which enables VT to have a good performance in a long exposure time.
Ground Based Instruments
The ground follow-up instruments constitute an important part of the mission. Three instruments are developed for the follow-up of SVOM GRBs: a wide angle camera that surveys a significant fraction of the sky for transients, and two robotic telescopes. In addition to these dedicated instruments, the SVOM collaboration will seek agreements with various existing telescopes or networks willing to contribute to the follow-up of SVOM GRBs.
GWAC (Ground Wide Angle Camera)
GWAC (Figure 25) provides a unique way to survey a large field of view for optical transients. The instrument will monitor 63% of ECLAIRs field of view, looking for optical transients occurring before, during and after GRBs. GWAC will also have its own trigger system, providing alerts to the world. GWAC is a complex system: the heart of the system is a set of 36 wide angle cameras with a diameter of 18 cm and a focal length of 22 cm, together these cameras cover a field of view of 60 º x 60º. They use 4k x 4k CCD detectors, sensitive in the range of wavelength 500-800 nm. These cameras reach a limiting magnitude V=16 (5σ) in a typical 10 second exposure. This set of cameras is completed by two 60 cm robotic telescopes. equipped with EMCCD cameras. These telescopes will provide multicolour photometry of the transients discovered by GWAC with a temporal resolution ≤ 1 second.
Figure 25: First prototype of one GWAC module. The final system will be composed by 9 modules. Right: the figure shows the discovery space of an instrument dedicated to short time-scale optical transients (image credit: P.R. Wozniak, et al., 2009)
GFTs (Ground Follow-up Telescopes)
The ground follow-up telescopes have two main goals. Firstly, they measure the photometric evolution of the optical afterglow in the first minutes after the trigger in a broad range of visible and NIR wavelengths, with a temporal resolution of few seconds. Secondly, when an afterglow is detected, they provide its position with arc second precision within 5 minutes of the trigger. Some essential features of the GFTs are their field of view, their size, and their sensitivity in the near infrared. The field of view (~30 arc minutes) enables observing quickly the entire error boxes of ECLAIRs. The size, typically 1 meter, allows the detection of all visible (i.e. non-dark) afterglows at the condition to arrive within few minutes after the trigger. 20) 21)
Finally, the near infrared sensitivity permits the detection of high-z GRBs and GRBs extinct by dust, whose afterglow are obscured in the visible domain. 22) GFTs are especially useful for the study of the early afterglow during the slew of the satellite, and for the rapid identification of the optical afterglow in various cases: when SVOM cannot slew to the burst or when the slew is delayed due to pointing constraints, and when the optical afterglow is only visible in the NIR. One telescope is located in China at Xinglong Observatory and the other one will be located in Mexico at San Pedro Martir.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).