The advent of space faring in the 1960s undoubtedly opened a new era in the exploration of the Universe. Access to space now allowed scientists to study radiation that was impossible to measure from the ground and to benefit from special observing conditions, such as the absence of atmospheric turbulence.
Nevertheless, the complementarity of ground and space-based observations quickly became a necessity owing to the need for multi-wavelength coverage of celestial targets. Today, any space project must include a ground-based component in its preparation and support from terrestrial observatories for follow-up observations. The implementation of synergized ground and space-based observations is however subject to several types of constraints imposed by the mode of operation of the satellite and the nature of the ground-based operations.
SVOM is a low earth orbit satellite, orbiting at 650 km. At this altitude, the satellite travels around the Earth in just over 95 minutes (15 revolutions per day). The orbit is inclined by 30° with respect to the terrestrial equatorial plane and as a result, the satellite’s trajectory oscillates between latitudes of +30° and -30°. The orbit parameters are the result of a compromise between the launch site, the power of the launcher (and its cost) and the satellite’s stabilization system. The video below shows the track of the satellite (in green) over several revolutions; We also distinguish the day zone and the night zone, credit: CNES.
The first constraint related to the low earth orbit is to ensure permanent contact with the satellite. One solution is to deploy a network of antennas under its track. The choice of the satellite’s inclination then fixes the number of stations. In the case of SVOM and its 30° inclination, this means appropriately distributing 47 antennas between latitudes -30° and +30°.
The second constraint is imposed by the need to maintain the satellite and its instruments at suitable temperatures. One solution consists in defining a cold face of the satellite so as to evacuate via heaters the calories produced by the electronics. This cold face must be maintained at more than 90 degrees from the Sun. As a result, half the sky is not observable at any given time.
The third constraint linked to the scientific objective of the SVOM mission is to always observe areas of the sky accessible by ground telescopes during their local nighttime. This implies that the optical axis of the instruments on board the satellite point in the direction opposite to the Sun. This strategy comes at a price however: the Earth will hide the field of view of the instruments once per orbit, up to 50% of the period of revolution i.e. 45 minutes.
The fourth constraint is determined by the vital access to large ground-based telescopes (VLT, Hawaii, La Palma). In order to benefit from optimal observations, the area of the sky targeted by the onboard instruments must be close to the zenith of these large telescopes.
Other constraints are imposed by our galaxy: the Milky Way. Indeed, our galaxy harbors many transient X and gamma-ray sources able to mimic a burst and to trigger the detection chain. Furthermore, if a burst is detected through the Galaxy, its ground follow-up will be strongly affected by interstellar absorption. Finally, the Scorpius X-1 source located outside of the galactic plane is extremely bright in the energy range studied by SVOM. In order not to disturb the measurements, it must be avoided.
These strongly coupled constraints led to the definition of the cold face of the satellite, the arrangement of the onboard instruments and the pointing strategy. The latter is commonly referred to as the “attitude law”.
The Ground/Space Synergy
The study of astronomical objects requires many resources, in particular from a material point of view. It is necessary to be able to acquire the most precise data possible, all the while launching a satellite with the flexibility of execution required for the observation of transient phenomena. The key word is optimization. To best meet the mission’s objectives, the synergy between the onboard instruments and those present on Earth is crucial.
SVOM thus implements an appealing combination of instruments.
Step 1: Detect and Locate
Detection takes place in space because gamma rays are stopped by Earth’s atmosphere. This task is devoted to the ECLAIR telescope, whose detection rate is estimated to be about 80 per year, with approximately 20% of very distant events with redshift greater than 6.An estimate of the event’s position in the sky is then transmitted in a few tens of seconds to the scientific community, transiting through the VHF alert network, whose relay antennas are strewn along the intertropical zone.
Step 2: Observe the Prompt Emission
During the time taken to localize the burst, the GRM gamma monitor will be able to provide an estimate of the peak energy (Epeak) of the burst. The peak energy is defined as the energy at which the burst radiates the maximum energy. Several studies seem to indicate a correlation between Epeak and the absolute brightness of the burst. On Earth, the GWAC, wide-angle telescopes, will give an observation of the prompt emission in the visible field.
Step 3: Increase Location Accuracy to Enable Follow-up
After the satellite’s automatic slewing maneuver in less than 5 minutes, the MXT and the VT, supplemented by the two GFTs on the ground, will ensure multi-wavelength follow-up systematically for several hours. (The VT will in particular allow the detection of nearly 75% of gamma-ray bursts in the visible domain, and for the first time, explore the area of dark bursts, for which the optical counterpart is not detected.) This marks the beginning of the search for afterglows.
Step 4: Redistribute the Alert
All these instruments are part of a cascade of operations designed to refine the location of the burst from a few arc minutes to a few arc seconds. The alerts will then be redistributed to the scientific community in real time through the GCN alert network or other networks available at launch.
Step 5: Determine the Distance
In the event of a good position estimate, the large ground-based general-purpose telescopes with a smaller field of view (such as the ESO VLT in Chile) will provide the possibility to acquire a spectrum. It will then be possible to measure the redshift in order to estimate the distance to the source.