Abstract
The SVOM multi-wavelength observatory is dedicated to the study of transient phenomena in the Universe, in particular gamma-ray bursts (GRBs), which are extremely luminous and energetic phenomena at the core of multi-messenger astrophysics. GRBs are extra-galactic sources and result from the gravitational collapse of a fast rotating massive star, or the coalescence of two compact objects in a binary system. My work focuses on the on-orbit calibration of the ECLAIRs coded aperture telescope (4-150 keV) and the GRM gamma-ray monitor (0.05-5 MeV) onbard SVOM, by observing known astrophysical sources, and on the spectral analysis of the brightest GRBs, which will trigger a satellite repointing. I contributed to the software used to simulate observational data, in particular I built an event bank with GEANT4 for the GRM, which includes the three main components of instrumental background (cosmic X-ray background, Earth albedo and reflection). With these tools, I have tested different methods of background modeling using the counting technique, which will be necessary for the analysis of the GRBs detected by ECLAIRs and the GRM. Firstly, I have shown that the usual fitting method that is based on a simple temporal model does not work in various scenarios, such as platform repointings, or for the study of non-transient sources. Secondly, I have developed two models to reproduce the temporal variations of the background during a repointing, in particular a physical model with two free parameters. For a medium- to high-fluence burst, this model leads to an unbiased integral flux with an accuracy of 1% to 9% when combining ECLAIRs and GRM. These spectroscopic performances fully satisfy the mission scientific requirements. Finally, only the physical model is suited to the study of non-transient bright sources for ECLAIRs / GRM inter-calibration. I have simulated and analyzed the three best candidate sources around their emersions and occultations by the Earth. The Crab nebula is very intense, but requires the use of the GRM background model. Its spectral analysis shows very good results for ECLAIRs, with a flux accuracy of 2%. The background estimation introduces a systematic bias in the spectrum measured with one of the three GRM detectors, which is however not significant when the three detectors are combined. In the case of Cygnus X-1, whose intensity is less than half of the Crab nebula, the reconstruction of the source flux with the GRM fulfills the mission requirements, with a bias limited to 12%, only by assuming that the uncertainties on the background are known to within a few percent, which seems unrealistic. Finally, as the analysis of the Crab pulsar does not require the application of the background model, the flux measured by each instrument is accurate to within a few percent over a dozen orbits. My work shows that it will be possible to study these three sources with reasonable exposure, especially the Crab nebula and pulsar. Their spectral analysis should achieve the statistical accuracy that is needed to reveal then to correct potential systematic errors due to uncertainties in instrument response. However, the background model will have to be validated with real data after the satellite launch, as will the simulation tools.