Measuring of Channel-Energy relationship for the SVOM/ECLAIRs camera

The French-Chinese Space based multi-band Variable astronomical Object Monitor(SVOM) mission will be dedicated to the study of the high-energy transient sky with a particular interest for Gamma-ray bursts (GRBs). The mission is due to be launched in 2021. These cosmological and very powerful stellar explosions signal the catastrophic formation of a compact object (a black hole or a neutron star). GRBs appear as short-lived, highly variable and intense bursts of Gamma-ray light, followed by a long-lived multi wavelength afterglow emission characterized by a rapid flux decay over time. To study GRBs, SVOM will embark two wide-field instruments and two narrow-field telescopes on-board an agile platform. The wild field instruments will study the GRB prompt emission from 4 keV to 5 MeV and the narrow-field telescopes will study the afterglow emission in X-ray (0.2 10 keV) as well as in the visible once the satellite repoints towards the GRB position calculated by the ECLAIRs instrument. ECLAIRs, the prime instrument on board SVOM, is a 4-150 keV 2-D coded mask telescope in charge of autonomously detecting and providing the first GRB position. ECLAIRs will measure the arrival date, the position on the detection plane (6400 CdTe pixels) and the energy of each detected photon. The measured energy will be encoded over a linear dynamics (210) as a number (channel) by the readout electronics. To reconstruct the energy scale in physical units (keV), it is necessary to calibrate the linear channel-energy relationship (i.e to measure the gain and offset coefficients) for each of the 6400 ECLAIRs detectors. Experimental on-ground measures on the detection plane irradiated using radioactive sources will be performed to calibrate the relationship. In flight, due to the aging of electronics and detectors induced by the radiation environment along the SVOM orbit, the reconstruction of the energy scale may change for some pixels over time, making it necessary to re-calibrate the relationship for these pixels. To do so, we will make use of the measured ECLAIRs background, in particular the instrumental lines present below 90 keV. My thesis work focuses on investigating a robust and CPU time efficient method to derive updated values of the gain and offset coefficients for the 6400 ECLAIRs pixels using in-flight simulated ECLAIRs background spectra. I introduce the SVOMmission and the problem definition in section 1. In section 2.2, I explain the spectral fitting process of both the most intense instrumental lines and their underlying continuum using various models through an iterative χ2 minimization process. In section 3.1, I describe the method used to estimate the channel-energy relationship by measuring gain and offset parameters. The above method needs to be performed across all 6400 ECLAIRs detectors and hence, it is necessary to optimize the method to perform the analysis in the shortest possible time that is compliant with the ECLAIRs scientific specifications. In section 3.2, I explain how this optimization is achieved for the spectra produced with the minimum required event counts, while meeting the 1-sigma error criteria. The 1-sigma error criteria is met if the measured line centroid is within ± 1 channel from the line of interest.