Gamma-ray burst: from the central engine to future observations with athena

Gamma-Ray Bursts (GRBs) are among the most luminous and distant sources in the Universe. In the 33% of the GRBs afterglow light curves is observed the presence of one or multiple X-ray flares, which energy ranges from a percent of, up to a value comparable to, the one of prompt emission. X-ray flare are believed to mark the late-time activity of the central engine. The temporal properties of X-ray flares make it difficult to interpret the observed emission in the framework of the external shock scenario. The comprehension of the X-ray flare origin enables to obtain information about the central engine that cannot be directly observed, indeed the first signal detection, due to the prompt emission, occurs at high distances from the central engine it- self. The guideline will be the study of the time variability of the flares observed with XRT instrument on board of the Swift satellite. After that the results from the XRT data analysis will be compared with observations of the prompt events made by the BAT instrument. This will allow to link prompt emission variability with the eventual flares ones. In fact the prompt emission has an high variability with time and if the same variability is confirmed by both BAT and XRT data, it will be a strong proof that X-ray flares are connected with the residual activity of the central engine, and a confirm of the current theoretical model for X-ray flares. The Athena observatory class mission will address the study of GRBs in the X-ray band, allowing the diagnostic of the metal content around the central engine, will answer the open problem of X-ray flares origin at very high redshifts, and finally will address for the first time the detection and characterization of the WHIM. In this regard it is vital for the implementation of the observations to analyze and optimize the performances of the instruments in terms of background and spectroscopy. Therefore the second part of this work is addressed to the study how the performances of the X-IFU instrument could be affected during a Solar Energetic Particle event (SEP)