By Sara Walsh

At low redshifts, the observed baryonic density falls far short of the total number of baryons predicted. It is predicted that these ‘missing’ baryons reside in large scale gaseous filaments known as the warm-hot intergalactic medium, or WHIM. Athena aims to measure the local cosmological baryon density in the WHIM to better than 10% and to constrain structure formation models in the low-density regime by measuring the redshift distribution and the physical parameters of 200 filaments against bright background sources. To achieve this, Athena aims to detect 200 filaments in the WHIM through absorption, 100 towards active galactic nuclei (AGN) and 100 towards bright gamma-ray burst (GRB) afterglows. We analyse GRB afterglow spectra, generated using the SIXTE simulator, to determine the probability of detecting the WHIM through OVII (574 eV) and OVIII (654 eV) absorbers with the X-IFU instrument of Athena. We provide limits to the brightness and location of the GRB afterglows that can be observed to reveal the missing baryons of the WHIM with Athena and show that Athena may detect 45 - 137 absorbers through OVII-OVIII absorption features during the four-year mission lifetime.

Access to the manuscript in ADS.