Simulating the Dark Matter Decay Signal from the Perseus Galaxy Cluster

The nearby Perseus galaxy cluster is a key target for indirect detection searches for decaying dark matter. We use the C-EAGLE simulations of galaxy clusters to predict the flux, width, and shape of a dark matter decay line, paying particular attention to the unexplained 3.55. keV line detected in the spectra of some galaxies and clusters, and the upcoming XRISM X-ray observatory mission. We show that the line width in C-EAGLE clusters similar to Perseus is typically [600-800] km s(-1), and therefore narrower than the amplitude of the velocity dispersion of galaxies in the cluster. Halos that are significantly disturbed can, however, exhibit galaxy velocity dispersions higher than 1000. km s(-1), and in this case will show a large difference between the line profiles of on- and off-center observations. We show that the line profile is likely to be slightly asymmetric, but still well approximated by a Gaussian at the 10% level, and that the halo asymmetry can lead to fluxes that vary by a factor of two. In summary, we predict that, if the previously reported 3.55. keV line detections do originate from dark matter decay, the XRISM mission will detect a line with a roughly Gaussian profile at a rest-frame energy of 3.55. keV, with a width > 600 km s(-1). and flux approximately in the range [4-9] x 10(-8) counts s(-1) cm(-2).