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ABSTRACT The association of GRB170817A with a binary neutron star (BNS) merger has revealed that BNSs produce at least a fraction of short gamma-ray bursts (SGRBs). As gravitational wave (GW) detectors push their horizons, it is important to assess coupled electromagnetic (EM)/GW probabilities and maximize observational prospects. Here, we perform BNS population synthesis calculations with the code mobse, seeding the binaries in galaxies at three representative redshifts, $$z$$ = 0.01, 0.1, and 1 of the Illustris TNG50 simulation. The binaries are evolved and their locations numerically tracked in the host galactic potentials until merger. Adopting the microphysics parameters of GRB170817A, we numerically compute the broad-band light curves of jets from BNS mergers, with the afterglow brightness dependent on the local medium density at the merger site. We perform Monte Carlo simulations of the resulting EM population assuming either a random viewing angle with respect to the jet, or a jet aligned with the orbital angular momentum of the binary, which biases the viewing angle probability for GW-triggered events. We find a gamma-ray detection probability of $$\sim\!2{{\rm per\ cent}},10{{\rm per\ cent}},\mathrm{and}\ 40{{\rm per\ cent}}$$ for BNSs at $$z$$ = 1, 0.1, and 0.01, respectively, for the random case, rising to $$\sim\!75{{\rm per\ cent}}$$ for the $$z$$ = 0.01, GW-triggered aligned case. Afterglow detection probabilities of GW-triggered BNS mergers vary in the range of $$\sim \! 0.3 \!-\! 0.5{{\rm per\ cent}}$$, with higher values for aligned jets, and are comparable across the high- and low-energy bands, unlike gamma-ray-triggered events (cosmological SGRBs) which are significantly brighter at higher energies. We further quantify observational biases with respect to host galaxy masses. 

Abstract The Einstein Telescope (ET), the European project for a third-generation gravitational-wave detector, has a reference configuration based on a triangular shape consisting of three nested detectors with 10 km arms, where each detector has a 'xylophone' configuration made of an interferometer tuned toward high frequencies, and an interferometer tuned toward low frequencies and working at cryogenic temperature. Here, we examine the scientific perspectives under possible variations of this reference design. We perform a detailed evaluation of the science case for a single triangular geometry observatory, and we compare it with the results obtained for a network of two L-shaped detectors (either parallel or misaligned) located in Europe, considering different choices of arm-length for both the triangle and the 2L geometries. We also study how the science output changes in the absence of the low-frequency instrument, both for the triangle and the 2L configurations. We examine a broad class of simple 'metrics' that quantify the science output, related to compact binary coalescences, multi-messenger astronomy and stochastic backgrounds, and we then examine the impact of different detector designs on a more specific set of scientific objectives.