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  1. Abstract The science objectives of the LISA mission have been defined under the implicit assumption of a 4-years continuous data stream. Based on the performance of LISA Pathfinder, it is now expected that LISA will have a duty cycle of $$\approx 0.75$$ ≈ 0.75 , which would reduce the effective span of usable data to 3 years. This paper reports the results of a study by the LISA Science Group, which was charged with assessing the additional science return of increasing the mission lifetime. We explore various observational scenarios to assess the impact of mission duration on the main science objectives of the mission. We find that the science investigations most affected by mission duration concern the search for seed black holes at cosmic dawn, as well as the study of stellar-origin black holes and of their formation channels via multi-band and multi-messenger observations. We conclude that an extension to 6 years of mission operations is recommended.
  2. We study the thermal evolution of UV-irradiated atomic cooling haloes using high-resolution three-dimensional hydrodynamic simulations. We consider the effect of H^- photodetachment by Ly{\alpha} cooling radiation trapped in the optically-thick cores of three such haloes, a process which has not been included in previous simulations. H^- is a precursor of molecular hydrogen, and therefore, its destruction can diminish the H2 abundance and cooling. Using a simple high-end estimate for the trapped Ly{\alpha} energy density, we find that H^- photodetachment by Ly{\alpha} decreases the critical UV flux for suppressing H2-cooling by up to a factor of \approx 5. With a more conservative estimate of the Ly{\alpha} energy density, we find the critical flux is decreased only by ~15-50 percent. Our results suggest that Ly{\alpha} radiation may have an important effect on the thermal evolution of UV-irradiated haloes, and therefore on the potential for massive black hole formation.