Additional astrophysical objectives for LISA follow-on missions

Abstract

The main LISA follow-on mission currently being discussed is one that would search with high sensitivity for a primordial gravitational wave background. To extend the discussion of possible astrophysical objectives, two other types of LISA follow-on missions will be considered here. In both cases the expected level of performance improvement is substantial. One is a mission that emphasizes higher sensitivity at frequencies of 0.003 to 10 Hz. The other aims for improved sensitivity at frequencies from 1 to 100 µHz. For the higher frequency mission, a factor 30 increase in the product of telescope area times the square root of laser power would be the goal. This could be achieved with 1 m diameter telescopes and 10 W of laser power. The arm length would be reduced to 0.5 million km, so that the total increase in GW sensitivity above 0.1 Hz would be a factor of 300. A factor 10 reduction in the test mass spurious accelerations at frequencies above 1 mHz would be included also. The main scientific objective would be to observe interactions of 10 M⊙ BHs with IMBHs out to large redshifts, if MBHs such as the few million M⊙ BHs observed now grew through the IMBH mass range. For the low frequency mission, 15 million km baselines and a factor 10 or more reduction in the spurious acceleration levels from 1 to 100 µHz would be the goals. The main objective would be the observation of many more MBH–MBH binaries to learn about galaxy formation.