  The multi-wavelength detection of GW170817 has inaugurated multi-messenger
astronomy. The next step consists in interpreting observations coming from
population of gravitational wave sources. We introduce saprEMo, a tool aimed at
predicting the number of electromagnetic signals characterised by a specific
light curve and spectrum, expected in a particular sky survey. By looking at
past surveys, saprEMo allows us to constrain models of electromagnetic emission
or event rates. Applying saprEMo to proposed astronomical missions/observing
campaigns provides a perspective on their scientific impact and tests the
effect of adopting different observational strategies. For our first case
study, we adopt a model of spindown-powered X-ray emission predicted for a
binary neutron star merger producing a long-lived neutron star. We apply
saprEMo on data collected by XMM-Newton and Chandra and during $10^4$ s of
observations with the mission concept THESEUS. We demonstrate that our emission
model and binary neutron star merger rate imply the presence of some signals in
the XMM-Newton catalogs. We also show that the new class of X-ray transients
found by Bauer et al. in the Chandra Deep Field-South is marginally consistent
with the expected rate. Finally, by studying the mission concept THESEUS, we
demonstrate the substantial impact of a much larger field of view in searches
of X-ray transients.
