  We show that for detections of gravitational-wave transients, constraints can
be given on physical parameters of the source without using any specific
astrophysical models. Relying only on fundamental principles of general
relativity, we can set upper limits on the size, mass, and distance of the
source solely from characteristics of the observed waveform. If the distance of
the source is known from independent (e.g. electromagnetic) observations, we
can also set lower limits on the mass and size. As a demonstration, we tested
these constraints on binary black hole signals observed by the LIGO and Virgo
detectors during their first and second observing runs, as well as on simulated
binary black hole and core-collapse supernova signals reconstructed from
simulated detector data. We have found that our constraints are valid for all
analyzed source types, but their efficiency (namely, how far they are from the
true parameter values) strongly depends on the source type, ranging from being
in the same order of magnitude to a several orders of magnitude difference. In
cases when a gravitational-wave signal is reconstructed without waveform
templates and no astrophysical model on the source is available, these
constraints provide the only quantitative characterization of the source that
can guide the astrophysical modeling process.
