On constraining the speed of gravitational waves following GW150914
  We point out that the observed time delay between the detection of the signal
at the Hanford and Livingston LIGO sites from the gravitational wave event
GW150914 places an upper bound on the speed of propagation of gravitational
waves, $c_{gw}\lesssim 1.7$ in the units of speed of light. Combined with the
lower bound from the absence of gravitational Cherenkov losses by cosmic rays
that rules out most of subluminal velocities, this gives a model-independent
double-sided constraint $1\lesssim c_{gw}\lesssim 1.7$. We compare this result
to model-specific constraints from pulsar timing and cosmology.
