What's the actual speed of electromagnetic radiation in space? The speed of EM radiation is very slightly less than $c$, because space is not quite a vacuum.  Say EM travels at $(1-\varepsilon)c$.

For example, this results in a slight delay between receiving a gravitational wave and detecting an associated EM emission from the body that caused it.  For an object about 130 million light years away (GW170817) I've seen this delay quoted as everything from $1.74$ seconds (so $\varepsilon\approx 4\times 10^{-16}$) to 27 minutes (so $\varepsilon\approx 4\times 10^{-13}$).  I'm sure these figures depend on all sorts of interesting astronomy to do with black hole mergers and gamma ray bursts, but the actual value of $\varepsilon$ doesn't need to know any of this.

So I'm asking here because anyone working on LIGO must surely know our best estimate for the value of $\varepsilon$.
I believe this question was addressed in the paper , Abbot, et. al., Gravitational Waves and Gamma-Rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A , ApJ. Lett., 848:L13, 2017 October 16.


  "We use the observed time delay of ($+1.74 \pm 0.05$) s between GRB 170817A and GW170817 to: (i) constrain the difference between the speed of gravity and the speed of light to be between $-3 \times  10^{-15}$ and $+7 \times 10^{-16}$ times the speed of light, ..."


In Section 2.2 of the paper they discuss the uncertainty of delay between GW and GRB emission, predicted to be a few seconds. In Section 4 of the paper they compute the range of speed uncertainty quoted above. They conservatively use 26 MPc, their lower bound on the 90% credible distance interval, and for the upper bound on speed difference they assume the GW and GRB were emitted simultanously, and use the 1.74 s $\Delta t$. For a lower bound on speed of gravity, they assume the GRB were emitted 10 s after the GW, and a faster EM signal made up some of the difference.

I believe that X-rays were also detected several (~8-9) days later. This delay was attributed to time needed for the shock wave from the merger to interact with surrounding matter. I was unable to find any reference to signal arrival 27 minutes after GW detection in the LIGO papers.