# In GR what is a "source-frame mass"?

In the recent LIGO paper,

they refer to the masses in some models as:

"... a nearly equal mass black-hole binary system of source-frame masses $36^{+5}_{−4} M_{\odot}$ and $29^{+4}_{−4} M_{\odot}$ (median and 90% credible range)."

"... a stellar mass binary black hole system with source-frame component masses $23^{+18}_{-5} M_{\odot}$ and $13^{+4}_{-5}M_{\odot}$"

In General Relativity I've heard of different types of mass parameters such as Komar, Bondi, or ADM mass. But what is the source-frame mass? How are they getting a localized measure of mass in such a dynamic spacetime?

It's just an effect of living in an expanding universe, just as wavelengths from a source will be stretched by a factor of $1+z$ for redshift $z$ between when they are emitted and when we receive them.
The observed frequency of the signal is redshifted by a factor of $(1+z)$, where $z$ is the cosmological redshift. There is no intrinsic mass or length scale in vacuum general relativity, and the dimensionless quantity that incorporates frequency is $fGm/c^3$. Consequently, a redshifting of frequency is indistinguishable from a rescaling of the masses by the same factor. We therefore measure redshifted masses $m$, which are related to source frame masses by $m = (1+z) m^\text{source}$.
That is, the naive mass measurement assuming a Minkowski universe is going to be wrong by a factor of $1+z$ compared to being right next to the black holes and weighing them. The team corrects for this given the redshift of $z = 0.09$.