Numerator: 2GM

The passage around this formula

The relation itself is old and simple. For a spherical, non-rotating exterior, the surface redshift $z_s$ follows from the mass M and radius R as $z_s = \left(1 - \frac{2GM}{Rc^2}\right)^{-1/2} - 1$. — a function of the star’s compactness alone, so that whatever fraction of a percent of uncertainty NICER puts on M and R propagates directly into the redshift. The paper works through six NICER-mapped pulsars this way, and two of them mark the range worth remembering. PSR J0740+6620, the most massive precisely-timed neutron star known, comes in at roughly 2.07\,$M_\odot$ with a radio-timing mass fixed independently by Shapiro delay to 2.08$\pm$0.07\,$M_\odot$ [ 4 ] , giving a compactness near 0.25 and a surface redshift $z_s$$\simeq$0.40…

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