2025ApJ...988...35S

I measure and collect timings of phase markers (like eclipse times) for the orbits of 25 X-ray binaries (XRBs) so as to calculate the steady evolutionary period change ($\dot{P}$). I combine these with my observed $\dot{P}$ measures from 52 cataclysmic variables (CVs). Further, I subtract out the contributions from gravitational radiation (${\dot{P}}_{{\rm{GR}}}$) and mass transfer (${\dot{P}}_{{\rm{mt}}}$), deriving the period change from the residual unknown angular momentum loss (${\dot{P}}_{{\rm{AML}}}$ = $\dot{P}$-${\dot{P}}_{{\rm{GR}}}$-${\dot{P}}_{{\rm{mt}}}$). I have ${\dot{P}}_{{\rm{AML}}}$ measures for 77 XRBs and CVs, with these being direct measures of the driver of binary evolution. The venerable magnetic braking model of binary evolution has its most fundamental predictions tested, with most systems having predictions wrong by over 1 order of magnitude. Other proposed mechanisms to explain the angular momentum loss (AML) also fail, so we are left with no known mechanism that dominates the AML. An alternative path to the AML law is empirical, where my ${\dot{P}}_{{\rm{AML}}}$ measures are fitted to a power law involving the fundamental binary properties. With this, the dominant AML law for systems with orbital periods (P) from 0.13-1.0 day is ${\dot{P}}_{{\rm{AML}}}=-1500\times 1{0}^{-12}{P}^{1.29}{M}_{{\rm{prim}}}^{2.75}{M}_{{\rm{comp}}}^{-1.00}{\dot{M}}_{-8}^{0.43}$, in appropriate units. Similar AML laws for binaries below the period gap and for binaries with P > 1.0 day are derived. These three AML laws are of good accuracy and are the best representations of the actual evolution for all 77 XRBs and CVs of all classes, so the three taken together can be called “universal.”