Current and secular accretion rates of EX Hydrae

We report an observed accretion rate of $\dot M_1 = (3.86\pm0.60)\times 10^{-11}$ $M_{\odot}$yr$^{-1}$ for the white dwarf in the short-period, intermediate polar EX Hya. This result is based upon the accretion-induced $4\pi$-averaged energy flux from 2.45 $\mu$m to 100 keV and the corresponding luminosity at the Gaia distance of 56.77 pc. Our result is in perfect agreement with the theoretical mass transfer rate from the secondary star induced by gravitational radiation (GR) and the spin-up of the white dwarf, $-\dot M_2 = (3.90\pm0.35)\times 10^{-11}$ $M_{\odot}$yr$^{-1}$; 24% of it is caused by the spin-up. The agreement indicates that mass transfer is conservative. The measured $\dot M_1$ obviates the need for angular momentum loss (AML) by any process other than GR. We complemented this result with an estimate of the mean secular mass transfer rate over $\sim 10^7$ yr by interpreting the non-equilibrium radius of the secondary star in EX Hya based on published evolutionary calculations. This suggests a time-averaged mass transfer rate enhanced over GR by a factor $f_{\mathrm{GR}} \gtrsim 2$. Combined with the present-day lack of such an excess, we suggest that an enhanced secular AML is due to an intermittently active process, such as the proposed frictional motion of the binary in the remnants of nova outbursts. We argue that EX Hya, despite its weakly magnetic nature, has evolved in a very similar way to non-magnetic CVs. We speculate that the discontinuous nature of an enhanced secular AML may similarly apply to the latter.

K. Beuermann, K. Reinsch · 2024