No Period Change in Two Long-Period AM CVn Binaries

Abstract

Ultracompact binary systems, consisting of two compact objects in an orbit $\lesssim 0.5R_{\odot }$, should exhibit measurable rates of orbital period change ($\dot{P}\ne 0$) due to the emission of gravitational waves (GWs). Measurements of \pdot\ have so far been limited to the shortest-period ultracompact binaries ($\lesssim 20$\,min). Among the AM\,CVn-type subclass, several works have proposed the presence of extra angular momentum loss beyond GW emission, with magnetic braking being a widely discussed mechanism. If present, this magnetic braking would dominate the angular momentum loss of AM\,CVn-type binaries with orbital periods $\gtrsim 30$\,min. In this work, we present a long-term eclipse timing study of two AM\,CVn-type binaries, YZ\,LMi and Gaia14aae, with respective orbital periods of 28.3\,min and 49.7\,min and continuous observations since 2006 and 2015. Both systems show $\dot{P}$ consistent with zero within $2\ddot{I}ƒ$. Their $3\ddot{I}ƒ$ upper limits are $1.1\times 10^{-13}{\mathrm{s}\mathrm{s}}^{-1}$ and $9.7\times 10^{-14}{\mathrm{s}\mathrm{s}}^{-1}$ respectively. These non-detections are most simply explained by a scenario in which secular angular momentum loss is not substantially stronger than GW emission at all orbital periods, but is combined with deviations from the secular $\dot{P}$ whose timescales span decades but whose amplitude is $\lesssim 10^{-13}{\mathrm{s}\mathrm{s}}^{-1}$. %, orders of magnitude smaller than the eclipse timing variations seen in hydrogen-dominated cataclysmic variables. Our non-detections of $\dot{P}$ represent a limit on the strength of any enhanced angular momentum loss beyond pure GW emission.

16 pages, 10 figures, 2 appendices. Accepted for publication in MNRAS