高能暂现源 · 每日文献综述

Ultracompact binary systems, consisting of two compact objects in an orbit <inline-formula><tex-math>$\lesssim 0.5 {\rm R}_\odot$</tex-math></inline-formula>, should exhibit measurable rates of orbital period change (<inline-formula><tex-math>$\dot{P} \ne 0$</tex-math></inline-formula>) due to the emission of gravitational waves (GWs). Measurements of <inline-formula><tex-math>$\dot{P}$</tex-math></inline-formula> have so far been limited to the shortest-period ultracompact binaries (<inline-formula><tex-math>$\lesssim 20$</tex-math></inline-formula> 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 <inline-formula><tex-math>$\gtrsim 30$</tex-math></inline-formula> 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 <inline-formula><tex-math>$\dot{P}$</tex-math></inline-formula> consistent with zero within <inline-formula><tex-math>$2\sigma$</tex-math></inline-formula>. Their <inline-formula><tex-math>$3\sigma$</tex-math></inline-formula> upper limits are <inline-formula><tex-math>$1.1 \times 10^{-13}\, {\rm s \, s}^{-1}$</tex-math></inline-formula> and <inline-formula><tex-math>$9.7 \times 10^{-14}\, {\rm s \, s}^{-1}$</tex-math></inline-formula>, 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 <inline-formula><tex-math>$\dot{P}$</tex-math></inline-formula> whose time-scales span decades but whose amplitude is <inline-formula><tex-math>$\lesssim 10^{-13}\, {\rm s \, s}^{-1}$</tex-math></inline-formula>. Our non-detections of <inline-formula><tex-math>$\dot{P}$</tex-math></inline-formula> represent a limit on the strength of any enhanced angular momentum loss beyond pure GW emission.