We present ULTRACAM multiband optical photometry of two transitional millisecond pulsars (tMSPs), PSR J1023+0038 and PSR J1227−4853, taken while both were in their radio pulsar states. The light curves show significant asymmetry about the flux maxima in all observed bands, suggesting an asymmetric source of heating in the system. We model the light curves using the icarus binary code, using models with an additional ‘hotspot’ heating contribution and an anisotropic heat redistribution model to treat the asymmetry. Our modelling reveals companion stars with underfilled Roche lobes in both PSRs J1023+0038 and J1227−4853, with Roche lobe filling factors in the range f ∼ 0.82−0.92. While the volume-averaged filling factors are closer to unity, significant underfilling is unexpected from tMSPs as they must rapidly overfill their Roche lobes to start transferring mass, which occurs on time-scale of weeks or months. We discuss the motivation and validity of our extensions to the models and the implications of the underfilled Roche lobe, and suggest future work to further investigate the role of the filling factor in the tMSP cycle.

中文翻译：

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射电脉冲星状态下两颗过渡毫秒脉冲星的光度测量

我们展示了两个过渡毫秒脉冲星 (tMSP) PSR J1023+0038 和 PSR J1227−4853 的 ULTRACAM 多波段光学光度法，它们都是在它们都处于无线电脉冲星状态时拍摄的。光变曲线在所有观察到的波段中显示出关于通量最大值的显着不对称性，表明系统中存在不对称的加热源。我们使用 icarus 二进制代码对光变曲线进行建模，使用具有额外“热点”加热贡献的模型和各向异性热再分配模型来处理不对称性。我们的模型揭示了 PSR J1023+0038 和 J1227-4853 中罗氏叶填充不足的伴星，罗氏叶填充因子在 f ∼ 0.82-0.92 范围内。虽然体积平均填充因子更接近统一，tMSP 出乎意料会出现显着的填充不足，因为它们必须迅速过度填充其罗氏叶才能开始转移质量，这发生在数周或数月的时间尺度上。我们讨论了模型扩展的动机和有效性以及未填充的罗氏叶的影响，并建议未来的工作以进一步研究填充因子在 tMSP 循环中的作用。