We propose a new observing method for single-dish millimeter and submillimeter spectroscopy using a heterodyne receiver equipped with a frequency-modulating local oscillator (FMLO). Unlike conventional switching methods, which extract astronomical signals by subtracting the reference spectra of off-sources from those of on-sources, the FMLO method does not need to obtain any off-source spectra; rather, it estimates them from the on-source spectra themselves. The principle is a high dump-rate (10 Hz) spectroscopy with radio frequency modulation (FM) achieved by fast sweeping of a local oscillator (LO) of a heterodyne receiver: Because sky emission (i.e., off-source) fluctuates as $1/f$-type and is spectrally correlated, it can be estimated and subtracted from time-series spectra (a timestream) by principal component analysis. Meanwhile astronomical signals remain in the timestream since they are modulated to a higher time-frequency domain. The FMLO method therefore achieves (1) a remarkably high observation efficiency, (2) reduced spectral baseline wiggles, and (3) software-based sideband separation. We developed an FMLO system for the Nobeyama 45-m telescope and a data reduction procedure for it. Frequency modulation was realized by a tunable and programmable first local oscillator. With observations of Galactic sources, we demonstrate that the observation efficiency of the FMLO method is dramatically improved compared to conventional switching methods. Specifically, we find that the time to achieve the same noise level is reduced by a factor of 3.0 in single-pointed observations and by a factor of 1.2 in mapping observations. The FMLO method can be applied to observations of fainter ($\sim$mK) spectral lines and larger ($\sim$deg$^{2}$) mapping. It would offer much more efficient and baseline-stable observations compared to conventional switching methods.

中文翻译：

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一种基于调频本振的毫米波和亚毫米波光谱无偏点观测新方法

我们提出了一种使用配备有调频本地振荡器 (FMLO) 的外差接收器进行单盘毫米波和亚毫米波谱观测的新方法。与传统的切换方法通过从源上的参考光谱中减去源外的参考光谱来提取天文信号不同，FMLO方法不需要获得任何离源光谱；相反，它从源上光谱本身估计它们。原理是通过快速扫描外差接收器的本地振荡器 (LO) 实现具有射频调制 (FM) 的高转储率 (10 Hz) 光谱：因为天空发射（即离源）波动为 $1/ f$-type 并且是谱相关的，它可以通过主成分分析从时间序列谱（时间流）中估计和减去。同时，天文信号保留在时间流中，因为它们被调制到更高的时频域。因此，FMLO 方法实现了 (1) 非常高的观测效率，(2) 减少了光谱基线摆动，以及 (3) 基于软件的边带分离。我们为 Nobeyama 45 米望远镜开发了一个 FMLO 系统和一个数据缩减程序。频率调制是通过可调和可编程的第一本机振荡器实现的。通过对银河源的观测，我们证明与传统的切换方法相比，FMLO 方法的观测效率得到了显着提高。具体来说，我们发现在单点观测中达到相同噪声水平的时间减少了 3.0 倍，在映射观测中减少了 1.2 倍。FMLO 方法可应用于较暗 ($\sim$mK) 谱线和较大 ($\sim$deg$^{2}$) 映射的观测。与传统的切换方法相比，它将提供更有效和基线稳定的观察结果。