A new off-point-less observing method for millimeter and submillimeter spectroscopy with a frequency-modulating local oscillator (FMLO)

Abstract

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.

22 pages, 15 figures, 2 tables, accepted for publication in Publications of the Astronomical Society of Japan

References in corpus (21)

• Scikit-learn: Machine Learning in Python
• Astropy: A Community Python Package for Astronomy
• The NumPy array: a structure for efficient numerical computation
• The Astropy Project: Building an inclusive, open-science project and status of the v2.0 core package
• SCUBA-2: iterative map-making with the Sub-Millimetre User Reduction Facility
• High-resolution wide-band Fast Fourier Transform spectrometers
• AzTEC Millimetre Survey of the COSMOS Field: I. Data Reduction and Source Catalogue
• Molecular abundances in the inner layers of IRC +10216
• On-The-Fly Observing System of the Nobeyama 45-m and ASTE 10-m Telescopes
• Principal Component Analysis with Noisy and/or Missing Data
• The Bolocam Lockman Hole Millimeter-Wave Galaxy Survey: Galaxy Candidates and Number Counts
• A high resolution line survey of IRC+10216 with Herschel. First results: Detection of warm silicon dicarbide SiC2
• Digital Spectro-Correlator System for the Atacama Compact Array of the Atacama Large Millimeter/submillimeter Array
• High abundance ratio of ${}^{13}$CO to C${}^{18}$O toward photon-dominated regions in the Orion-A giant molecular cloud
• The Impact of Atmospheric Fluctuations on Degree-scale Imaging of the Cosmic Microwave Background
• Early Science with the Large Millimeter Telescope: CO and [C II] Emission in the z=4.3 AzTEC J095942.9+022938 (COSMOS AzTEC-1)
• New Panoramic View of ${}^{12}$CO and 1.1 mm Continuum Emission in the Orion A Molecular Cloud. I. Survey Overview and Possible External Triggers of Star Formation
• CRUSH: fast and scalable data reduction for imaging arrays
• A New 100-GHz Band Two-Beam Sideband-Separating SIS Receiver for Z-Machine on the NRO 45-m Radio Telescope
• Optimization by Smoothed Bandpass Calibration in Radio Spectroscopy
• A New Technique for Heterodyne Spectroscopy: Least-Squares Frequency Switching (LSFS)

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• The LMT 2 Millimeter Receiver System (B4R). I. Overview and Results of Science Demonstration