Institute of Astronomy (University of Latvia) with Ventspils International Radio Astronomy Centre (Ventspils University of Applied Sciences) participation is implementing the scientific project “Complex investigations of the small bodies in the Solar system” related to the research of the small bodies in the Solar system (mainly, focusing on asteroids and comets) using methods of radio astronomy and signal processing. One of the research activities is hydroxyl radical (OH) observation in the radio range - single antenna observations and VLBI (Very Long Baseline Interferometry) observation. To detect weak (0.1 Jy) OH masers of astronomical objects using radio methods, a research group in Ventspils adapted the Irbene RT-32 radio telescope working at 1665.402 and 1667.359 MHz frequencies. Novel data processing methods were used to acquire weak signals. Spectral analysis using Fourier transform and continuous wavelet transform were applied to radio astronomical data from multiple observations related to weak OH maser detection. Multiple comets (Comet C/2017 T2 (PANSTARRS), Comet C/2019 Y4 (ATLAS), Comet C/2020 F8 (SWAN)) observations were carried out in 2019-2020.
There are four known (1612.231, 1665.402, 1667.359 and 1720.530 MHz) hyperfine transitions of OH at 18 cm wavelength which have been used for 40 years, historically to observe comets. In 1973, the molecule OH in comet Kahoutek  was observed from the Nancay 30 meter telescope. The 18 cm line is the result of an excitation from resonance fluorescence, whereby molecules absorb solar radiation and then reradiate the energy. The OH molecule absorbs the UV solar photons and cascades back to the ground state Lambda doublet, where the relative populations of the upper and lower levels strongly depend upon the heliocentric radial velocity (the “Swings effect”) . The result of comets observations in 1.6GHz frequency band made by other astronomy groups ,,, and others - show that the typical peak source flux densities of the comet are in the range of 4 to 40 mJy. Weakness of the radio signal is the main challenging factor. Assuming that the detection threshold is 3*σ, at least 1.3 to 13 mJy noise floor is required. Significant work was invested to prepare the instrumentation of Irbene 32-meter antenna for spectral line observation at L band. This includes improvement of receiver system sensitivity at 1.665 and 1.667 GHz, by building and installing new secondary focus front-end .
Observations and data processing
To detect OH masers of the comets, multiple observation sessions were performed using Irbene radio telescope RT32 at 1665.402 and 1667.359 MHz frequencies. Comet Atlas C/2019 Y4 was observed 133 hours, Panstarrs C/2017 - 149 hours, Swan C/2020 F8 - 110 hours. Data calibration and processing methods were necessary to filter out weak OH maser signals from radio astronomical data sets. A programmed USRP X300/310+TwinRX spectrometer is used to record data using 16bit+16bit (real + imag part) per sample. For spectral data calibration, the frequency switching method  was integrated in the observation process and data processing was implemented to collect data using long integration time, consequently to perform the compensation of the Doppler shift. For data filtering Fourier transforms, Blackman-Harris window function, Butterworth Low Pass, Locally Weighted Scatterplot Smoothing functions and wavelet transforms were used. Observations of small bodies are possible with the best available accuracy when optical (using the optical Schmidt telescope of Institute of Astronomy) and radio methods are combined . Data processing from two independent simultaneous measurements (using specific Kalman filters) allows one to reduce human errors in sporadic sources.
Summary and Conclusions
Observations of OH masers of comets can be a very challenging task. The upgrade of the L frequency band receiver was performed in Irbene, Latvia to observe OH masers of comets. Multiple data processing methods were developed to acquire a weak signal. OH masers of the comets (Comet C/2017 T2 (PANSTARRS), Comet C/2019 Y4 (ATLAS), Comet C/2020 F8 (SWAN)) were observed, and the observation process of Comet C/2019 U6, Comet 2P/Encke and Comet C/2020 F3 (NEOWISE) are ongoing in summer 2020.
This research is funded by the Latvian Council of Science, project„Complex investigations of the small bodies in the Solar system”, project No. lzp-2018/1-0401.
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