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Arecibon planetaarisen tutkan Maan lähiasteroidihavainnot: 2017 joulukuu - 2019 joulukuu
We successfully observed 191 near-Earth asteroids using the Arecibo Observatory's S-band planetary radar system from 2017 December through 2019 December. We present radar cross sections for 167 asteroids; circular-polarization ratios for 112 asteroids based on Doppler-echo-power spectra measurements; and radar albedos, constraints on size and spin periods, and surface-feature and shape evaluation for 37 selected asteroids using delay-Doppler radar images with a range resolution of 75 m or finer. Out of 33 asteroids with an estimated effective diameter of at least 200 m and sufficient image quality to give clues of the shape, at least 4 (∼12%) are binary asteroids, including 1 equal-mass binary asteroid, 2017 YE5, and at least 10 (∼30%) are contact-binary asteroids. For 5 out of 112 asteroids with reliable measurements in both circular polarizations, we measured circular-polarization ratios greater than 1.0, which could indicate that they are E-type asteroids, while the mean and the 1σ standard deviation were 0.37 ± 0.23. Further, we find a mean opposite-sense circular-polarization radar albedo of 0.21 ± 0.11 for 41 asteroids (0.19 ± 0.06 for 11 S-complex asteroids). We identified two asteroids, 2011 WN15 and (505657) 2014 SR339, as possible metal-rich objects based on their unusually high radar albedos, and discuss possible evidence of water ice in 2017 YE5.Peer reviewe
A Search for Technosignatures Around 31 Sun-like Stars with the Green Bank Telescope at 1.15-1.73 GHz
We conducted a search for technosignatures in April of 2018 and 2019 with the
L-band receiver (1.15-1.73 GHz) of the 100 m diameter Green Bank Telescope.
These observations focused on regions surrounding 31 Sun-like stars near the
plane of the Galaxy. We present the results of our search for narrowband
signals in this data set as well as improvements to our data processing
pipeline. Specifically, we applied an improved candidate signal detection
procedure that relies on the topographic prominence of the signal power, which
nearly doubles the signal detection count of some previously analyzed data
sets. We also improved the direction-of-origin filters that remove most radio
frequency interference (RFI) to ensure that they uniquely link signals observed
in separate scans. We performed a preliminary signal injection and recovery
analysis to test the performance of our pipeline. We found that our pipeline
recovers 93% of the injected signals over the usable frequency range of the
receiver and 98% if we exclude regions with dense RFI. In this analysis, 99.73%
of the recovered signals were correctly classified as technosignature
candidates. Our improved data processing pipeline classified over 99.84% of the
~26 million signals detected in our data as RFI. Of the remaining candidates,
4539 were detected outside of known RFI frequency regions. The remaining
candidates were visually inspected and verified to be of anthropogenic nature.
Our search compares favorably to other recent searches in terms of end-to-end
sensitivity, frequency drift rate coverage, and signal detection count per unit
bandwidth per unit integration time.Comment: 20 pages, 8 figures, in press at the Astronomical Journal (submitted
on Sept. 9, 2020; reviews received Nov. 6; re-submitted Nov. 6; accepted Nov.
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