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Black Hole Astrophysics with VLBI: Past, Present, and Future
- Workshop in honor of Professor INOUE Makoto's retirement -
March 27(Mon)-29(Wed), 2017
Mitaka Campus, NAOJ, Tokyo, Japan

Oral Presentation

3.5 yr Monitoring of 225 GHz Opacity at the Summit of Greenland

Author(s): Satoki Matsushita, Keiichi Asada, Pierre L. Martin-Cocher, Ming-Tang Chen, Paul T. P. Ho, Makoto Inoue, Patrick M. Koch (ASIAA), Scott N. Paine (CfA/SAO), David D. Turner (NOAA)

Presenter: Satoki Matsushita (ASIAA)

We present the 3.5-yr monitoring results of 225 GHz opacity at the summit of the Greenland ice sheet (Greenland Summit Camp) at an altitude of 3200 m using a tipping radiometer. We chose this site as our Greenland Telescope (GLT) site, because conditions are expected to have low submillimeter opacity and because its location offers favorable baselines to existing submillimeter telescopes for VLBI. The site shows a clear seasonal variation with the average opacity lower by a factor of two during winter. Estimated atmospheric transmission spectra in winter season are similar to the ALMA site at lower frequencies (<450 GHz), but 10%-25% higher opacities at higher frequencies (>450 GHz). This is due to the lower altitude of the Greenland site. Nevertheless, half of the winter time at the Greenland Summit Camp can be used for astronomical observations at frequencies between 450 GHz and 1000 GHz with opacities <1.2, and 10% of the time show >10% transmittance in the THz windows. One major advantage of the Greenland Summit Camp site in winter is that there is no diurnal variation due to the polar night condition, and therefore the durations of low-opacity conditions are significantly longer than at the ALMA site. Opacities lower than 0.05 or 0.04 can continue for more than 100 hours. Such long stable opacity conditions do not occur as often even at the South Pole. Since the opacity variation is directly related to the sky temperature (background) variation, the Greenland Summit Camp is suitable for astronomical observations that need unusually stable sky background.

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