9:15 AM - 9:30 AM
[ACG38-02] Utility of the meteorological satellite Himawari-8 for astronomy: monitoring observation of stars
Keywords:Meteorological satellite observation, Himawari-8, Astronomy
The geostationary meteorological satellite Himawari-8 scans space within about one degree from the Earth's rim during its full disk scan every 10 minutes, and sometimes captures bright celestial objects such as stars, the Moon, and planets. The Advanced Himawari Imager (AHI) onboard the Himawari-8 has 16 bands from visible light to near- and mid-infrared wavelengths, so it can obtain a multi-color photometric image of the captured objects simultaneously. In particular, it covers the water vapor band at around 6 – 7 µm and the ozone band at 9.6 µm, which is difficult to observe from the ground. Moreover, the observing frequency and the number of bands have been greatly increased compared to its predecessor MTSAT-2. Therefore, it is expected that Himawari-8, a meteorological satellite, can be utilized for astronomical research. In this study, as the first demonstration of the application of a meteorological satellite to observational astronomy, we attempted to monitor stars in the Himawari-8 images.
Since the sensitivity of the AHI is optimized for imaging the Earth's surface, observable stars are limited to those brighter than about 2 magnitudes in visible light. Therefore, we first extracted and measured bright stars that can be detected in visible light. Using the calibrated Himawari Standard Data, aperture photometry was performed for each star that appeared in the image about once every two days. We found that the background intensity depends on the radial distance from the Earth's rim and the north-south coordinate. Therefore, to reduce the bias in photometry, the background was subtracted by fitting a 2D polynomial function to radiances around the stars.
In this way, we obtained light curves covering a period of six years from the beginning of the Himawari-8 operation, that is, from July 2015 to June 2021. We evaluated the systematic variability of the photometric values by using several stars whose luminosity are known to be constant, and found that the flux values were systematically large until the first half of 2016. Therefore, we decided to use the data from 2017 to 2021 in the subsequent analysis. For Rigel, one of the non-variable stars, the obtained spectral energy distribution averaged over this period is found to be consistent with the literature values in each band.
Himawari-8 also captured the famous red supergiant star Betelgeuse in its field of view. In early 2020, Betelgeuse temporarily faded by about one magnitude. The cause of this phenomenon, called “the Great Dimming,” has been still under debate. We compared the light curves of all 16 bands of Betelgeuse obtained by Himawari-8 with model spectra, and showed that both the decreased surface temperature and the increased dust extinction may contribute to the Great Dimming to the same extent (Taniguchi, Yamazaki, Uno, submitted).
Furthermore, we have been making a catalog of the detected sources in order to search for transient events in infrared wavelengths. Using the Himawari Standard Data of January 2020, we extracted the coordinates of the bright pixels detected simultaneously in several consecutive infrared bands, and investigated the presence or absence of the corresponding sources in the celestial coordinates. As a result, about 80 known stars were detected and identified in the infrared bands 5 – 7, which have relatively high sensitivity. Although there were no significant detections of unknown sources, we are continuing our analysis by extending the observation period and the number of bands.
As described above, the utilization of an advanced meteorological satellites as a "space telescope" is bringing scientific results not only in the field of global environment, but also in those of astronomy and planetary science. In this presentation, we will discuss the utility of Himawari-8 in observational astronomy.
Since the sensitivity of the AHI is optimized for imaging the Earth's surface, observable stars are limited to those brighter than about 2 magnitudes in visible light. Therefore, we first extracted and measured bright stars that can be detected in visible light. Using the calibrated Himawari Standard Data, aperture photometry was performed for each star that appeared in the image about once every two days. We found that the background intensity depends on the radial distance from the Earth's rim and the north-south coordinate. Therefore, to reduce the bias in photometry, the background was subtracted by fitting a 2D polynomial function to radiances around the stars.
In this way, we obtained light curves covering a period of six years from the beginning of the Himawari-8 operation, that is, from July 2015 to June 2021. We evaluated the systematic variability of the photometric values by using several stars whose luminosity are known to be constant, and found that the flux values were systematically large until the first half of 2016. Therefore, we decided to use the data from 2017 to 2021 in the subsequent analysis. For Rigel, one of the non-variable stars, the obtained spectral energy distribution averaged over this period is found to be consistent with the literature values in each band.
Himawari-8 also captured the famous red supergiant star Betelgeuse in its field of view. In early 2020, Betelgeuse temporarily faded by about one magnitude. The cause of this phenomenon, called “the Great Dimming,” has been still under debate. We compared the light curves of all 16 bands of Betelgeuse obtained by Himawari-8 with model spectra, and showed that both the decreased surface temperature and the increased dust extinction may contribute to the Great Dimming to the same extent (Taniguchi, Yamazaki, Uno, submitted).
Furthermore, we have been making a catalog of the detected sources in order to search for transient events in infrared wavelengths. Using the Himawari Standard Data of January 2020, we extracted the coordinates of the bright pixels detected simultaneously in several consecutive infrared bands, and investigated the presence or absence of the corresponding sources in the celestial coordinates. As a result, about 80 known stars were detected and identified in the infrared bands 5 – 7, which have relatively high sensitivity. Although there were no significant detections of unknown sources, we are continuing our analysis by extending the observation period and the number of bands.
As described above, the utilization of an advanced meteorological satellites as a "space telescope" is bringing scientific results not only in the field of global environment, but also in those of astronomy and planetary science. In this presentation, we will discuss the utility of Himawari-8 in observational astronomy.