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Transit observation by Ritsumeikan astronomical observatory Takeshi Okuda, Shouta Maruki, Harutaka Takebe, Kouichi Nishiyama, Masaki Mori Department of Physical Sciences, Ritsumeikan University, Shiga, JAPAN Observatory The Ritsumeikan astronomical observatory was built mainly for student research in Biwako- Kusatsu Campus Ritsumeikan University in Japan, as 20th anniversary of opening the cam- pus. The coordinate is 34.983 ◦ N 135.965 ◦ E, 150m above sea level. Figure 1: The telescope is stored in astronomical dome on university campus building. The campus is close to the city, so the night sky is not so dark. The telescope is a classical Cassegrain type with 60cm diameter(F/10), manufactured by Nishimura Co., LTD. The camera is a cooled CCD with 52.1mm diagonal, produced by Finger Lake Instrumentation, normally operated at -20 ◦ C and 2048×2048 pixel mode. Sup- porting operation and analysis, a weather station, a sky quality meter and a large angle sky camera are installed near the astronomical dome. Observation The filter for transit observation is Astrodon Exo-Planet. Many transits were observed in 2017, and a transit of WASP-52b in 2017/09/24UTC is reported here. The observed image is shown in Figure 2, where four available reference stars are also shown. The object and observation condition are summarized in Table 1. The number of LightFrame is 175 and DarkFrame is 60. For the photometric analysis, one DarkFrame is used for two or three LightFrame. Figure 2: Observed image (logADC) of the sky around WASP-52 with reference stars. Figure 3: Profile of the sky brightness measured by sky quality meter (SQM-LE). Table 1: Information of WASP-52, WASP-52b and observation. Target WASP-52[1] RA 23h 13m 58.76s Dec +08 ◦ 45’ 40.6” period(day) 1.7497798 epoch(HJD) 2455793.68143 V(mag) 12 depth(mag) 0.0290 duration(min) 108.58 Filter Exo-Planet Exposure(s) 60 Figure 4: The maximum ADC value of the pixel which consist of target and reference stars. The sky condition in observation was not super clear. There was very thin cloud at very high altitude by the sky camera. The status of the sky brightness is shown in Figure 3. The linearity of the CCD camera is ensured up to 60000 ADC value at least, which was measured beforehand. The Figure 4 shows all four reference stars are available to evaluate target darkening by transit. Photometric Analysis In the first place, the night sky pixels are selected (Figure 5). Peak region(11bin) is fitted by Gaussian, then non peak region(21bin), 6σ apart from Gaussian center, is fitted by Expo- nential. The ADC value of higher crossing point of Gaussian and Exponential is set to x t 1 . Then, the ADC value symmetric to x t 1 for Gaussian center is set to x t 2 . The pixels whose ADC is between x t 2 and x t 1 are night sky pixels. The ADC bin size of histogram follows the night sky brightness to keep similar figure for various night sky brightness. Figure 5: Schematic procedure to select night sky pix- els. Figure 6: Cross section of model fit for night sky back- ground. The selected night sky pixels are fitted by model function (Figure 6). Errors for fit are evaluated by the Poisson of photo-, thermal-electrons and electrons converted from read- out noise. Subscript L is LightFrame, D is DarkFrame, B is BiasFrame. ¯ G is averaged conversion factor from electrons to ADC. ¯ G, ¯ ADC B and ¯ σ B 2 were measured beforehand. δ (x, y )= ADC L (x, y ) - ADC D (x, y ) - F model (x, y ) √ ¯ G ( ADC L (x, y )+ ADC D (x, y ) - 2 ¯ ADC B ) +2¯ σ 2 ADC B χ 2 n-m = 1 n - m n ∑ δ (x, y ) 2 n is the number of pixel and m is the number of parameter of model function. The fitting is the minimizing χ 2 n-m , and this reduced χ 2 is very stable and close to 1. In addition, it is con- firmed that the distribution of δ (x, y ) of selected night sky pixels corresponds to the standard normal. Therefore, the error evaluation and choice of model function is appropriate. In the second place, the star pixels are selected by δ (x, y ) and adjacent. The adjacent pixels whose δ (x, y ) is more than particular threshold H consist of a light spot. H means H times standard deviation apart from expected night sky background. The light spot is spreaded by the fluctuation of focus by unstable atmosphere with time. Therefore, the distribution of light spot is not same for each LightFrame. The higher H for integration of electrons within a light spot, the better S/N. But it is strongly affected by the fluctuation of focus. The lower H for integration of electrons within a light spot, the more robust. But is is low S/N because of larger fraction of background. Therefore, multiple integration for higher multiple H are done. Then the integration is extrapolated to lower H . The slope of the extrapolation follows the fluctuated distribution of light spot. These extrapolation of integration is done after correcting the natural vignetting, and errors too. Figure 7: Light curve of target and four reference stars. Figure 8: Relative photometry of target by four refer- ence stars. After photometric analysis above, the light curves of target and four reference stars are shown in Figure 7. And relative photometric profile is shown in Figure 8. The typical transit light curve was produced. The error bar consists of Poisson of CCD electrons and atmospheric scintillation[2]. Result with Simulation The atmospheric correction by zenith angle was done and normalized to 1 in out of transit, as shown from Figure 8 to Figure 9. It is shown that the 3% darkening caused by transit clearly. The light curve was fitted well (Figure 10), with simulated curve by three parameters (impact parameter, planet radius to the star and orbital radius to the star) with known orbital period (Table 1) and the Eddington limb darkening. Figure 9: Corrected transit profile. Figure 10: Fitted curve by transit simulation is added. Table 2: Comparison of the evaluated parameters Observation HJD or BJD Planet radius to Orbital radius to Impact -2450000 the star radius the star radius Parameter G.H´ ebrard et al[3] 5793.68 0.1646 ± 0.0012 7.38 ± 0.10 0.60 ± 0.02 T.Louden et al.[4] 6899.54 0.1639 ± 0.0030 7.18 ± 0.09 0.59 ± 0.03 G.Chen et al.[5] 7263.50 0.1608 ± 0.0018 7.14 ± 0.12 0.61 ± 0.03 This report 8021.15 0.1652 ± 0.0016 6.99 ± 0.08 0.60 ± 0.02 The comparison of the evaluated parameters with precedent reports is shown in Table 2. The planet radius and the impact parameter are consistent with precedent reports. However, the orbital radius is smaller than precedent reports and seems to be shrinking with time. References [1] Exoplanet Transit Database (2017), http://var2.astro.cz/ETD/ [2] D.Dravins et al., PASP 110, 610-633, 1998 [3] G.H´ ebrard et al., A&A 549, A134, 2013 [4] T.Louden et al., MNRAS 470, 742-754, 2017 [5] G.Chen et al., A&A 600, L11, 2017
