AstroNote 2023-175

In response to the call for SN 2023ixf images (AstroNote #2023-133, Yaron et al.), we have collected early-phase images of SN 2023ixf (2023TNSTR1158....1I, Itagaki) in M 101, a type II supernova (AstroNote #2023-119, Perley et al.) from amateur astronomers and school observatories in Taiwan. The observations were conducted from 2023-05-20.57 to 2023-05-21.63 UT. Being located at approximately 120°E longitude, Taiwan's geographical position allows us to contribute to the collective effort of global observations of SN 2023ixf by filling the time-zone gap. 

Due to the absence of standard photometric filters or the use of a color camera, we lack standard stars for magnitude zero point estimation, and we decide to use constant colours for magnitude meaurements instead. We used the Kinder pipeline (Yang et al. A&A 646, A22) with aperture photometry to measure these images without host galaxy template subtraction. We obtained the instrument magnitude (m) of a selection of field stars, cross-matched them with the PanSTARRS1 catalog to acquire their g and r-band magnitudes. By plotting their (g-r) versus (r-m) values together, we performed a linear fit to establish a relationship. Ｗe adopted a color value of (g-r) = -0.4 mag for the SN (AstroNote #2023-129, Kendurkar et al.) on May 20th and assumed -0.5 mag on May 21st. Using the SN colour we then obtain the (r-m) value for the zero point. Because of a significant scatter in the linear-fitted relation, we have included a large error bar in our measurements. Further standard star observations and calibration are required for a more accurate estimation of magnitudes.

We have provided the instrument configuration details and the converted PS1 r magnitude of SN 2023ixf in the table below:

Given magnitudes are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V) = 0.008 mag in the direction of the SN (Schlafly & Finkbeiner 2011). We converted the original Kuo's image, taken with an RGGB Bayer filter, into a monochromatic image by calculating the arithmetic mean of each 2x2 pixel array. Similarly, for the original Lee's image, which was a three-layer RGB data cube, we performed an arithmetic mean to reduce it to a 2D monochromatic image. We are not able to calibrate Wang's image due to the automatic contrast adjustments and other unknown processing performed by the phone. We note that Shiau's image was taken from the remote telescope at Utah, USA.

Overall, we observed a clear rising in the brightness of approximately 2 magnitude of the SN 2023ixf within the collected data points from 2023-05-20.57 to 2023-05-21.63 UT. We express our gratitude to the amateur astronomers and school observatories in Taiwan who responded to our collection and provided their images.