AstroNote 2023-150

The Type II supernova SN 2023ixf in the nearby galaxy M101 was discovered by Koichi Itagaki on 2023-05-19.7272 UT (ATel #16045, #16047, #16053, #16057). The supernova is the target of multiwavelength observations (ATel #16044, #16049, #16051, #16052, #16060) and IceCube neutrino search (ATel #16043). The candidate supergiant progenitor was identified in the archival HST images (ATel #16050, AstroNotes #2023-147). A number of pre-discovery images constraining the time of the explosion were reported in AstroNote #2023-120, #2023-123, #2023-124, #2023-125, #2023-127, #2023-128, #2023-130, #2023-133 (the AstroNotes are linked to the SN 2023ixf page at the TNS) and ATel #16054.

We add to the list of pre-discovery observations by reporting RGB color and unfiltered monochrome images obtained with three small telescopes: ED80 (D=80mm, F=600mm refractor and Canon EOS 600Da color CMOS camera with removed IR-cut filter), NP101 (D=101mm, F=550mm refractor and QHY600m monochrome CMOS camera with RGB filter wheel), and N130 (D=130mm, F=650mm Newton with unfiltered ZWO ASI 1600MM Pro monochrome CMOS camera). For the monochrome camera we used V magnitudes of comparison stars to set the magnitude scale zero-point, referring to the resulting magnitudes as CV. Following the AAVSO DSLR observing manual https://www.aavso.org/dslr-camera-photometry-guide we used Johnson-Cousins B magnitudes of comparison stars to calibrate tri-color Blue images referring to the resulting magnitudes as TB, V magnitudes for tri-color Green images (TG) and R magnitudes for tri-color Red (TR). The comparison star magnitudes were derived from APASS with the R-band magnitude color-transformed from APASS magnitudes following Jester et al. (2005, AJ, 130, 873). An ensemble of APASS stars within the field of view was used to calibrate the magnitude zero-points. We also applied the color transformation of Park et al. (2016, AdSpR, 57, 509) to derive approximate magnitudes of the supernova in the Johnson-Cousins system from the measured TB, TG, and TR values. The TB, TG, and TR measurements were performed on stacked images and the time corresponding to the middle of the imaging sequence is assigned to these measurements. The deeper CV frames were measured individually and their reported times correspond to the middle of a 400s exposure. The results are summarized below and given in the attached text file.

Date(UTC)          JD(UTC)  exposure telescope      magnitudes
2023-05-17.974   2460082.474   8x240s  NP101   TB>20.0          TG>19.0          TR>18.2

2023-05-18.81602 2460083.31602 1x400s   N130 CV>18.9
2023-05-18.82100 2460083.32100 1x400s   N130 CV>18.9
2023-05-18.82611 2460083.32611 1x400s   N130 CV=18.76 +/-0.25
2023-05-18.83118 2460083.33118 1x400s   N130 CV=19.12 +/-0.34
2023-05-18.83613 2460083.33613 1x400s   N130 CV>18.7
2023-05-18.84106 2460083.34106 1x400s   N130 CV>18.3
2023-05-18.84604 2460083.34604 1x400s   N130 CV>18.4
2023-05-18.85120 2460083.35120 1x400s   N130 CV>18.9
2023-05-18.85626 2460083.35626 1x400s   N130 CV=18.90 +/-0.28
2023-05-18.87420 2460083.37420 1x400s   N130 CV=18.53 +/-0.22

2023-05-18.877   2460083.377  39x300s   ED80   TB=19.35 +/-0.10 TG=18.35 +/-0.13 TR=17.72 +/-0.06 B=19.72 V=18.60 R=17.84

2023-05-18.87933 2460083.37933 1x400s   N130 CV=18.40 +/-0.22
2023-05-18.88434 2460083.38434 1x400s   N130 CV=19.12 +/-0.32
2023-05-18.88926 2460083.38926 1x400s   N130 CV=18.80 +/-0.26
2023-05-18.89426 2460083.39426 1x400s   N130 CV=18.68 +/-0.24
2023-05-18.89944 2460083.39944 1x400s   N130 CV=18.84 +/-0.27
2023-05-18.90463 2460083.40463 1x400s   N130 CV=18.17 +/-0.19

2023-05-18.941   2460083.441   8x240s  NP101   TB=18.03 +/-0.03 TG=17.88 +/-0.05 TR=17.27 +/-0.04 B=18.15 V=17.67 R=17.34

2023-05-19.798   2460084.298  19x300s   ED80   TB=15.13 +/-0.01 TG=14.69 +/-0.01 TR=14.43 +/-0.01 B=15.12 V=14.66 R=14.35

The N130 (see the times of the individual images) and ED80 (exposed between 2023-05-18.805 and 2023-05-19.075) image series overlap with the latest non-detection (2023-05-18.8139) and the earliest detection (2023-05-18.8535) of the explosion reported in AstroNote #2023-130 (see also AstroNote #2023-133). The N130 images allow us to further constrain the eruption time. The first detection of the supernova with SNR>4 is obtained at the third frame of the N130 series on 2023-05-18.82611 (19:50 UTC mid. exp.), however visual inspection of the first two frames obtained at 19:35 and 19:42 (mid. exp.) suggest the supernova may be present at these frames.

The supernova is too faint to be measured reliably on the individual ED80 images, however their visual inspection suggests the supernova is first visible around 19:52 UT, just after the first N130 detection. A video constructed from the ED80 images is available at https://youtu.be/hhsV_lhdnNk

The N130 photometry hints that the initial rise in brightness of the supernova might have been non-monotonic as the first two detections are followed by a series of four non-detections. In principle, a similar feature in an early lightcurve of a supernova might be expected from the shock breakout. However, variable seeing (resulting from changes in the atmosphere and the guider performance) may temporarily push a faint object below the detection limit. A careful forced photometry analysis is needed to test if the lightcurve is consistent with a monotonic rise. We note that both N130 and ED80 images were obtained with dithering. This technique reduces the probability of a repeated false detection at any given sky position due to image artifacts.

The ED80, NP101, and N130 images may be found at http://scan.sai.msu.ru/~kirx/ATel/SN2023ixf_May17-19/