Evidence for Shock Power re-brightening in the SiFAP2 light curves of SN 2023ixf and upper-limits from the Swift satellite

ATel #17302; Andrea Simongini (INAF-OAR-UniRoma2), Filippo Ambrosino (INAF-OAR), Fabio Ragosta (UniNa), Silvia Piranomonte (INAF-OAR), Andrea Melandri (INAF-OAR), Giulia Illiano (INAF-OAB), Matteo Imbrogno (ICE-CSIC), Adriano Ghedina (FGG), Massimo Cecconi (FGG), Francesco Leone (Univ. Catania), Manuel Gonzalez (FGG), Hector Perez Ventura (FGG), Marcos Hernandez Diaz (FGG), Jose San Juan (FGG)
on 23 Jul 2025; 15:26 UT
Credential Certification: Andrea Melandri (andrea.melandri@inaf.it)

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We report evidence for a shift in the powering mechanism of the Type II SN 2023ixf, discovered on MJD 60083 in the M101 galaxy at 6.4 Mpc (Perley et al. 2023). We observed the event ~2 years after discovery, at MJD 60797, 60798, 60854 with the Silicon Fast Photometer and Polarimeter (SiFAP2; Meddi et al. 2012, Ghedina et al. 2018) and at MJD 60870, 60877 with the Swift Ultra Violet Optical Telescope (UVOT; Roming et al. 2005).

SiFAP2 clearly detects a point-source at the location of SN 2023ixf in all observed epochs. Photon count rates were converted into V-band magnitudes after background subtraction and calibration using standard curves. We report the following preliminary results:

Epoch(MJD)	V-band magnitude
60797	19.95 (31)
60798	20.01 (31)
60854	19.83 (31)

UVOT does not detect a significant signal at the SN location in the U, B, or V bands. The limited exposure time (1265 s and 1665 s) and high background of the first map, possibly due to a bright transient event during the observations, limit the sensitivity of the UVOT images. A source is marginally detected in the coadded image with B = 18.54 +/- 0.14 and U = 18.76 +/- 0.08, while no detection is found in V > 18.79.

Assuming a radioactive decay-powered light curve, we would expect the magnitude to be under the sensitivity limit of our telescope, at V > 22 mag. However, the observed brightness exceeds this expectation, suggesting an additional energy input is sustaining the luminosity at late times. This deviation is consistent with a transition to interaction-powered emission, likely driven by ongoing ejecta–circumstellar material (CSM) interaction (Jacobson-Galán, W. V. 2025). The photometric evolution between MJD 60798 and MJD 60854 shows no significant fading (dV = –0.18 +/- 0.44 mag), further supporting a flattening of the light curve that is inconsistent with Co-56 decay alone.

Additional Swift-UVOT observations are already scheduled. We encourage further multi-wavelength follow-up observations to monitor the ongoing evolution of the source and to constrain the nature of its late-time emission better.