AstroNote 2024-372

We observed the field of the AT 2024aecx (ATLAS24rkq) (AstroNote 2024-371, Stevance et al.) as part of the Kinder collaboration (Chen & Yang et al., 2024 arXiv:2406.09270), using the 40cm-SLT at Lulin Observatory, Taiwan, to obtain images in g, r, i, and z filters with 150 second exposure time for each.

Observations started at 16:48 UT on 16 of December 2024 (MJD = 60660.7), 3.37 hours after the ATLAS discovery. The images were taken under varying and poor observing conditions: 3" seeing at an average airmass of 2.7.

We derived the following preliminary magnitudes using PSF photometry (all in the AB system) with template subtraction using SDSS templates:

g = 14.20 +/- 0.07 mag,

r = 14.74 +/- 0.05 mag,

i = 15.03 +/- 0.06 mag, and

z = 15.09 +/- 0.04 mag.

The presented magnitudes are calibrated using the field stars from the SDSS DR18 (Almeida et al., 2023) catalog and are not corrected for the expected Galactic foreground extinction.

The current magnitude is r = 14.74 +/- 0.05 mag at MJD 60660.704, corresponding to an absolute magnitude of M_r = -15.6 (assuming m-M = 30.25 and A_r = 0.13). Comparing our r-band and i-band observations to the ATLAS forced photometry for the o-band discovery magnitude, we estimate a fade in the o-band brightness by ~0.14 mag in 3.37 hours. The object appears very blue, with colours (g-r) = -0.6 and (r-i) = -0.3 after correcting for Milky Way extinction.

A shock cooling signature of IIb supernova is plausible given the sudden rise to peak brightness, quick decline and blue colour. Further photometric follow-up and spectroscopic classification are strongly encouraged.