AstroNote 2025-3

On UT 2024 December 27 the Very Large Array (VLA) observed the position of AT2024aehp, a luminous fast blue optical transient candidate at z=0.170 (AstroNote 2024-386), using the X-band (10 GHz) receiver. The X-band image RMS was 4 uJy in 36 minutes of on-source time. No source is detected at the position of AT2024aehp, resulting in a 3 sigma upper limit on the luminosity of 1e28 erg/s/Hz. This limit is a factor of 10 higher than the radio luminosity of AT2018cow at a similar observation frequency and epoch (Margutti et al. 2019, ApJ, 872, 18). 

The source was observed by Swift XRT on UT 2024-12-31 12:01:07.478 in a 3.3 ks observation and was not detected to a flux upper limit of 1.2e-13 erg/s/cm^2. The source was observed again on UT 2024-01-03 00:19:34.69 in a 350 s observation, and was not detected to 0.3 - 10 keV flux upper limit of 5.7e-12 erg/s/cm^2. We assume a canonical counts to flux conversion of 4e-11 to convert from counts to flux, in the absence of a source spectrum. Using the luminosity distance, this corresponds to an X-ray luminosity upper limit of 1e43 erg/s for the first epoch of observations. This limit is similar to the X-ray luminosity of AT2018cow at a similar epoch (Margutti et al. 2019, ApJ, 872, 18).

In addition, new optical photometry from ZTF survey data and follow-up observations by the Spectral Energy Distribution machine on the 60-inch telescope at Palomar Observatory indicate that the previously observed fast decline stopped and that the optical light curve flattened out in the past week. The r-band light curve data is listed below. The photometric evolution of AT2024aehp is atypical of luminous fast blue optical transients.  

MJD filter magnitude

60671.37822 ztf-r 20.09+/-0.13

60673.46547 ztf-r 20.13+/-0.11

60675.40816 sdss-r 20.16+/-0.06

60676.28695 sdss-r 20.30+/-0.08

60677.20583 sdss-r 20.25+/-0.08 

Further observations are planned. We thank the observatory staff for rapidly scheduling and executing these observations.

SED Machine is based upon work supported by the National Science Foundation under Grant No. 1106171