AstroNote 2023-41

At MJD 59979.3274769 (2023-02-04 07:51 UTC) ZTF detected a new transient candidate (ZTF23aabtjjc/AT2023bbu) coincident with a faint galaxy in the PS1 catalog.  The source was saved to the ZTF Bright Transient Survey program by an on-duty scanner.   The magnitude at this time was g = 18.22 +/- 0.14.  The most recent ZTF observation of the field prior to this was seven days earlier at MJD 59972.41977, resulting in an upper limit of g > 20.37 (from forced photometry).  It had previously been detected by ATLAS at MJD 59974.27547 (o = 18.60) and reported to TNS as ATLAS23bnt, following an upper limit of o > 19.5 the previous night (MJD 59973.28015), suggesting a very fast rise possibly as short as 1-2 days.

The source faded rapidly after the first ZTF detection, dropping over the next five days to g = 18.60 +/- 0.18 at MJD 59984.40050 and g=18.80 +/- 0.16 at MJD 59985.34056.  ZTF has not detected the transient since, but follow-up imaging with the P60 Rainbow Camera shows the rapid fading to have continued, reaching g = 20.54 +/- 0.09 by MJD 59995.39043.  The transient remains quite blue throughout its evolution (g-r ~ 0 and r-i ~ 0, correcting for Galactic extinction).

We acquired spectroscopy of the candidate using GMOS-S at Gemini Observatory on 2023-02-21, under the target-of-opportunity program GS-2023A-Q-127. The spectrum shows a largely featureless blue continuum with host-galaxy emission features corresponding to H-alpha and [OIII] at z=0.0688.  At this redshift, the peak magnitude of the transient would be M_g ~ -19.2 or possibly slightly brighter given our sparse coverage around this phase.  While slightly less luminous than the known population of luminous fast blue optical transients (LFBOTs) such as AT2018cow, the timescales and color evolution are similar, suggesting a possible relation to this class.  A foreground CV origin has not yet been ruled out (although we do not detect any z=0 emission features in the spectrum) and multiwavelength follow-up observations are encouraged.

We thank the Gemini Observatory staff for assistance in rapidly obtaining these observations.

This report is based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Grant No. AST-2034437 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, Deutsches Elektronen-Synchrotron and Humboldt University, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, Trinity College Dublin, Lawrence Livermore National Laboratories, and IN2P3, France. Operations are conducted by COO, IPAC, and UW.