AstroNote 2023-93

At MJD 60044.20379 (2023-04-10 04:53 UTC) ZTF detected a new transient candidate (ZTF23aaeozpp/AT2023fhn) 5” from a galaxy in SDSS. The magnitude at this time was r = 19.74 +/- 0.15. Two nights later, the transient had brightened by 0.4 magnitudes, and exhibited very blue colors (g - r = -0.47 mag, corrected for Milky Way extinction). The transient was saved as part of the ZTF Bright Transient Survey (Fremling et al. 2020, ApJ, 895, 32F; Perley et al. 2020, ApJ, 904, 35P) and reported to the Transient Name Server. Five nights later, the transient had faded by 1.4 magnitudes, to g~20.2 mag; the rapid fading was flagged by the ZTFReST project (Andreoni & Coughlin et al. 2020, ApJ, 904, 155A).

The fast timescale (duration above half-maximum light of 6 days) and implied high peak luminosity from the photometric redshift of the nearby galaxy (SDSS z_ph = 0.159 ± 0.039, implying M < -20 mag) motivated us to trigger follow-up observations. We acquired spectroscopy of the candidate using GMOS-S at Gemini Observatory on 2023-04-19, under the target-of-opportunity program GS-2023A-Q-127. The spectrum shows a featureless blue continuum. The timescale and persistent blue colors of the light curve resemble fast blue optical transients such as AT2018cow, AT2020xnd, and AT2022tsd. 

The following night (2023-04-20), we obtained a spectrum of the nearby galaxy using the Double Beam Spectrograph on the Palomar 200-inch telescope. We identify H-alpha, NII, and SII emission lines at a consistent redshift of z~0.24. If the transient is at the redshift of the galaxy, the peak luminosity is M_g = -21.5 mag.

A foreground dwarf nova origin cannot be ruled out. In particular, the ZTF position of the transient is close (1.5”) to a catalogued source in Legacy Survey that has a PSF morphological classification. However, the offset would be unusually large for CVs in ZTF, and the Gemini spectrum does not show any distinct features at z=0. 

We encourage additional follow-up observations of this transient, and 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.