AstroNote 2023-114

At MJD 60065.1895833 (2023-05-01 04:33 UTC), ZTF detected a new transient candidate (ZTF23aaimsja) at (α,δ= 160.5739646,  52.4887015; J2000), ~3.3” from the catalogued galaxy SDSS J104217.64+522916.2. The magnitude at discovery (first alert) was r = 19.97 +/- 0.16 mag. The transient was saved as part of the ZTF Bright Transient Survey on 2023-05-01 (Fremling et al. 2020, ApJ, 895, 32; Perley et al. 2020, ApJ, 904, 35) and reported to the Transient Name Server, where it was assigned the name AT2023hkw.

AT2023hkw was flagged by several filters designed to detect rapidly evolving transients, including ZTFReST (Andreoni & Coughlin et al., 2021). Forced photometry recovered additional detections prior to discovery, which revealed a fast rise rate of 1.2 magnitudes in ~1 day. LT follow-up photometry showed that 3.6 days after the peak, the transient faded by 0.7 magnitudes to g = 20.14 +/- 0.11. The fast timescale (duration above half-maximum light of six days) of AT2023hkw motivated us to trigger follow-up observations. We acquired an optical spectrum of the candidate using DEIMOS at Keck Observatory on 2023 May 12 (PI Filippenko). The spectrum showed narrow H-alpha and [N II] emission lines from the host galaxy at a consistent redshift of z = 0.339. If the transient is at the redshift of the galaxy, the implied absolute magnitude is M_g = -21  and the offset from the galaxy nucleus is 14 kpc. 

We encourage multiwavelength follow-up observations of this transient. 

This report is based on observations obtained with the Samuel Oschin 48-inch telescope  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 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.