AstroNote 2024-155

ZTF24aapvieu was discovered at a position (J2000) of:

RA = 12:02:37.27

Dec = +35:23:35.22

The first ZTF detection was at g = 19.93 +/- 0.08 mag on 2024-06-01 05:21:18.432 UTC (MJD 60462.22). Lowell Discovery Telescope (LDT) photometry five days later confirmed that the transient faded by about 1.5 mag, implying a fast fade rate of about 0.3 mag/day. There is a bright (g~19, r~18) SDSS galaxy 0.7 arcseconds away from AT2024kmq, with photo-z = 0.21 +/- 0.01.  Correcting for a foreground extinction of E_(g-r) = 0.02 at this location [1], we derive an implied red color of g-r = 0.87 +/- 0.15 mag from the LDT photometry.

From the rapid evolution and implied extragalactic nature, we identified this as a fast optical transient.  We reported the ZTF24aapvieu to the Transient Name Server and it was designated AT2024kmq. 

We obtained a spectrum using the Gemini Multi-Object Spectrograph on Gemini South at 2024-06-08 07:04:27 UTC (MJD 60469.29), seven days after peak light.  The spectrum appears to be dominated by the host galaxy.  Detections of Na I, Mg I, Hɑ, and Hꞵ absorption lines yielded the spectroscopic redshift of z = 0.192 and an implied absolute peak magnitude (with a cosmological k-correction) of M_g = -19.97 +/- 0.08 mag.  The lack of any emission lines in the host galaxy implies that it has a low star formation rate, which would be unusual for a fast extragalactic transient (e.g., [2]) but not unprecedented [3].

Due to its fast evolution and unusual properties of its host, we encourage additional follow-up observations of this transient. 

[1] Schlafly, E. F., & Finkbeiner, D. P. (2011), The Astrophysical Journal, 737, 103 [2] Wiseman et al. 2020, MNRAS, 498, 2575 [3] Nicholl et al. 2023, ApJL 954, L28

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.