AstroNote 2021-205

During the night of 2021 July 15 (UT), the Zwicky Transient Facility (ZTF) detected a new optical transient, internally designated ZTF21abmdcbi, at the following position (J2000):

01:41:20.10 +23:01:49.6

25.333731 +23.030432

The detection was made during routine observations as part of the partnership survey (SRG shadow program) and the public two-day cadence survey. The magnitude at the time of first detection (JD 2459410.9312) was r = 19.55 +/- 0.13 (all magnitudes AB, not corrected for Galactic extinction). It was subsequently detected in g-band on the same night (g = 19.42 +/- 0.14 at JD 2459410.9628). The last constraining ZTF nondetection was recorded 33 days prior (r > 21.30 at JD 2459377.9653). 

The source was saved to TNS on July 16 by ALeRCE (Forster et al 2021) and received the name AT2021tnj.

A significant (1 mag) decay in flux was observed over the next five days. Because of the relatively fast decline and blue colors during the decline, the source was flagged during daily scanning for fast transients on 2021 July 20. LT follow-up imaging on July 23 (g-r=-0.11 +/- 0.11 mag) and July 27 (g-r=-0.41 +/- 0.42 mag) confirmed the blue colors and fast decline.

We ran ATLAS forced photometry (Smith et al. 2020), which provided a c-band detection at 19.25 +/- 0.24 (JD 2459408.0824), three days before the first ZTF detection. The last ATLAS nondetection was recorded at o > 19.54 (JD 2459392.1027).

On July 23 we obtained a spectrum using our ToO program on Gemini South. The spectrum is blue and featureless except for emission lines from the host galaxy (H-alpha, H beta, OIII) at a redshift of z=0.228. The redshift makes the transient highly luminous, with a peak absolute magnitude of c < -21.1 mag (no K-correction).

Further follow-up with the Liverpool Telescope on July 29 UT (JD 2459424.65), while affected by moonlight and poor seeing, shows only a marginal detection (2-sigma) in the latest observation, and confirms that the source continues to fade rapidly.  The upper limits in the stacked observations are g>21.5, r>21.5, i>21.8.

The luminosity, color, and featureless spectrum are consistent with the population of fast, luminous blue transients typefied by AT2018cow (Ho et al. 2021, https://arxiv.org/abs/2105.08811), although the rate of fading is somewhat slower.  We lack a constraint on the rise time of this transient.  Swift and VLA observations have been requested.

We thank the Gemini-South staff for their assistance scheduling and executing the ToO observation. 

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.