Discovery of a fast, luminous transient at z=0.244 with the Zwicky Transient Facility, the Liverpool Telescope, and Keck Observatory

ATel #14105; Daniel A. Perley (LJMU), Anna Ho (UC Berkeley), Christoffer Fremling (Caltech), Yuhan Yao (Caltech)
on 20 Oct 2020; 23:08 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Daniel Perley (d.a.perley@ljmu.ac.uk)

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During the night of 2020 Oct 12 (UT), the Zwicky Transient Facility (ZTF; Kulkarni et al., ATEL #11266; Bellm et al. 2019a, PASP, 131, 18002) detected a new optical transient, internally designated ZTF20acigmel, during routine observations as part of the two-day cadence public survey. The magnitude at the time of first detection (JD 2459134.6708) was r = 20.50 +/- 0.31 (all magnitudes AB). It was subsequently detected in g-band on the same night (g = 19.68 +/- 0.15 at JD 2459134.7279). The last constraining nondetection was recorded four days prior (g > 20.45 at JD 2459130.7350). A slight rise in flux was observed over the next two days (peaking at g = 19.56 +/- 0.13 at JD 2459136.7137). At t+4 days post-discovery the transient showed a significant decline in g-band (g = 20.04 +/- 0.22 at JD 2459138.6937).

ZTF20acigmel was automatically identified as a fast transient candidate by a software filter developed by A. Ho and Y. Yao on the basis of its blue color, fast evolution, and coincidence with a faint extended object (g=24.85, r=24.08) in the Legacy Survey (DR8; Dey et al. 2019, AJ, 157, 168). It was marked as a source of interest on Oct 16 following nightly scanning of the filter products. Follow-up imaging was obtained with IO:O on the robotic Liverpool Telescope (Steele et al., SPIE, 5489, 679) in the ugriz filters the same night and the following night. The color of the transient is moderately blue in both sets of observations in the optical/NIR filters, but shows a very blue u-g color of -0.4 mag. The source also exhibited a rapid decay with respect to the earlier ZTF observations, reaching g = 20.52 +/- 0.07 as of JD 2459140.3575. Subsequent imaging from the Palomar 60-inch telescope shows the transient to have now faded below 21st mag. The total time above half-max is between 4 and 8 days, placing this clearly in the regime of fast optical transients (see e.g. Drout et al. 2014, ApJ, 794, 23 or Pursiainen et al. 2018, MNRAS, 481, 894).

We acquired spectroscopy of ZTF20acigmel using the Low Resolution Imaging Spectrometer (LRIS; Oke et al. 1995, PASP, 107, 375) on the Keck I 10m telescope on Oct 19. The spectrum shows a featureless blue continuum with a series of weak absorption lines in the NUV region of the spectrum. The wavelengths of these lines are consistent with Mg II 2796+2803, Mg I 2852, and Fe 2600 at a common redshift of z=0.2442. There is also evidence of [OIII] 5007 and H-alpha emission at consistent redshift, although these lines are not individually significant. At this redshift the inferred peak absolute magnitude of the transient is approximately -20.9 (at 3900 Angstroms in the rest frame).

Based on these characteristics we suggest that ZTF20acigmel (reported to the TNS as AT2020xnd) is a member of the high-luminosity fast transient population and may be an analog of AT2018cow (Smartt al., ATEL#11727; Prentice et al. 2018, ApJ, 865, 3). We have requested Swift observations and are pursuing additional radio and millimeter observations to search for evidence of a central engine, as was seen in previous low-redshift members of this class (e.g. Ho et al. 2019, ApJ, 871, 73; Margutti et al. 2019, ApJ, 872, 18; Ho et al. 2020, ApJ, 895, 49; Coppejans et al. 2020, ApJ, 895, 23) Additional observations are encouraged, although we caution that the much higher redshift will make follow-up more challenging than was the case for AT2018cow itself.

The coordinates of the transient (J2000), measured from the Liverpool Telescope imaging, are RA=22:20:02.04, Dec=-02:50:25.1 (+/- 0.3").