AstroNote 2021-89

We report the discovery of the fast, low-luminosity optical transient ZTF21aapkbav/AT2021gca with the Zwicky Transient Facility (ZTF, Bellm et al. 2019, Graham et al. 2019) at coordinates:

RA = 14:28:07.332 (217.0305508d)
Dec = +33:29:49.38 (33.4970497d)

ZTF21aapkbav was first detected on 2021-03-17 09:23 UT at r=19.74+-0.06 mag. The last ZTF upper limit before the first detection was measured on 2021-03-08 06:27 UT, which constrains the onset time to be within < 9.1 days. ZTF21aapkbav faded by ~0.3 mag/day in both g- and r-band. The transient was last observed with ZTF on 2021-03-19 10:18 UT at g=20.59+-0.12 mag. Photometric measurements were obtained using the ZTF forced-photometry service (Masci et al., 2018).

The color of ZTF21aapkbav appears to be red, with g-r~0.2 mag. The Galactic extinction on the line of sight is small, with E(B-V)=0.014 mag (Planck Collaboration et al., 2014). 

ZTF21aapkbav is spatially consistent with the possible host galaxy GALEXMSC J142807.45+332950.0 at redshift z=0.036 (Kochanek et al., 2012), as tabulated by NED, with a separation of about 2 arcsec from the cataloged coordinates. Assuming an association between the galaxy and ZTF21aapkbav, the absolute magnitude of the transient first (brightest) detection would be M_r ~ -16.3 mag, which implies an intrinsically low luminosity. We caution that the onset time was poorly constrained (due to a streak of bad weather at Palomar Observatory), so it is possible that the peak of ZTF21aapkbav was brighter.

Follow-up observations are encouraged.

ZTF21aapkbav was found by the ''ZTF Realtime Search and Triggering'' (ZTFReST) project, which aims at near real-time identification of compelling kilonova candidates in ZTF data using the methods described in Andreoni et al. (2020, ApJ, 904, 2), independently of gravitational-wave or gamma-ray triggers. ZTF21aapkbav was also identified as part of the Census of the Local Universe project (De et al., ApJ, 905, 1).

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. The ZTF forced-photometry service was funded under the Heising-Simons Foundation grant #12540303 (PI: Graham)