AstroNote 2022-199

We report Keck/LRIS spectroscopic observations of AT2022tsd, a fast-evolving optical transient from the Zwicky Transient Facility (ZTF) public all-sky survey (internal name ZTF22abftjko).

AT2022tsd was discovered at the position (J2000) of

RA, Dec = 03:20:10.87, +08:44:55.94  (degrees: 50.04528195, +8.7488773; Galactic latitude -39 deg.)

on 2022 Sep. 7 UT by ZTF at r = 20.36 +/- 0.23 mag (JD = 2,459,829.9731) and g = 20.19 +/- 0.17 mag (JD=2,459,829.9879), and reported to the TNS by ALerCE (Munoz-Arancibia et al. 2022). The previous upper limits (5-sigma) were on 2022 Sept 04 (r>20.37 mag) and on 2022 Sept 02 (g>20.59 mag, r>20.54 mag). 

Forced photometry on ZTF public images place deeper pre-detection upper limits (3-sigma) of r>20.9, g>21.2, and r>21.2 for the three nondetection observations above, respectively, implying a fast rise.  Due to inclement weather and technical issues, no further observations were acquired by P48 during the ten days after the first detection.  The next observation, obtained on 2022 Sep 18 (11 days since discovery), showed that the transient had faded by over a magnitude (g ~ 21.5 mag from forced photometry). The transient position is 1.4’’ in projection from a galaxy visible in PS1 imaging.

A 40-minute Keck/LRIS spectrum (PI A. V. Filippenko) at the transient position acquired on 2022 Sep 23 showed a blue continuum with prominent narrow emission lines (Halpha, Hbeta, [N II], [O II], [O III], [S II]) from the host galaxy at a consistent redshift of z = 0.256. If the transient originated from within the galaxy, the discovery absolute magnitude would be M = -20.1 mag at a rest-frame wavelength of 3870 Angstroms (close to the rest-frame u-band) and M = -20.0 at a rest-frame wavelength of 5010 Angstroms (close to the rest-frame g-band).  Due to the lack of subsequent observations, these are lower limits on the true peak luminosity.

While the rise time is not well-constrained, it is faster than almost all well-established SN types of comparable luminosity except for AT2018cow-like objects and some strongly-interacting SNe (e.g. Ibn, Icn). The transient fade rate is however significantly slower than AT2018cow, and also slower than that of AT2020mrf, a radio- and X-ray-loud fast optical transient with a similar peak luminosity (Yao et al. 2022, ApJ, 934, 104). 

We encourage additional follow-up observations to determine the nature of this transient.