AstroNote 2022-275

ZTF22abvrxkk was an optical transient discovered by the Zwicky Transient Facility (ZTF) public all-sky survey. Two alerts were issued, both on 2022 Nov 21 UT: r = 19.46 +/- 0.19 mag (JD ​​= 2459904.8443) and g = 19.53 +/- 0.21 mag (JD = 2459904.8774). The transient was reported to the TNS (Fremling et al. 2022, Discovery Report 2022-3387) and named AT2022abfc. The most recent upper limit (5-sigma) was the previous night (Nov 20) at r > 19.88 mag. 

Forced photometry on ZTF public images places a deeper pre-detection upper limit (3-sigma) of r>20.4 mag for the nondetection observation above, implying a fast rise. Forced photometry also reveals a fast fade over the next five nights, to r~g~20.4 mag. The transient position is 0.6'' in projection from a galaxy visible in PS1 imaging.

A GMOS-S spectrum (PI Ho; Program ID GS-2022B-Q-126) at the transient position acquired on 2022 Dec 1 showed prominent lines (H-alpha in emission, Ca H&K and Mg I in absorption) from the host galaxy at a consistent redshift of z = 0.212. If the transient originated from within the galaxy, the discovery absolute magnitude would be M = -20.7 mag at a rest-frame wavelength of 3896 Angstroms (close to the rest-frame u-band).

A VLA observation (PI Perley; Program ID 22A-405) on Dec 6 revealed faint (13 uJy; 4-sigma) emission at 10 GHz (X-band). In an observation two weeks later (Dec 23), the 10 GHz emission had brightened to 36 uJy, implying a transient origin. The VLA radio position is consistent with the optical transient position and the implied luminosity is ~5E28 erg/s/Hz.

A Swift/XRT observation on Dec 8 resulted in a non-detection, with an upper limit of 0.006 ct/s, or ~2E-13 erg/cm2/s. At the redshift of AT2022abfc this implies an upper limit of ~3E43 erg/s. 

The optical light curve and the luminous radio counterpart resemble that of luminous fast blue optical transients such as AT2018cow and AT2020xnd; the Swift/XRT upper limit does not rule out X-ray emission identical to AT2018cow. However, the peak color of the transient is not well constrained (extinction-corrected g - r = 0.08 +/- 0.28) and the lack of Balmer absorption in the host-galaxy spectrum suggests a relatively old stellar population. 

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