AstroNote 2021-62

Our rapid transient identification pipeline (Ho et al. 2020) identified the candidate ZTF21aakilyd on the basis of its rapid rise compared to recent limits (g = 18.11 at MJD 59256.5084, versus g > 20.29 two nights prior at MJD 2459255.0254).  Confirmation of the transient was provided by i-band ZTF imaging the same night from the partnership survey.   It was noted as a candidate of interest by an on-duty scanner (Yao) on this basis, its coincidence with a possibly distant galaxy, and its very blue color (g-i = -0.90 +/- 0.15 mag).  The source has been registered with TNS as AT2021csp.

We triggered Liverpool Telescope follow-up observations (ugriz imaging and SPRAT spectroscopy) the following night at La Palma.  The source has brightened slightly (to g = 17.90) and is confirmed to be very blue; it is particularly bright in the near-UV (u = 17.47 at MJD 59257.20252).

The SPRAT spectrum is largely featureless but shows a strong, narrow P-cygni feature with the emission component centered at 5040 Angstroms in the observer frame.  Another emission line is visible at 6160 Angstroms.  We identify these features as C III (4650,5696) at a common redshift of z=0.081.  There are no other strong lines visible in the spectrum.

Given this redshift, the absolute magnitude of the transient at the time of the LT observation is M = -20 (at 4500 Angstroms in the rest frame), having risen to this magnitude in less than 3 days.  These properties are consistent with the empirical class of fast blue optical transients (e.g., Drout et al. 2014, Pursianen et al. 2018), although the peak has not yet been reached.   

We have triggered Swift, Gemini, and HST observations, and encourage additional follow-up of this unusual event.