AstroNote 2021-211

AT2021tnj/ZTF21abmdcbi is a fast-fading luminous transient discovered by the Zwicky Transient Facility (ZTF) on 2021 July 15 (Ho et al. AstroNote 2021-205).

We observed the field of AT2021tnj with the Very Large Array (VLA) on July 30 using K (18--26.5 GHz, on-source integration time 41.6 minutes) and Ku (12--18 GHz, 39.6 min) bands (Project ID 21A-308, PI Ho). Data was calibrated using the VLA CASA Calibration Pipeline and imaged using the CASA CLEAN function, using 3C147 as the flux calibrator and J0152+2207 as the gain calibrator. The source was not detected, with K band rms = 7.0 uJy and Ku band rms = 3.3 uJy. At z=0.228, the 3-sigma upper limits on the radio luminosity are <7.2e+38 erg/s (K band) and <2.3e+38 erg/s (Ku band). 

The position of the AT2021tnj was scanned by the Spektrum-Roentgen-Gamma (SRG) X-ray satellite on July 20-21. The source was not detected by the eROSITA instrument with a 0.3-2.2 keV 3-sigma upper limit of 2.3e-14 erg/s/cm^2. Assuming a power-law spectrum with photon index =1.7, at z=0.228, the 0.3-10 keV X-ray luminosity is <1e+43 erg/s.

The Neil Gehrels Swift Observatory observed AT2021tnj on Aug 1 (on source time = 2.01 ks). AT2021tnj was not detected by the X-Ray Telescope (XRT) with a 0.3-10 keV background count rate of 5.8e-7 count/pixel/s. Using a circular radius of 7.63 pixels, the 3-sigma upper limit is 0.0015 count/s. Assuming a power-law spectrum with photon index =1.7, at z=0.228, the 0.3-10 keV X-ray luminosity is <1e+43 erg/s. 

On Aug 1, AT2021tnj was also observed by the UV filters of the Ultra-Violet/Optical Telescope (UVOT) onboard Swift. It is detected with UVW2 = 21.96 +/- 0.27 mag, UVM2 = 22.09 +/- 0.37 mag, and UVW1 = 21.75 +/- 0.61 mag (AB system, not corrected for Galactic extinction). The UVOT observations are consistent with the blue color and decay rate reported in Ho et al. AstroNote 2021-205.

In summary, assuming that the explosion epoch of AT2021tnj is <5 days of the first optical detection (on 2021 July 12), our VLA upper limits indicate that the radio luminosity of AT2021tnj is fainter than that observed in AT2018cow and its analog AT2020xnd (Margutti et al. 2019 ApJ, 872, 18; Ho et al. ApJ, 871, 73, Ho et al. in prep). The X-ray upper limits are consistent with the X-ray luminosities of the two objects.

We would like to thank the VLA and Swift staff for scheduling and carrying out these observations.