An 18 GHz ATCA detection of AT2020xnd
ATel #14148; Joe Bright (Northwestern), Mark Wieringa (CSIRO), Tanmoy Laskar (Bath), Raffaella Margutti, Deanne Coppejans, Kate D. Alexander, Lindsay DeMarchi, David Matthews (Northwestern)
on 4 Nov 2020; 18:23 UT
Credential Certification: Joe Bright (joe.bright@physics.ox.ac.uk)
Subjects: Radio, Supernovae, Transient
Referred to by ATel #: 14249
We report further observations of the fast transient AT2020xnd (ATel #14105) with the Australia Telescope Compact Array (ATCA). We observed AT2020xnd between 05:00 and 09:00 UTC on 29/10/2020 with the 15 mm receiver, which is simultaneously sensitive at 17 and 19 GHz with a 2 GHz bandwidth centred on each frequency.
Combining our observations at these frequencies we detect AT2020xnd at 110 uJy +/- 20 uJy at 18 GHz and measure an image RMS noise of around 15 uJy/beam near the location of the source.
Based on a redshift z=0.2442 (ATel #14105) or angular distance 799 Mpc, and assuming (based on AstroNote 2020-204) that we are measuring emission from the optically thick region of the synchrotron spectrum we infer a source size from our 18 GHz detection >~2e16 cm at 17-21 days. This corresponds to an inferred outflow velocity >=0.3c (as Gamma*Beta*c=R/delta_t). This is faster than the one found in AT2018cow (Margutti+2019, ApJ, 872, 18 and Ho+2019, ApJ, 871, 73) and more similar to those seen in CSS161010 and ZTF18abvkwla (Coppejans+2020, ApJ, 895, L23 and Ho+2020, ApJ, 895, 49). These calculations assume that the spectral peak is caused by synchrotron self absorption and that equipartition applies (see e.g. Scott & Readhead 1977, MNRAS, 180, 539; Chevalier1982, ApJ, 258, 790; Chevalier1998, ApJ, 499, 810 for more details).
We thank the ATCA director and staff for approving and carrying out these observations.