TITLE:   GCN CIRCULAR
NUMBER:  40143
SUBJECT: EP250304a: ATCA radio observations
DATE:    25/04/14 19:06:17 GMT
FROM:    Yuhan Yao at UC Berkeley <yuhanyao@berkeley.edu>

Authors: Yuhan Yao, AJ Nayana, Eli Wiston, Raffaella Margutti, Ryan Chornock, Wenbin Lu (UC Berkeley), Elizabeth Mahony (CSIRO S&A, Marsfield), Tanmoy Laskar (U of Utah) 

We observed the Fast X-ray Transient (FXT) EP250304a discovered by Einstein Probe (Chen et al. GCN 39580; Zhang et al. GCN 39591) at the optical counterpart position (Liu et al., GCN 39583) with the Australia Telescope Compact Array at 5.5 and 9 GHz in continuum mode on 2025 March 21st 13:01-17:48 UT (i.e. 17.6 days after the EP trigger) under NAPA program C3623 (PI: Yao). At the redshift of z=0.2 (Saccardi et al., GCN 39585), EP250304a has been associated with a  type Ic broad line (Ic-BL) supernova (Izzo et al. GCN 39851). 

At the optical transient position, we did not detect a radio source at 5.5 GHz. The rms noise in the final map is 15 microJy/beam. The 3-sigma luminosity limit at z=0.2 is Lnu < 5.2 x 10^{28} erg s^{-1} Hz^{-1}.

Compared with known Ic-BL SNe with low-luminosity GRB counterparts, the radio upper limit of EP250304a is fainter than that of GRB171205a/SN2017iuk at a similar phase (Leung et al. 2021, MNRAS, 503, 1847), but consistent with those of GRB980425/SN1998bw (Kulkarni et al. 1998, Nature, 395, 663), GRB060218/SN2006aj (Soderberg et al. 2006, Nature, 442, 1014), and GRB100316D/SN2010bh (Margutti et al. 2013, ApJ, 778, 18).