GRB221009A/Swift J1913.1+1946: ATA follow-up observations

After the detection of a bright radio counterpart to GRB221009A at 15.5 GHz was reported in ATel #15653 /GCN #32653, we conducted follow-up observations with the Allen Telescope Array (ATA) at 1.5, 3, 4, 6, 8, and 10 GHz. Observations were carried out simultaneously at 1.5 + 3, 4 + 6 and 8 + 10 GHz, with a 672 MHz bandwidth centered on each frequency, using a newly deployed wideband correlator (Farah et al., in prep.), at spectral and temporal resolution of 0.5MHz and 10s, respectively. The target field was observed for 1 hour per pair of frequencies. The primary flux standard 3c286 was used to calibrate the absolute flux scale and bandpass response of the array, and J1925+2106 was used to calibrate the time dependent complex gains. Data flagging was performed with AOFlagger (Offringa+2012) while calibration and imaging were performed using standard techniques in CASA (McMullin+2007). We clearly detect an unresolved radio source at a position consistent with the one reported in ATel #15650 at 1.5, 3, 6, 8, and 10 GHz. Preliminary flux densities at 3, 6, 8, and 10 GHz are reported below, and include a 10% absolute flux scale uncertainty in addition to the statistical error from the fit. Observations conducted at 4 GHz were severely corrupted by radio frequency interference and were not processed. Our observations indicate that the radio counterpart to GRB221009A/Swift J1913.1+1946 is self-absorbed below around 10 GHz, although there is apparently significant flux density evolution between observing epochs. MJD Frequency (GHz) Flux density (mJy) 59862.0492 3.0 3.6 +/- 0.7 59861.9414 6.0 9.0 +/- 1 59862.1242 8.0 28 +/- 3 59862.1242 10.0 38 +/- 4 The Allen Telescope Array is a 42-element radio interferometer located at the Hat Creek Radio Observatory in Northern California. The facility is fully operated by the SETI Institute.