TITLE:   GCN CIRCULAR
NUMBER:  21850
SUBJECT: LIGO/Virgo G298048: VLBA upper limits at 8.7GHz for the GW counterpart SSS17a/DLT17ck on August 20-21 UT
DATE:    17/09/08 11:16:27 GMT
FROM:    Igor Andreoni at Swinburne U of Tech  <igor.andreoni@gmail.com>

A. Deller (Swinburne/OzGrav), M. Bailes (Swinburne/OzGrav), I. Andreoni (Swinburne/OzGrav/AAO), K. Bannister (CSIRO), J. Cooke (Swinburne/OzGrav), D. Dobie (University of Sydney), D. Kaplan (UWM), C. Lynch (University of Sydney), T. Murphy (University of Sydney), on behalf of a joint effort between OzGrav and the VAST collaboration.


Radio observations with the Very Long Baseline Array from 2.5 weeks ago (Deller et al., LVC GCN 21588) covering the field surrounding the optical transient SSS17a/DLT17ck (e.g., Coulter et al., LVC GCN 21529; Allam et al., LVC GCN 21530; Yang et al., LVC GCN 21531) have now been correlated and analysed.  

Each observation spanned 6.5 hours, with 256 MHz of bandwidth (dual polarisation) centered on 8.7 GHz, giving an image rms of approximately 22 microJy.  A 12500x12500 pixel image (2.5 x 2.5") image was produced, centered on SSS17a/DLT17ck.  As expected given non-detections with the VLA (Alexander at al., LVC GCN  21589; Corsi et al.,  LVC GCN 21613; Corsi et al., LVC GCN 21614; Corsi et al., LVC GCN  21636) and ATCA (Bannister at al., LVC GCN 21559; Kaplan et al., LVC GCN 21574; Lynch et al.,  LVC GCN 21670) close in time to these VLBA observations, we do not detect any radio emission, and place 5.5 sigma upper limits as follows:

Observation center MJD 57985.9: 5.5 sigma upper limit 125 microJy.
Observation center MJD 57986.9: 5.5 sigma upper limit 120 microJy.

Combining these two observations places a deeper limit, with image rms 16 microJy: 5.5 sigma upper limit 88 microJy.

We thank the staff of the Long Baseline Observatory for supporting these observations.