TITLE:   GCN CIRCULAR
NUMBER:  22968
SUBJECT: GRB 180715A: Gemini imaging upper limits
DATE:    18/07/19 22:55:09 GMT
FROM:    Wen-fai Fong at Northwestern U  <wfong@northwestern.edu>

W. Fong (Northwestern Univ.), A. J. Levan (U. Warwick), R. Chornock (Ohio Univ.) report on behalf of a larger collaboration:

"We observed the location of the short-duration GRB 180715A (Ukwatta et al., GCN 22947) with the twin Gemini-North and Gemini-South 8-meter telescopes located on Mauna Kea, Hawaii and Cerro Pachon, Chile, respectively. We used the pair of Gemini Multi-Object Spectrographs (GMOS) to obtain r-band imaging at mid-times of 5.9 hr, 13.0 hr and 1.2 days post-burst, spanning 2018 Jul 16-Jul 17 UT. Our observational coverage of the refined Swift/BAT position (90% containment; Ukwatta et al., GCN 22951) as well as the uncatalogued Swift/XRT sources (Kennea et al., GCN 22953) is as follows:

5.9 hr: ~70% of BAT position, XRT S7 and S11
13.0 hr: 100% of BAT position, XRT S8
1.2 days: 100% of BAT position, XRT S8

We identify optical sources in and around each of these 3 XRT sources (90% containment; Kennea et al., GCN 22953) in our imaging; however, all sources are clearly visible in archival PS1 or SDSS imaging and have measured magnitudes within +/- 0.3 mag of known values. Furthermore, we do not detect any uncatalogued optical sources in or around these 3 XRT sources to 3-sigma limits of r>23.6 AB mag (S7 and S11 at 5.9 hr) and r>25.2 AB mag (S8 at 13.0 hr). The photometry is calibrated to SDSS DR12 and is not corrected for Galactic extinction in the direction of the burst.

Finally, we perform image subtraction between the observations at 13.0 hr and 1.2 days, which each provide full coverage of the refined BAT position (90% containment). We note that there is significant contamination from the 8th mag star that affects ~20% of the BAT position in the subtraction. For the remaining, uncontaminated region, we place a 3-sigma limit of r>24 AB mag on the optical afterglow of GRB 180715A at 13.0 hr after the burst.

We thank the Gemini-North and Gemini-South queue observers and staff for their rapid assistance with these observations."