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GCN Circular 34704

GRB 230906A: optical counterpart
2023-09-15T21:21:52Z (6 days ago)
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C. Peña (Univ. of Utah), D.  B. Malesani (Radboud univ. and DAWN/NBI), A. Rossi (INAF), A. J. Levan (Radboud univ.), and G. Pugliese (API, Amsterdam), report on behalf of the Stargate collaboration:

Following the Chandra X-ray localization (O’Connor et al., GCN 34672) of the Fermi/GBM short GRB 230906A (Fermi GBM team, GCN 34631; Frederiks et al., GCN 34638), we observed the GRB afterglow with the ESO Very Large Telescope equipped with the HAWK-I near infrared camera and with the FORS2 optical camera. Observations in the HAWK-I Ks band consisted in 30 minute exposure with a mean time of 6.84 days after the GRB. Observations in the FORS2 R band consisted in 40 minute exposure with a mean time of 6.76 days after the GRB. Sky conditions were modest, with a delivered seeing of ~2" in the R band.

Consistent with the position of the X-ray afterglow discovered by Chandra (O’Connor et al., GCN 34672), we detect a weak source in the stacked R-band image, at J2000 coordinates (~0.5" error):

RA = 05:19:01.57
Dec = -47:53:32.3

We measure for this source an AB magnitude R = 25.44 ± 0.25, calibrated against nearby SkyMapper stars. In the Ks band, we can only set an upper limit with AB magnitude > 23.3. As noted by O’Connor et al. (GCN 34672), there is no detected host galaxy in the Legacy Survey, which has however a depth shallower than our measurement.

While the spatial association with the X-ray counterpart suggests a physical connection with the GRB, it is unclear whether the source is dominated by transient light or is a faint host galaxy. We note that the R-K color is not as red as was AT 2017gfo at a comparable epoch after explosion (e.g. Villar et al. 2017, ApJ, 851, L21).

We acknowledge the support of the observing staff at Paranal, in particular Ana Escorza, Linda Schmidtobreick, and Fuyan Bian.

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