TITLE: GCN CIRCULAR NUMBER: 30748 SUBJECT: GRB 210901A: LCO Optical Upper Limit DATE: 21/09/01 20:02:31 GMT FROM: Robert Strausbaugh at U. of the Virgin Islands robert.strausbaugh@uvi.edu

R. Strausbaugh (U. of the Virgin Islands), A. Cucchiara (U. of the Virgin Islands/College of Marin) report on behalf of a larger collaboration:

We observed Swift GRB 210901A (Laha, et al., GCN 30746) with the LCO 1-m Sinistro instrument at the South African Astronomical Observatory site, on September 1, from 17:14 to 17:36 UT (corresponding to 2.17 to 2.53 hours from the GRB trigger time) with the Bessel R and I filters.

We performed a series of 3x300s exposures in R and I. In the XRT detection region (Laha, et al., GCN 30746) no optical counterpart could be detected down to the following upper limits, calculated using the USNO-B.1 catalog as reference:

R>21.55

I>21.16

These magnitudes are not corrected for galactic extinction.

R.S. is funded by NSF AST grant #1831682