TITLE: GCN CIRCULAR NUMBER: 30755 SUBJECT: GRB 210901A: GROND observations DATE: 21/09/03 16:41:33 GMT FROM: Ana Nicuesa at TLS Tautenburg ana@tls-tautenburg.de
A. Nicuesa Guelbenzu, S. Klose (both TLS Tautenburg), and A. Rau (MPE Garching) report:
We observed the field of GRB 210901A (Swift trigger #1071488; Laha et al., GCN 30746) simultaneously in g'r'i'z'JHK with GROND mounted at the 2.2m MPG telescope at La Silla Observatory (Chile).
Observations started at 23:43 UT on September 1, about 8.5 hr after the GRB trigger. They were performed at an average seeing of 2.3 arcsec and at an average airmass of 1.5.
Inside the 2.5 arcsec enhanced XRT error circle (Osborne et al., GCN 30749), we do not detect any source. Based on an observation with 23.4 minutes integration time in g'r'i'z' and 20 minutes in JHK, at a midtime of 23:43 UT on September 1, we measure the following preliminary 3-sigma upper limits (AB mags):
g' > 23.5, r' > 23.6, i' > 22.6, J > 20.1, H > 19.7, and K > 16.3.
We note the presence of an object at the north-western border of the XRT error circle at coordinates RA, DEC (J2000) = 19:41:05.98, -76:55:33.0 for which we measure r' = 21.33 +/- 0.05 mag (AB). Due to the relatively bad seeing at present we cannot decide with certainty if this is a galaxy or a star.
Given magnitudes are calibrated against the SkyMapper catalog (Wolf et al., 2018, PASA 35, e010) in g'r'i'z' and 2MASS field stars in JHK (Skrutskie et al., 2006, AJ 131, 1163).
We thank Maren Hempel for excellent support and for performing the observations.