TITLE: GCN CIRCULAR NUMBER: 12192 SUBJECT: GRB 110721A: potential counterpart candidate DATE: 11/07/23 20:37:50 GMT FROM: Jochen Greiner at MPI J. Greiner (MPE), A.C. Updike (NASA/GSFC), T. Kruehler (DARK/NBI), V. Sudilovsky (MPE) report: We have analyzed the first 4 tiles of the Swift/XRT observations on the 0.75 deg Fermi/LAT radius error box (Vasileiou et al., GCN 12188) of the bright Fermi/GBM GRB 110721A (Tierney and von Kienlin, GCN 12187). In the pointing with OBS-ID 00020169 we find a single X-ray source at RA(2000.0)=22h14m38.27s, Decl(2000.0)= -38:35:36.3 (+-6.8 arcsec). We observed this position simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPI/ESO telescope at La Silla Observatory (Chile). Observations started at 06:33 UT on July 22, about 26 hrs after the GRB trigger. They were performed at an average seeing of 1.0" and at an average airmass of 1. We found a single point source within the 6.8" Swift-XRT error circle, at coordinates RA(2000.0) = 22h 14m 38.19s, Decl(2000.0) = -38d 35' 35.7" with an uncertainty of +-0.3". Based on 1840 s of total exposures in g'r'i'z' and 1920 s in JHK, we estimate preliminary magnitudes (all AB system) of g = 24.3 +/- 0.2 r = 22.9 +/- 0.1 i = 22.3 +/- 0.1 z = 21.8 +/- 0.1 J = 21.0 +/- 0.2 H = 20.5 +/- 0.2 K > 18.8 These optical and NIR magnitudes are calibrated against GROND zeropoints and 2MASS field stars, respectively, and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E(B-V)=0.016 mag in the direction of the burst (Schlegel et al. 1998). The spectral energy distribution (SED) is well fit by a powerlaw, typical for GRB afterglows. The low-level g'-band flux can either be fit with Ly-alpha at a redshift of z~3.2, or host-intrinsic extinction. We obtained another epoch of observations starting at 6:21 UT on Jul 23, at an average seeing of 1.4". Within the errors (+-0.2 mag), the object exhibits the same brightness in both observations. We note that the chance coincidence to find 1 X-ray source in 4 Swift/XRT pointings of the given exposure is nearly 100%. Also, the 4 tiles only cover 30% of the area of the Fermi/LAT error circle. However, the powerlaw shape of the SED of the optical/NIR source make this a good candidate for the counterpart of GRB 110721A. Our second observation is inconclusive, unfortunately - the non-fading behaviour could either be due to a plateau of the emission from the afterglow, or indicate a non-GRB nature. We suggest spectroscopic observations of this object to reveal its nature.