TITLE: GCN GRB OBSERVATION REPORT NUMBER: 4604 SUBJECT: GRB 060121: Probable Detection of NIR Afterglow DATE: 06/01/26 22:50:50 GMT FROM: Don Lamb at U.Chicago GRB 060121: Probable Detection of NIR Afterglow F. Hearty (Colorado), M. Bayliss (Chicago), D. Q. Lamb (Chicago), G. Gyuk (Chicago), M. Hammergren (Adler Planetarium), A. Puckett (Chicago), B. Ketzeback (APO), J. Barentine (APO), J. Dembicky (APO), R. McMillan (APO), and D. G. York (Chicago) report: We observed the afterglow (Malesani et al. GCN 4561, Levan et al. GCN 4562) of GRB 060121, a bright short/hard burst localized by HETE-2 (Arimoto et al. GCN 4550, Prigozhin et al. GCN 4551, Boer et al. GCN 4552), on the night of January 22nd, using NIC-FPS on the ARC 3.5-meter telescope at Apache Point Observatory. The observation began at 7.25 UT on 23 January (32.83 hours after the burst) and ended at 8.63 UT on 23 January (34.21 hours after the burst). The observation consisted of a series of 180 20-second exposures in Ks. We have constructed from them a stacked image of the GRB field, corresponding to a 60-minute exposure. We detect an object at the 5-sigma confidence level within 1" of the location of the optical afterglow and well within the revised XRT error circle (Mangano et al. GCN 4560). The PSF of the object is consistent with that of a point source. We therefore identify the object as the probable NIR afterglow of GRB 060121. We measure Ks ~ 20 mag, calibrated relative to four 2MASS stars in the field. Extended emission is visible about 1" SE and NE of the probable NIR afterglow, but is detected at less than the 2-sigma confidence level. Therefore the emission might be due to nearby galaxies (possibly including the host of GRB060121) or merely to a background fluctuation. Further observations are in progress. NIC-FPS is currently in its commissioning phase.