TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4611
SUBJECT: GRB 060121: Confirmation of NIR Afterglow
DATE:    06/01/28 00:58:25 GMT
FROM:    Don Lamb at U.Chicago  <lamb@oddjob.uchicago.edu>

GRB 060121: Confirmation 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 have obtained a second-epoch observation of 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), using NIC-FPS on the
ARC 3.5-meter telescope at Apache Point Observatory.  The second-epoch
observation began at 5.50 UT on 27 January (5.30 days after the burst)
and ended at 6.92 UT on 27 January (5.35 days after the burst).  The
observation consisted of a series of 195 20-second exposures in Ks. 
Using these exposures, we have constructed a stacked image of the GRB
field, corresponding to a 65-minute exposure.

In this image, we no longer detect the Ks-band source that we reported
in GCN 4604.  Therefore, the source has clearly faded between the two
observations, confirming that it is the NIR afterglow of GRB 060121

The extended emission features that were visible in the first epoch 
image about 1" SE and about 1" NE of the NIR afterglow are detected in 
the second epoch image, but at lower significance than in the first 
epoch image because of somewhat poorer seeing.  We are therefore
unable to establish with confidence the reality of either emission
feature.

NIC-FPS is currently in its commissioning phase.