[vsnet-grb-info 15817] GRB 141121A: Continued RATIR Optical Observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  17105
SUBJECT: GRB 141121A: Continued RATIR Optical Observations
DATE:    14/11/27 16:16:55 GMT
FROM:    Alan M. Watson at Instituto de Astronomia UNAM  <alan at astro.unam.mx>

Alan M. Watson (UNAM), Nat Butler (ASU), Alexander Kutyrev (GSFC), William H.
Lee (UNAM), Michael G. Richer (UNAM), Chris Klein (UCB), Ori Fox (UCB), J.
Xavier Prochaska (UCSC), Josh Bloom (UCB), Antonino Cucchiara (ORAU/GSFC),
Eleonora Troja (GSFC), Owen Littlejohns (ASU), Enrico Ramirez-Ruiz (UCSC), José
A. de Diego (UNAM), Leonid Georgiev (UNAM), Jesús González (UNAM), Carlos
Román-Zúñiga (UNAM), Neil Gehrels (GSFC), and Harvey Moseley (GSFC) report:

We observed the field of GRB 141121A (Lien et al., GCN 17075) with the
Reionization and Transients Infrared Camera (RATIR; www.ratir.org) on the 1.5m
Harold Johnson Telescope at the Observatorio Astronómico Nacional on Sierra San
Pedro Mártir from 2014/11 27.31 to 2014/11 27.54 UTC (147.50 to 153.22 hours
after the BAT trigger), obtaining a total of 4.62 hours exposure in the r, i and
z bands.

For a source within the Swift-XRT error circle, in comparison with the SDSS DR9,
we obtain the following detections:

r = 21.59 +/- 0.04
i = 21.39 +/- 0.04
z = 20.89 +/- 0.26

These magnitudes are in the AB system and are not corrected for Galactic
extinction in the direction of the GRB.

The afterglow has faded by about 0.33 magnitudes in r and i compared to our
observations at 126 hours (Butler et al., GCN 17101). This corresponds to a
further steepening of the temporal power law from t^-1.3 between 102 and 126
hours (Watson et al., GCN 17100) to t^-1.8 between 126 and 150 hours.

We detect a source 2.3 arcsec to the east of the optical transient, with r =
23.22 +/- 0.13, i = 23.11 +/- 0.13, and z > 21.02. This source is visible in
earlier images, but due to its proximity to the brighter optical transient our
automatic pipeline failed to produce photometry for it from those earlier
images. We expect better photometry for this source once the afterglow has faded
further.

For a flat universe with H0 = 70 km/s/Mpc and OmegaM = 0.29, and at a redshift
of 1.47 (Perley et al., GCN 17081) the angular separation of the afterglow and
this source corresponds to a projected separation of 20 kpc. This separation is
consistent with the source being the host galaxy of the GRB.

If this source is the host galaxy, our photometry suggests that it is among the
brightest ever observed at this redshift for optically bright GRBs (Hjorth et
al. 2012, ApJ, 756, 187; Perley et al. 2013, ApJ, 778, 128).

We thank the staff of the Observatorio Astronómico Nacional in San Pedro Mártir.