L. P. Singer (Caltech), S. B. Cenko (NASA/GSFC), and M. M. Kasliwal
(Carnegie Observatories) report on behalf of the intermediate Palomar
Transient Factory (iPTF) collaboration:
Upon receiving the final GBM localization for GRB 131231A, we
selected ten fields to observe with the Palomar 48-inch Oschin
telescope (P48) covering 62 deg^2, most of the 1-sigma GBM error
circle. Due to the short remaining visibility window at Palomar, we
obtained only one epoch of images of five of the ten fields covering
30 deg^2. This includes ~90% of the LAT error localization (Sonbas et
al., GCN 15640).
Sifting through 50 candidate variable sources near the LAT error
circle using standard iPTF vetting procedures, at 1.45 hours after
the burst we identify multiple uncatalogued sources, including
iPTF13ekl at R=15.7 mag, at the coordinates
RA(J2000) = 00h 42m 21.67s
DEC(J2000) = -01d 39' 11.0"
This position coincides with the Nanshan optical afterglow candidate
at 7.91 hours after the burst (Xu et al., GCN 15641). There are no
coincident sources in SDSS and no associated minor planets in the IAU
Minor Planet Center database at this location.
Despite the low ecliptic latitude of -5.7 deg, the two coincident
optical detections separated by 6.46 hours strongly disfavor a solar
system origin. These two observations fit a power-law decay with an
index of alpha=-1.03, suggesting that the source is indeed associated
with the GRB trigger.
See <http://www.its.caltech.edu/~lsinger/iptf/Fermi410157919.pdf> for
a depiction of the LAT error circle in relation to the P48 optical
candidates, with iPTF13ekl highlighted, and a selection of the P48