TITLE:   GCN CIRCULAR
NUMBER:  15887
SUBJECT: PTF14yb: GROND Transient Detection
DATE:    14/02/27 22:00:43 GMT
FROM:    John Graham at STScI  <graham@stsci.edu>

K. Varela, J. F. Graham, J. Greiner, (all MPE Garching) and D. A.
Kann (TLS Tautenburg) report on behalf of the GROND team:

We observed the iPTF-detected GRB optical afterglow-like transient 
iPTF14yb (Cenko et al. GCN 15883) simultaneously in g'r'i'z'JHK 
with the GROND instrument (Greiner et al. 2008, PASP 120, 405) 
mounted at the 2.2m MPG telescope at La Silla Observatory (Chile).

Observations started at 08:09 UT on February 27th 2014 
(approximately 22 hours after initial detection). We clearly 
detect the source in the g'r'i'z' bands at the following (AB) 
magnitudes and errors:

g = 22.37 +/- 0.04
r = 22.11 +/- 0.03
i = 22.07 +/- 0.06
z = 22.11 +/- 0.03


We do not detect the source in the JHK bands above our 3-sigma AB 
limits given below:

J > 21.6
H > 21.0
K > 19.7


The values we measure are comparable with the concurrent RATIR 
observations (Cucchiara et al., GCN 15886). Given magnitudes are 
calibrated against SDSS field stars (g'r'i'z') as well as 2MASS 
field stars (JHK) and are not corrected for the expected Galactic 
foreground extinction corresponding to a reddening of E_(B-V)=0.03 
mag in the direction of the burst (Schlegel et al. 1998).

Given the unusually flat spectral slope of 0.25 +/- 0.07 (after 
correcting for Galactic extinction) apparent in our photometry 
and the flat spectra noted in Cenko et al. (GCN 15883) with the 
present data we cannot distinguish between a powerlaw or a thermal 
spectrum.  Deep NIR imaging is encouraged to allow this distinction.



John Graham