TITLE:   GCN CIRCULAR
NUMBER:  8223
SUBJECT: GRB 080913: GROND photo-z
DATE:    08/09/13 16:39:12 GMT
FROM:    Jochen Greiner at MPI  <jcg@mpe.mpg.de>

J. Greiner, T. Kruehler (both MPE), A. Rossi (Tautenburg Obs.)
report on behalf of the GROND team: 

We have analyzed more data of our GROND observing run (Rossi et al. 2008, 
GCN #8218) of GRB 080913 (Schady et al. 2008, GCN #8217; Stamatikos et al. 
2008, GCN #8222). The object reported previously is clearly detected in 
several epochs. The position is

RA (2000.0) = 04:22:54.74 
Decl. (2000.) = -25:07:46.2

with an uncertainty of 0.5 arcsec. This position is consistent with the 
astrometrically corrected Swift X-ray position (Beardmore et al. 2008, 
GCN #8219).

z'-band data spanning the time period from 3 min to 3 hrs after the burst
clearly show a fading source and are well fit with a power law
of decay slope 0.6 +- 0.3. This confirms the fading seen with the VLT
(Vreeswijk et al. 2008, GCN #8221). If this fading continues, we predict
z(AB) ~ 25 mag at 24 hrs after the GRB.

The afterglow is detected in z'JHK, but not in g'r'i-bands. After
correction for foreground extinction of A_V=0.129, we obtain (AB system)
g > 23.3 +- 0.1
r > 23.3 +- 0.1
i > 23.0 +- 0.1
z = 22.52 +- 0.15
J = 20.90 +- 0.06
H = 20.67 +- 0.09
K = 20.54 +- 0.21
at 07:13 UT (mid-time of 12 min exposure), 26 min after the GRB.
A fit to this simultaneously obtained 7-filter spectral energy distribution, 
using Hyper-z (Bolzonella et al. 2000) results in a photometric redshift of
z = 6.44 +- 0.3. The SED-fit is shown on http://www.mpe.mpg.de/~jcg/grb080913.html