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GCN Circular 17336

Subject
GRB 150120B: GROND Afterglow Detection
Date
2015-01-21T09:14:34Z (9 years ago)
From
Philip Wiseman at MPE/Swift <wiseman@mpe.mpg.de>
P Wiseman, J Graham, P Schady and J Greiner (All MPE Garching) report on
behalf of
the GROND team:

We observed the field of GRB 150120B (Swift trigger 627170; D'Elia et al.,
GCN 17314) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008,
PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La Silla
Observatory (Chile).

Observations started at 01:25 UT on 21.01.2015, ~18hrs after the GRB
trigger. They were performed at an average seeing of 1.1" and at an
average airmass of 1.7.

We find a single point source within the 1.4" Swift-XRT error circle (Goad
et al., GCN 17320), consistent with that
reported by Perley et al. (GCN 17315), Guidorzi et al. (GCN 17316), at

RA (J2000.0) = 02 h 37 m 09.77 s

DEC (J2000.0) = +08d 04' 40.49"

with an uncertainty of 0.3" in each coordinate.

Based on 25 min of total exposures in g'r'i'z'JHK, we estimate preliminary
magnitudes and 3 sigma upper limits (all in AB system) of

g' > 24.2
r' = 23.6 +/- 0.1 mag,
i' = 22.7 +/- 0.2 mag,
z' = 22.4 +/- 0.40 mag,
J = 20.2 +/- 0.3 mag,
H = 19.9 +/- 0.5 mag, and
K = 18.7 +/- 0.4 mag.

Based on these magnitudes, the best-fit SED has a break in the g' band
suggesting a photometric redshift of 3.5 +/-0.5.

Given magnitudes are calibrated against GROND zeropoints as well as 2MASS
field stars and are not corrected for the expected Galactic foreground
extinction corresponding to a reddening of E_(B-V)=0.17 mag in the
direction of the burst (Schlegel et al. 1998).
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