GCN Circular 17732

K. Varela, F. Knust, and J. Greiner (all MPE Garching)
report on behalf of the GROND team:

Further to Varela et al. (GCN #17729
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 ), we report continued observations 
of the field of GRB 150423A (Swift trigger 638808; Pagani et al., GCN #17728
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 ) 
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). The 
observations were performed at an average seeing of 1.0 and at an average 
airmass of 1.5.

We detect 2 faint sources within or close to the 2.2" XRT error circle:

Source #1, within the XRT error circle:
 RA (2000.0)   =  14h 46m 18.86s
 Decl (2000.0) = +12d 17' 00.7"
with an error of 0.3".

Source #2, just outside the XRT error circle:
 RA (2000.0)   =  14h 46m 19.04s
 Decl (2000.0) = +12d 17' 03.4
with an error of 0.3". This source matches an object visible in the SDSS.

Based on 25.1 min of total exposure time in g'r'i'z' and 20.0 min in JHK, 
centered 58 min after the GRB, we estimate the following preliminary magnitudes 
(all in AB system).

Source #1:
g' = 23.2 +/- 0.2 mag,
r' = 23.1 +/- 0.2 mag,
i' = 22.8 +/- 0.2 mag,
z' = 22.6 +/- 0.2 mag,
J  > 21.6 mag,
H  > 21.0 mag, and
Ks > 18.5 mag,

Source #2:
g' = 24.2 +/- 0.2 mag,
r' = 23.3 +/- 0.2 mag,
i' = 23.4 +/- 0.3 mag,
z' = 22.9 +/- 0.2 mag,
J  > 21.6 mag,
H  > 21.0 mag, and
Ks > 18.5 mag.

The spectral energy distribution of source #1 is best fit with a 
power-law of slope beta = 0.8+-0.3 (without significant dust extinction), 
typical for GRB afterglows. Also, source #1 is brighter than source #2,
but not visible in the SDSS. While we cannot establish clear fading, we 
suggest this object as the afterglow of GRB 150423A.

Given magnitudes are calibrated against SDSS (g'r'i'z') as well as 2MASS
field stars and are not corrected for the expected Galactic foreground
extinction corresponding to a reddening of E_(B-V)=0.03 mag in the
direction of the burst (Schlegel et al. 1998).