[vsnet-grb-info 7707] GRB 090423: GROND second epoch imaging and refined SED

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9261
SUBJECT: GRB 090423: GROND second epoch imaging and refined SED
DATE:    09/04/26 18:38:40 GMT
FROM:    Thomas Kruehler at MPE/MPI  <kruehler at mpe.mpg.de>

T. Kruehler, J. Greiner and F. Olivares (all MPE Garching) report on behalf
of the GROND team:

We obtained a second epoch of imaging of the field of GRB 090423 (Swift 
trigger 350184, H. Krimm et al., GCN #9198), which started on 25 Apr. 2009 
at 00:44 UT, 40.8 h after the trigger.

In the second epoch, the NIR afterglow (Tanvir et al. GCN #9202) is 
undetected in stacked images with a total integration time of 4800~s in 
each of J, H and K down to limiting AB magnitudes of J_AB > 22.7, H_AB > 
22.3 and K_AB > 21.6.

Together with the magnitudes reported in Olivares et al. (GCN #9215) this 
implies a fading with a power law index faster than 1.2. This lower limit 
on the temporal decay index is consistent with that found for the X-ray 
afterglow at a similar time interval (G. Stratta & M. Perri, GCN #9212).

In addition we report on a refined analysis of the GROND photo-z from 
Olivares et al. (GCN #9215). Using an improved photometric calibration and 
including the deep z' band upper limit from Perley et al. (GCN #9217) which 
is contemporaneous to our GROND imaging, we find a refined photometric 
redshift of 8.0+0.4-0.8 (90% confidence level). The error range is unlikely 
to improve due to the gap between the z' and J bands (for the negative 
error), and the width of the J filter band (for the positive error).

This range covers the previous claims from photometric (GCNs #9213, #9215, 
#9217) and spectroscopic observations (GCNs #9219, #9222). The SED is well 
described with a power law of index ~1. No signatures of intrinsic dust are 
evident with an upper limit of A_V < 0.5, assuming a SMC dust reddening 
template.