GCN Circular 9261
Subject
GRB 090423: GROND second epoch imaging and refined SED
Event
Date
2009-04-26T18:38:40Z (17 years ago)
From
Thomas Kruehler at MPE/MPI <kruehler@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.