[vsnet-grb-info 7663] GRB 090423: Deep z-band upper limit and confirmation of photo-z

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9217
SUBJECT: GRB 090423: Deep z-band upper limit and confirmation of photo-z
DATE:    09/04/24 03:51:52 GMT
FROM:    Daniel Perley at U.C. Berkeley  <dperley at astro.berkeley.edu>

D. A. Perley, S. B. Cenko, and J. S. Bloom (UC Berkeley) report on
behalf of GRAASP:

We initiated a series of 15 x 200 second exposures of the field of GRB
090423 (Krimm et al, GCN 9198) in z-band using GMOS on Gemini-South
starting at 23:38 UT on 2009-04-23, 15.71 hours after the trigger.  No
source is visible in the XRT error circle (Evans et al., GCN 9205) or
the NIR position (Tanvir et al., GCN 9202) in the co-added image.  We
estimate a limiting magnitude (3-sigma) of:

z > 26.0 mag (t_mid = 16.25 hr)

This observation is nearly co-eval with the GROND photometry (Olivares
et al., GCN 9215) which detects the counterpart with a magnitude of J_AB
= 21.6, implying a color of J-z > 4.4 (AB).  This, along with the
non-detection of a lower-redshift host galaxy candidate, the TNG
spectrum (Thoene at al., GCN 9216), and the nearly flat JHK SED,
confirms the very high redshift (z > 7) of this event.

We thank Eleazar Carrasco and the Gemini observing staff for acquiring
these observations.