[vsnet-grb-info 7655] GRB 090423: NIR photometry and evidence for spectral break

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9209
SUBJECT: GRB 090423:  NIR photometry and evidence for spectral break
DATE:    09/04/23 15:02:30 GMT
FROM:    Derek Fox at PSU  <dfox at astro.psu.edu>

A. Cucchiara (PSU), D. B. Fox (PSU), and E. Berger (Harvard) report:

"On April 23.33 UT we imaged the field of Swift GRB 090423 (Krimm et
al., GCN 9198) with NIRI on the Gemini-North telescope.  We obtained
9x60 sec exposures in Y,J,H.  In the coadded J and H images we clearly
identify the object noted by Tanvir et al. (GCN 9202) and it appears
point-like (see also Levan et al., GCN 9206). The object is not
detected in Y-band.

We derive the following magnitudes, calibrated using three 2MASS stars
in the field:

J mag: 19.29 +- 0.07
H mag: 17.71 +- 0.14

The resulting J-H color of 1.1 AB mag is very red, corresponding to a
steep spectral slope of beta=3.8.  Taken in conjunction with a flat
H-K color (see GCN 9202), our non-detection in Y-band, and a red I-J
color of 1.7 AB mag in comparison to P60 limits (GCN 9201), we suggest
that there is a likely break in the spectrum at about 1.2 microns,
with extinction an unlikely explanation due to the sharpness of the
feature and the shallow slope to redder wavelengths.  If this is due
to Ly-alpha absorption, then the resulting redshift for GRB 090423 is
z~9.

Further analysis is in progress and additional observations are
planned."