[vsnet-grb-info 2075] GRB 050904: Possible High-Redshift GRB

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3914
SUBJECT: GRB 050904: Possible High-Redshift GRB
DATE:    05/09/04 12:04:56 GMT
FROM:    Daniel E. Reichart at U.North Carolina  <reichart at physics.unc.edu>

J. Haislip, D. Reichart, E. Cypriano, S. Pizzaro, A. LaCluyze, J. Rhoads,
E. Figueredo report on behalf of the UNC team of the FUN GRB Collaboration.

With SOAR, we have continued to image the NIR afterglow (Haislip et al.,
GCN 3913) of GRB 050904 (Cummings et al., GCN 3910) in JHK.  Between 3.0
and 7.4 hours after the burst, we measure the temporal index to be -1.20.
At 7.4 hours after the burst, we measure J - K = 1.2 mag, corresponding to
a spectral index of -0.35.

Scaling the i and R limits of Fox et al. (GCN 3912) to 3.0 hours after the
burst, we measure i - J > 1.85 mag and R - J > 3.05 mag, corresponding to
spectral indexes of <-1.90 and <-3.35, respectively.

This is too steep to be explained by even heavy extinction.  If
interpretted as dropout, this corresponds to 5.3 < z < 9.0 (calculated
using filter centers).

Further NIR and especially z observations are strongly encouraged.