GRB 090423, June 5-10 2009, Spitzer 3.6 microns
R. Chary, J. Surace, S. Carey (SSC/Caltech), E. Berger (Harvard), and G. Fazio (SAO/Harvard) report on behalf of a larger collaboration:
"As part of the IRAC Warm Instrument Characterization Campaign (IWIC), we observed the field of the z=8.3 GRB090423 (GCN 9198) for 72 hours using Spitzer/IRAC at 3.6 microns. The observations took place between 2009 June 5.01 and 10.81 UT, corresponding to about 46 days after the burst in the observer frame, or about 5 days in the rest-frame. We detect a weak source at the location of the near-IR afterglow (Tanvir et al. 2009 arXiv:0906.1577), confirmed through an astrometric tie to the Gemini-North near-IR images. Aperture-corrected photometry of the source results in a 3.6 micron flux density of 46+/-17 nJy (27.2+/-0.3 AB mag). The spectral energy distribution and power-law decay of the afterglow presented in Tanvir et al. predicts a 3.6 micron flux density of ~27.3 AB mag at the time of our observations. The detected source is thus consistent with being the afterglow.
An image of the region can be found at: http://www.its.caltech.edu/~rchary/grb090423/
Light colors are bright flux, dark colors are faint. The pixel scale in the IRAC image (left) is 0.4"/pixel. The right-hand image is the Gemini J-band image from Cucchiara et al. (GCN 9209). The red circles are 1" radius and show the position of the afterglow.
Further analysis is ongoing, and a second epoch of observations is planned for February 2010 to assess the contribution of an underlying host galaxy to the measured flux."