TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3532 SUBJECT: GRB 050525a, Mid-Infrared Observations DATE: 05/06/07 22:53:44 GMT FROM: Peter Garnavich at U of Notre Dame P. Garnavich (Notre Dame), M. Pahre (CfA), A. Noriega-Crespo (CalTech), K.Z. Stanek (CfA, OSU), S.T. Holland (GSFC), D. Bersier (STScI), T. Matheson (NOAO), R. Perna (U. Colorado), K. Krisciunas (Notre Dame) After confirmation of a bright optical afterglow associated with GRB 050525 (Band et al. GCN 3466; Rykoff et al. GCN 3465;3468) we triggered a "target-of-opportunity" (ToO) program on the Spitzer Space Telescope. Observations began on May 27.26 (UT), or 2.26 days after the burst, with IRAC imaging followed by IRS spectroscopy, MIPS mapping and finally another epoch of IRAC images ending on May 27.56 (UT). Soon after triggering the ToO, the brightness of the afterglow switched to a steeper decline rate (Mirabal et al. GCN 3488; Kaplan et al. GCN 3507) making a rapid response essential. The first epoch of imaging shows point source at the position of the afterglow (Rykoff et al. GCN 3468; Yanagisawa et al. GCN 3489) in the 3.6,4.5 and 8.0 micron IRAC channels and in the 24 micron MIPS band. A second epoch of IRAC imaging and MIPS mapping was performed on May 29. The source is seen to fade in IRAC between the two epochs of the first visit and is beyond the detection limit during the second visit. The MIPS 24 micron source is also seen to fade between visits. This is the first confirmed detection of a GRB afterglow at mid-infrared wavelengths. Preliminary estimate of the flux in the IRAC channels combined with K-band observations of Kaplan et al. (GCN 3507) suggests a spectral energy distribution consistent with a power-law. The index of the power-law between 2 and 8 microns is 1.3+/-0.2. This is surprisingly steep and further analysis is continuing. The IRAC 3.6 micron images taken at two epochs can be viewed at: http://www.nd.edu/~pgarnavi/grb050525/irac.jpg We thank Nancy Silbermann and the Spitzer Science Center for rapidly responding to a complex target-of-opportunity observation. This message may be cited.