GCN Circular 3532
Subject
GRB 050525a, Mid-Infrared Observations
Event
Date
2005-06-07T22:53:44Z (20 years ago)
From
Peter Garnavich at U of Notre Dame <pgarnavi@nd.edu>
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.