Spitzer ToO observations of a short gamma-ray burst

An understanding of the origin of the short gamma-ray bursts remains an elusive and exciting pursuit. A great leap forward has been made over the past three years with the first rapid localizations and afterglow detections of such events, but follow-up has yet to reveal a detailed understanding of the progenitors and the nature of the afterglow light. We propose an ambitious multiwavelength approach to the problem, leveraging Spitzer with Chandra as well as numerous ground-based telescopes. By measuring the broad-band spectrum of the afterglow and any concurrent 'mini-supernova ' over a wide range of wavelengths at several epochs, we can distinguish between models proposed to explain this type of burst. We will constrain the energetics of the explosion and the short GRB bursting rate (an important number for gravitational wave observatories), and measure with unprecedented detail the stellar content of a short burst host galaxy. Given the high impact nature of these observations and the rarity of short bursts, we are requesting multiepoch Target of Opportunity observations on a single event in Cycle 5. The wavelengths observed by Spitzer, when used in coordination with these other instruments, can make a crucial contribution to understanding the nature of short duration GRBs, particularly by removing the degeneracies among the models due to dust extinction. This is a resubmission of our AO-4 ToO proposal, which has not been called yet. However, even if that observation is carried out, we are requesting an AO-5 observation, because so little is known about the short bursts that each new detection adds a very significant amount of information. Harvey Tananbaum has agreed to grant us Chandra ToO time through November 2008 (the end of Chandra AO-9) if Spitzer observations are carried out. Following that, we will submit a Chandra AO-10 proposal for ToO time; if warranted, we will request Chandra Director's Discretionary Time to support our Spitzer observations.