Joint Spitzer/Chandra Accepted Cycle 8 Targets and Abstracts

Title: 70 Micron MIPS and Chandra ACIS-I Imaging of the IRAC Dark Field: Infrared AGN

Using data taken for the calibration the Infrared Array Camera on-board Spitzer, we have been able to discover a population of objects with mid-infrared variability on months-timescales and whose colors are characteristic of active galactic nuclei (AGN) at significantly high redshift. We request 25.5 hours of imaging at 70 microns with MIPS and 100 ksec of x-ray imaging with ACIS of the IRAC Dark Field. The 70 micron data are needed to provide a photometric data point near the peak of the spectral energy distribution, allowing an accurate determination of the spectral shape and bolometric luminosity. These are known to correlate locally with the nature of the energy source and will help provide a discriminant between AGN-like and starburst-like systems. The x-ray imaging provides an additional measure of the strength of AGN activity and the degree of obscuration to the energy source. This is an extragalactic field 15 arcminutes in diameter near the north ecliptic pole, and is the dark current calibration target for IRAC. Because it is observed frequently as part of routine operations, the field is now similar in size and depth to the infrared component of the GOODS program, and is confusion-limited in the mid-infrared. More importantly, due to the periodicity of the observations, the Spitzer data are sensitive to variability on week timescales, ultimately spanning a baseline of five to seven years. This cannot be achieved with an ordinary science GO program, and this is the only mid-infrared dataset that will have this capability at this depth for the foreseeable future. In the past year we have acquired deep multiband optical and near-IR imaging from the ground, and will soon acquire deep high spatial resolution HST imaging. All HST/Spitzer data in the field is publicly available, and the requested MIPS and Chandra data will add to this unique dataset.

While long gamma-ray bursts (GRBs) have been conclusively demonstrated over the past nine years to be distributed at great cosmological distances, the nature of short and hard GRBs has yet to be definitively determined. An understanding of the origin of mysterious short gamma-ray bursts remains an elusive and exciting pursuit. A great leap forward was made this past year with the first rapid localizations and afterglow detection 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 multiwavelenth 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, 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 3. The wavelengths observed by Spitzer, when used in coordination with these other instruments, can make a crucial contribution to understanding the nature of short/hard GRBs, particularly by removing the degeneracies among the models due to dust extinction.

Title: Nature vs. Nurture: The influence of OB star environments on proto-planetary disk evolution.

A natural approach for understanding the origin and diversity of planetary systems is to study the birth sites of planetary systems under varying environmental conditions. Dust grains in protoplanetary disks, the building blocks of planets, are structurally and chemically altered, and grow through coagulation into planetesimals. The disk geometry may change from a flaring to a more flattened structure, gaps may develop under the gravitational influence of protoplanets, and eventually the disk will dissipate, terminating the planet formation process. While the infrared properties of disks in quiet cloud environments have been extensively studied, investigations under the conditions of strong UV radiation and stellar winds in the proximity of OB stars have been limited. We propose a combined IRAC/IRS study of a large, well-defined and unbiased X-ray selected sample of pre-main-sequence stars in three OB associations: Pismis 24 in NGC 6357, NGC 2244 in the Rosette Nebula, and IC 1795 in the W3 complex. The samples are based on recent Chandra X-ray Observatory studies which reliably identify hundreds of cluster members and were carefully chosen to avoid high infrared nebular background. A new Chandra exposure of IC 1795 is requested, and an optical followup to characterise the host stars is planned. Modelling the Spitzer findings will provide the composition and size of dust present as well as the geometry, mass, and gaps in the global structure of the disk. As hundreds of cluster members will be covered with IRAC and dozens with IRS, good statistics on the disk evolution and dispersal as a function of location with respect to OB stars will be obtained. Comparison of disk properties within our sample and with existing Spitzer studies of quiescent star-forming regions should significantly advance the aim of characterising the influence of the environment on the evolution of protoplanetary disks. This effort relies on a powerful synergy between the Chandra and Spitzer Great Observatories.

We propose for coordinated Spitzer and Chandra observations of fourteen gamma-ray blazars. The blazars have two-peaked spectral energy distributions, where the infrared peak is synchtrotron emission produced by relativistic electrons, and the x-ray-gamma-ray peak is inverse-Compton scattered emission. The observations will map out the shape of the infrared synchrotron peak and its changes with two sets of independent observations about six months apart. All three Spitzer instruments will be used in normal, successive instrument campaigns, as we did during our 2005 observations of flaring blazar 3C454.3. The Chandra observations are used to obtain x-ray fluxes and spectral indices which will indicate relative contributions of synchrotron and inverse-Compton emission in the Chandra x-ray band. The Spitzer MIPS and Chandra observations need to be scheduled within a day of each other because the sources are highly variable.

Proposal Number	Subject Category	PI Name	Target Name	Total Chandra Time	Title
08701041	Extragalactic: AGN/quasars/radio galaxies	Jason Surace	IRAC Dark Field	100ks	70 Micron MIPS and Chandra ACIS-I Imaging of the IRAC Dark Field: Infrared AGN
08401042	Extragalactic: GRBs	Kevin Hurley	ToO TBD	30ks	Spitzer-Chandra ToO Observations of a Short/Hard GRB
08201040	Galactic: circumstellar/debris disks	Jeroen Bouwman	IC 1795	50ks	Nature vs. Nurture: The influence of OB star environments on proto-planetary disk evolution
08701043	Extragalactic: AGN/quasars/radio galaxies	Ann Wehrle	TBD	12ks	Coordinated Spitzer/Chandra Observations of Gamma Ray Blazars

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