STACKFAST is a software package that enables fast and efficient X-ray stacking analyses for X-ray surveys, optimized for use with Chandra observations.
STACKFAST works in two steps:
1. Extracting photons and exposure information from the Chandra data set for every object in a master source catalog, and
2. Allowing the user to coadd the emission from any subset within the master catalog.
The advantages of this method relative to other stacking codes is that
the computationally-intensive photon extraction and exposure calculations are
performed once at the start of the analysis. The subsequent
stacking in bins of source properties is extremely fast,
allowing the user maximum flexibility in performing X-ray stacking of
AGN, galaxies, or stars in bins of interesting properties.
Here we provide an example of STACKFAST works.
Here we will stack the X-ray photons from the positions of mid-infrared sources identified by Spitzer IRAC. Previous work has shown that active galactic nuclei (AGN) are preferentially found in specific regions of IRAC color-color space, so sources in this region will be expected to have higher average X-ray flux. This was confirmed by Hickox et al. (2006) using an earlier version of STACKFAST; as an example of how the latest version of the code works, we perform a similar analysis using deeper Chandra and IRAC data.
X-ray data set:
We use Chandra observations of the 9 deg Bootes field. These include the XBootes 5 ks mosaic over the full field (Murray et al. 2005; Kenter et al. 2005), the 100 ks LALA field (Wang et al. 2004), plus further pointed exposures from 10 to 40 ks (see full exposure map at right).
Input multiwavelength catalog:
We use the IRAC source catalog of the Spitzer Deep Wide-Field Survey (SDWFS; Ashby et al. 2008). This contains approximately 500,000 sources, of which 100,000 are detected at > 3 sigma significance in all four IRAC bands.
We extract photons and exposure times for the full 500,000 sources in the SDWFS catalog, to produced stacked database file.
Run time: 40 minutes on 8-core Mac Pro.
We bin the sources in IRAC color-color space and calculate the average X-ray flux for objects in each bin, performing various quality cuts and accounting for the varying exposure time and off-axis angle for each combination of source and X-ray observation.
Run time: approximately ~1 min initialization and ~0.25 seconds for each of 120 bins, or ~1.5 minutes total.
The results of the stacking analysis (average X-ray fluxes in many color-color bins) are shown below. This analysis shows that IR-selected AGN are significantly brighter in X-rays than other IR sources, providing a simple but powerful independent verification of AGN mid-IR color selection. This analysis also demonstrates the flexibility and speed for which STACKFAST is designed.
Average X-ray flux (in counts/sec/source) in bins of
IRAC color for ~100,000 four-band detected sources in the SDWFS IRAC
catalog. The left and right panels show results for the soft (0.5-2 keV) and hard (2-7 keV) bands, respectively. The red dashed line shows the IRAC AGN selection region
defined by Stern et al. (2005). The STACKFAST input parameters are set to only include X-ray data for which the source
is within 7 arcmin of the ACIS aimpoint (to avoid effects of the broad
PSF at large off-axis angles), and to exclude sources near bright detected X-ray sources (Kenter et al. 2005).
The code developed for STACKFAST has been used in several previous studies:
- Hickox et al. 2006: "
X-ray and infrared properties of galaxies and AGNs in the 9 square degree Bootes field"
- Hickox et al. 2007a: "Can Chandra resolve the remaining cosmic X-ray background?"
- Hickox et al. 2007b: "A large population of mid-infrared selected, obscured active galaxies in the Bootes field"
- Hickox et al. 2009: "Host galaxies, clustering, Eddington ratios, and evolution of radio, X-ray, and infrared-selected AGNs"
- Chen et al. 2013: "A correlation between star formation rate and average black hole accretion in starforming galaxies"
The STACKFAST team includes R.C. Hickox (Dartmouth), A.D. Goulding (Harvard-Smithsonian CfA), and S.S. Murray (Johns Hopkins). Past student developers include S. Marchesi, S. Griffis, M. Oulmakki (Dartmouth), S. Kanev (Harvard).
Please check this page for further updates and more information on STACKFAST.