ACIS Extract

Introduction

• The source extraction region are contours of the local PSF enclosing whatever PSF fraction is desired.  Crowded fields are handled by shrinking the extractions regions so they do not overlap.

• The "mask regions", which completely cover the sources, are used to construct a background data set.

• A "grouped light curve", i.e. source flux measured in a set of independent, unequally sized time bins with constant significance.

• The median energy of events in each time bin.

• An "adaptively smoothed light curve", i.e. source flux measured in a set of overlapping, unequally sized time intervals with constant significance.

• Source extraction region files help the observer review the observation for spurious and missed sources.

• A variety of interactive plots are produced showing various source properties across the catalog.

The authors happily acknowledge that AE is derived from the ideas, software, and testing efforts of David Alexander, George Chartas, Eric Feigelson,  Nicolas Grosso, Scott Koch, Bertil Olsson, Yohko Tsuboi, and the authors of various CIAO threads and manuals.   Support for this effort was provided by NASA contract NAS8-38252 to Gordon Garmire, the ACIS Principal Investigator

Other Notable Features

• Since early 2002 AE has been used successfully on many large ACIS projects, including observations of the Chandra Deep Field North, Orion, and the Galactic Center. 

• Hooks are provided for observer-supplied extraction and background regions to support special handling of contaminated sources.

• Maximum Likelihood reconstructions can be performed on source neighborhoods.

• Source positions are estimated by two methods: the mean of the event data, and the peak of the correlation between the event data and the PSF.

• Sources spanning multiple CCDs are correctly calibrated.

• Local background regions are individually sized to enclose a specified number of counts.

• Scaling of background spectra takes into account exposure differences (not simply geometric areas) in source and background regions, correctly accounting for masking applied to the data.

• RMFs can be computed either with mkacisrmf/mkrmf or by choosing among the library RMFs associated with the Townsley et al. CTI corrector .

• Grouping of spectra is based on a background-subtracted S/N threshold rather than on a fixed number of counts.  The observer has direct control over the spectral range modelled (i.e. the boundaries of the first and last spectral groups).

• XSPEC scripts are provided for three spectral models (1-temperature and 2-temperature thermal, powerlaw).   Both chi^2 and C-stat fitting statistics are supported.  The scripts can be customized both globally and for individual sources (e.g. to freeze a parameter).

• Source spectra with models are plotted in both the common grouped and less common cumulative forms.

• Multiple spectral models can be fit, and a tool for visually reviewing the fits is provided.

• We provide both "Getting Started" examples of basic AE usage, and the complex AE recipe that we use in our own multi-obsid extractions of fields with ~1000 sources.

• We provide the (undocumented) code we use to generate publishable LaTeX tables.

• We provide an example recipe which uses AE to analyze piled-up sources.