Tools for ACIS Data Analysis

Development of publicly-released tools and procedures to support analysis of X-ray data from the ACIS instrument aboard NASA's Chandra Observatory has been a continuous activity within the ACIS instrument team since well before the launch of Chandra in 1999.





Preparation of ACIS Data; Source Detection

Leisa Townsley, Patrick Broos

Our recipes and scripts for preparing archive ACIS data for analysis and for source detection are described by Broos et al. (2010); please contact Leisa Townsley if you would like the latest version.

Extracting and Analyzing Sources from Multiple ACIS Observations

Patrick Broos, Leisa Townsley, Konstantin Getman, Franz Bauer

ACIS Extract (AE) is a package of tools that can assist the observer in performing the many tasks involved in analyzing the spectra of large numbers of point and diffuse sources observed with the ACIS instrument on Chandra.   AE makes extensive use of standard CIAO tools, but also implements several innovative data analysis algorithms that are described in the extensive AE manual and by Broos et al. (2010).

 Please join the ACIS Extract email list to receive announcements of AE releases.

Simulation of ACIS CCD Detectors

Leisa Townsley, Patrick Broos

Near the time of the Chandra launch, we developed a Monte Carlo CCD simulator for use in characterizing and calibrating the X-ray CCD detectors in the ACIS instrument.  This simulation models the physics of X-ray photon interaction with the detector and the CCD readout process.

Soon after the launch of Chandra, the ACIS instrument suffered radiation damage that introduced significant charge transfer inefficiency (CTI).   We developed and released to the community (in Y2000) a tool to mitigate the effects of CTI in data from both front- and back-illuminated ACIS CCDs. 

Three papers have been published describing the CCD simulation and CTI correction work at Penn State University. 

Mitigating Charge Transfer Inefficiency in the Chandra X-Ray Observatory Advanced CCD Imaging Spectrometer

Simulating CCDs for the Chandra Advanced CCD Imaging Spectrometer

Modeling charge transfer inefficiency in the Chandra Advanced CCD Imaging Spectrometer



MIT members of the ACIS team also developed CCD simulation and CTI correction capabilities.  Both Penn State and MIT  approaches to CTI correction  informed the design of the CTI correction that was eventually adopted by the CXC for standard ACIS data processing.  Our CTI correction tool was retired after standard processing provided a correction.


Correction of Photon Pileup in ACIS Spectra

Patrick Broos, Leisa Townsley

We are currently developing a method and IDL tool for correcting the effects of photon pileup in ACIS observations of point sources.  A fitting procedure derives a non-physical "nuisance" model for the photon spectrum indicent on ACIS that will reproduce the (piled) observed spectrum of the source.  Detection of incident photons (with random arrival times) by ACIS is simulated with our CCD simulator, and the CTI correction applied by CIAO is simulated by our CTI corrector.  Once an adequte photon model is derived, a simulation of the pileup-free observation is performed by forcing photons to arrive

Methods for Estimating Fluxes and Absorptions of Faint X-ray Sources

Konstantin Getman

Getman et al. (2010) discuss a technique for estimating apparent and intrinsic broadband X-ray fluxes and soft X-ray absorption from gas along the line of sight to X-ray sources, using nonparametric methods.  This technique is well-suited for analysis of stars in young stellar clusters, which can often be detected in ACIS observations with too few X-ray events to support traditional spectral fitting.  An IDL implementation of this work, XPHOT.pro, has been publicly released.

Vizualization Tools

Patrick Broos, Scott Koch

Many of the tools and recipes above make use of some general-purpose interactive plotting tools in an IDL package called TARA that we developed prior to the launch of Chandra.  TARA also contains an application, Event Browser, that allows an observer to "browse" X-ray event data, interactively defining data filters and visualizing the data.  Event Browser is perhaps similar to the prism tool in CIAO.  Although Event Browser was written prior to the launch of Chandra and has no ability to apply calibration information (exposure maps, ARFs, etc.), we find it useful to this day to efficiently explore ACIS event data.