Effective Area Cross-calibration Results Involving the Chandra X-ray Observatory


Organization of this Web page

This web page is divided into several sections:

  1. Scope

  1. Observations Used

  1. Comparison with XMM-Newton

  1. Reports and Presentations

  1. Updates

  1. Upcoming work

First time visitors might want to examine the most recent report to the Chandra Users’ Committee (CUC) in the Reports section and read this page sequentially afterward.  Experts may want to view the details in the XMM comparison section and check the Updates section occasionally.



This page is offered to Chandra users with the objective of presenting the status of efforts to cross-calibrate Chandra with other X-ray telescopes.   This web page is a direct result of a request from the CUC. We will update this page as needed; older sections will be replaced by fixed documents from time to time.


Observations Used


Due to the different angular resolutions of the telescopes involved, most cross-calibration observations have involved point sources.  In order to avoid pileup and to check more components of the system, the transmission gratings on Chandra are inserted for most observations.  Because there are two grating assemblies that can be used and two detectors, the common combinations are usually implemented sequentially during cross-calibration campaigns.  In other telescopes such as XMM, several instruments are used at the same time.  Due to observing constraints that vary by observatory, cross-calibration campaigns are rare, usually performed once or twice per year.

Internal calibration

There are many internal checks that one can (and should) perform before comparing results between observatories.  These calibration efforts are reported elsewhere and updates are available on the web (see http://asc.harvard.edu/cal/ for links to Chandra calibration web pages).  Early work on cross-calibration was hindered by a time-dependent contaminant build-up , which is now relatively well characterized spatially and spectrally so that it can be effective areas can be corrected .  The calibration pages for other X-ray observatories should be consulted for details concerning their respective internal calibration efforts that may include cross-calibration several instruments.  See, for example, RXTE , ASCA , BeppoSAX , XMM-Newton , and Suzaku web pages.

Early Observations

The first cross-calibration observations were taken in January 2000, using 3C 273.  Several telescopes were involved: RXTE, ASCA, and SAX.   Early work showed that ASCA and the HETGS spectra of 3C 273 agreed to within 10% and that the derived spectral slopes were nearly identical (see Fig. 1 ).  Similarly, SAX and the HETGS agreed to 10% above 0.7 keV.  ASCA and SAX are no longer operating so work on these cross-calibration efforts has been suspended.

heg_asca.png.png Fig. 1   : Early results comparing the Chandra HETGS spectra to fits to the ASCA data taken simultaneously.  The MEG and HEG data are shown separately.  The ASCA fits were performed by T. Yaqoob.  Given the early uncertainties in the MEG and HEG efficiencies and the ACIS contamination, these fits provided a good agreement between these two X-ray observatories.

XMM-Newton Cross-cal Observations

Table 1 ( pdf ) summarizes observations obtained with XMM and Chandra that were intended for use in cross-calibration.  Some observations were not strictly simultaneous, so only specific combinations are useful for comparing XMM and Chandra.  For two observations, the gratings failed to insert due to a failure of the limit switches that was avoided thereafter.  The spectral fit parameters will be explained and compared in the next section.  Exposure times for Chandra observations are approximate.

Comparison with XMM -Newton

XMM fit parameters were taken from the XMM calibration document XMM-SOC-CAL-TN-0052 (Stuhlinger et al. 2006), which is available at the XMM calibration documentation page or from the updated cross calibration analysis .  The XMM cross cal page provides fits to two observations of PKS 2155-304 and one of 1H1426+428.  The parameters derived from the XMM pn and the Chandra grating spectrometers are generally consistent within error bars.  Additional sources have been observed simultaneously and analyzed by both the XMM and Chandra calibration teams.  For 3C 273, the fit is the sum of two power laws with independent normalizations, A, and photon spectral indices, Γ .  For PKS 2155-304 and 1H 1426+428, the fits are to broken power laws, so the normalization of the second component is computed by matching the two power laws at the break energy.  The neutral column densities (N H ) were fixed at 1.7e20, 1.24e20, and 1.36e20 cm -2 for 3C 273, PKS 2155-304, and 1H 1426+428, respectively.  The tb_abs function in xspec was used for the absorption model.  Only fits to EPIC PN data were used for this comparison.

The Chandra spectral fit results were taken from a recent HETGS effective area report , available from the HETGS calibration page and from LETGS fits as described by Marshall et al. (2003) and on the LETG-ACIS web page about measuring the growth of the ACIS contamination but updated to include more recent data.  The HETGS fits assumed a pure power law model.  The N H values were fixed at the same values as those used in the XMM analysis.

The same models were used for the LETGS and XMM fits to the 3C 273 data but A and Γ do not agree within individual parameter uncertainties.  The reason is most likely related to the different energy ranges of the instruments; the PN samples spectra in the 0.1-12 keV range, while the HETGS has good sensitivity for the energy range 0.5-8 keV and the LETGS can be used reliably from 0.25 to 7 keV.  Coupled with the correlation between the two spectral components, comparing fit values directly is problematic.  Because of this problem and that different models were used for the XMM and Chandra fits to the BL Lac data, the model spectra are compared in Fig. 2 , limiting the HETG fits to the 0.5-8 keV range and the LETGS models were limited to 0.25-7 keV range.  These comparisons suffer from several complications, including non-overlapping time intervals (so that variability gives different normalizations), different model assumptions for PKS 2155-304 and 1H 1426+428, and residuals from the fits are not shown.   Variability is particularly noticeable in PKS2155-304 observations.  There was no significant variability noted in the 3C 273 and 1H1426-428 observations.

In Fig. 3 , we account for residuals in the HETGS and LETGS fits.  The XMM fits are compared to the fluxes derived from the HETGS and LETGS data in energy bands that are 0.05E wide.  The agreement with XMM seems much better — particularly the 2001 and 2003 observations of 3C 273, where the XMM and HETGS data agree to within 10% from 0.5 keV to 7 keV.  Some comparisons are not as favorable, such as the 2003 and 2004 XMM-LETGS comparisons, where the LETGS data give a flatter overall slope than the XMM fits.  A detailed analysis of the differences is still needed.

Reports and Presentations

Previous status reports to the Chandra Users’ Committee are given on the CXC web site.  This web page is a direct result of a request from the CUC after a presentation given at the October 2005 CUC meeting .  The latest report to the CUC was on April 5, 2006.  Two other presentations on this topic are available; see the EPIC Calibration Meeting held on May 5-6, 2006, and this talk at the International Calibration Working Group meeting in Iceland held June 14-16, 2006.


This section will include updates and corrections.

Upcoming work

We will continue to compare spectral fits to XMM and Chandra observations and refine the models to obtain better comparisons between instruments.  We plan to compute fluxes for cross-calibration observations in several energy bands that can be compared directly between instruments.

The next calibration observations will involve the Suzaku X-ray telescope as well as XMM-Newton.  The target is PKS 2155-304, observed in early May, 2006.  One Chandra grating-detector combination was used due to observational constraints.  The LETG was used with the HRC-S in order to test models to very low energies.  Other observations are being obtained in July to verify the relative calibration of the Chandra grating-detector combinations.

Herman Marshall (hermanm at space dot mit dot edu)

Last updated: July 17 , 2006