Chandra X-Ray Observatory

Chandra Users' Committee Meeting (27-28 April 2010)

Report issued on 03 June 2010

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Luisa Rebull (by phone)
Matthias Ehle
Elizabeth Blanton
Joel Kastner
Martin Laming
Jon Miller (by phone)
Steve Allen (by phone)
Massimo Stiavelli
Joan Wrobel (byphone)

Summary of Meeting


  1. RPS

    A number of CUC members and their colleagues have expressed some concern about the RPS system, specifically the e-mail interface for handling proposals with more than five targets. It appears that the RPS system, and this aspect of it in particular, has not been updated since Chandra's launch. The e-mail interface seems to be vulnerable to hidden special characters that are inserted by modern e-mail handling applications, but that RPS cannot strip out. Given the fact that the vast majority of Chandra proposers appear to be happy and familiar with RPS (including some users of the e-mail interface) we are reluctant to recommend any big changes.


    Perhaps the best compromise would be to offer users a simplified front end to RPS that can fill in all fields required in the e-mail version. We are thinking of proposals with target lists where the only change from one target to the next will be in the coordinates and the count rate, with all other instrument parameters the same, where the user would be asked to upload a target list to clone RPS forms. It might also be worth looking at whether it is reasonable to reduce the number of supported ACIS modes in such circumstances to ease implementation.

  2. Calibration

    The CUC remains a little confused about the precise status of the Chandra calibration, especially in comparison to other missions like XMM. We attempt to summarize as follows:

    The comparison between ACIS, PN and MOS gives ACIS fluxes higher than those for PN by ~10%, and MOS fluxes higher than PN by ~4-7%. Above 2 keV, these three instruments give the same plasma temperatures. Below 2 keV, ACIS can still be fit with a single temperature, although the 0.5-2 and 2-7 keV temperatures are inconsistent by about 10%. PN and MOS however both need 2 temperature component to give acceptable fits to spectra from what are presumed to be isothermal sources (outer regions of galaxy clusters). The soft band in XMM generally gives a lower temperature than Chandra, indicating a soft excess, or a hard deficit, in XMM relative to Chandra.

    The comparison of spectral line fluxes (O VII, O VIII, Ne IX, Ne X) indicates higher fluxes at 1 keV and below with Chandra. A series of plots of blazar grating observations during the XMM and Chandra missions indicated the improved calibration in a series of energy bands, with the Chandra flux relative to XMM increasing slightly with energy.

    Is this accurate? Ideally we would have liked to see the temperature and flux for both the ACIS/pn hard band and the ACIS/pn soft band.

    The CXC's ongoing efforts to 'scrub' its web pages to remove obsolete and/or confusing documentation (which the CUC applauds!) should focus on the Cal group's areas. These are in particular need of updating and reorganization. Such update/reorganization should be aimed at clearly identifying areas that are well-understood and stable (e.g., CTI correction for all modes except CC graded) vs. areas that are 'under study' and in flux (mission cross-calibration work; ACIS filter contamination; CC graded mode), and should clarify the status of the latter. For example, the various memos concerning ACIS contamination could be 'bundled together,' and the obsolete documents among them should be clearly labeled as such.

    We are pleased to hear of cross calibration papers nearing readiness for submission. We also support the ongoing efforts in calibration of the ACIS CC mode.


    Calibration of ACIS-HETG (including CC mode) should aim for a goal of 5% between the various orders in MEG and HEG. This is driven by the need to accurately measure continua in a variety of applications from spectra of accreting binaries (black holes, neutron stars, white dwarfs) and AGN. In particular the measurement of absorption edges, existence/contribution of thermal bremsstrahlung components, and fitting the red wing of the Fe K alpha line from accreting black holes all depend critically on fixing the continuum level. To a lesser extent, the interpretation of emission line spectra from neutron star and black hole binaries also place demands on the calibration, since the dominant lines come from H-like ions for which the atomic data are accurate at this level. Other coronal sources which depend more on a wider selection of spectral line and less on the continuum do not stress the calibration so much, because atomic data used in interpretation is less accurate.

  3. CDFS Archive

    The CUC are mildly concerned about the reduced oversubscription in archive proposals to study the CDFS, compared with other archive and theory proposals. Of course there is no way to know a priori what will happen in response to the AO.


    For the time being we just comment that in the peer review, we hope that CDFS archive proposals will be held to the same standard for funding as other archive proposals.

  4. One/Two CUC meetings/year?

    As the Chandra mission matures, the necessity to have CUC meetings twice a year has been questioned.


    The CUC agrees that the meeting schedule could reasonably go to once a year, preferably in October after the peer review. We also agree that at each meeting, a time in April for a telecon should be set aside in case any issues arise that need discussion. Note that the telecon need not happen, but the CUC should be prepared in case it does become desirable.

  5. Catalog

    We are impressed with the ongoing progress. Our discussions prompted a few further questions: What is likely to happen to the source catalog when the Chandra mission ends? Is CIAO envisaged to be an interface to the catalog?


    Although solid progress is being made on the CSC, we remain concerned about the CSC's visibility in the wider astronomical community. We recommend that (1) CSC releases be announced via the AAS; and (2) when the manuscript defining the CSC is accepted, it be posted to astro-ph (we note with satisfaction that the accepted version of the Evans et al paper on the Chandra Source Catalog has been posted at We would also like to see a batch search engine.

  6. Low Energy Calibration

    The CUC is pleased with the progress that has been made.


    It would be desirable to have a physical model for the "gaussium" component, not just for the intellectual satisfaction, but to be able to predict how the contamination might behave in the future. We recognize that this is asking a lot of the resources of the CXC, and that the main aim of providing a calibration that observers can use has basically been met.

  7. Senior Review

    Second place to Planck in a review with the metric science/NASA$ is a very satisfactory outcome, and we commend Harvey and the team on a job well done.

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