XMM-Newton-NEWS


XMM-Newton-NEWS #25,    06-Nov-2002

ESA, XMM-Newton Science Operations Centre at
European Space Astronomy Centre (ESAC)
P.O. Box - Apdo. 50727, 28080 Madrid, Spain

SOC Home Page: http://xmm.esac.esa.int/
Helpdesk web interface: https://www.cosmos.esa.int/web/xmm-newton/xmm-newton-helpdesk
Helpdesk email address: xmmhelp@sciops.esa.int
News Mailing List: https://www.cosmos.esa.int/web/xmm-newton/newsletterindex.shtml

Contents:

User group meeting; minutes available online

The XMM-Newton users'group met for the 2nd time on 16/17-Sep-2002 at VILSPA.

The users group made a number of recommendations which will impact future AO review cycles; amongst which are; Please find the minutes of the meeting at;
    http://xmm.esac.esa.int/external/xmm_user_support/usersgroup/index.shtml

SAS web pages content reorganized

Following a recommendation of the XMM-Newton User Group, the content of the SAS web pages has been reorganized, to provide the users with an easier access to all the information necessary to reduce and analyze XMM-Newton data. The SAS entry page is available at the following URL;
    http://xmm.esac.esa.int/sas/

A top level, introductory presentation of SAS and its capabilities is available at the URL;
    http://xmm.esac.esa.int/sas/documentation/

New XMM-Newton Calibration Portal

The new XMM-Newton Calibration Portal is online and available at the following URL;
    http://xmm.esac.esa.int/external/xmm_sw_cal/calib/

The new portal provides a clearer structure of the XMM-Newton calibration situation. The page gives all the necessary calibration information for all XMM-Newton instruments (EPIC, RGS, OM) in order that a proper data reduction may be undertaken.

The XMM-Newton calibration scientists and instrument teams have also prepared documents describing the current status of the EPIC, RGS and OM calibration as implemented in SAS 5.3.3. These are linked on the new XMM-Newton Calibration Portal under the heading 'What is the current status of the calibration?'.

RGS/MOS cooling

Cooling of the RGS and EPIC MOS instruments

During November and December 2002 a series of operations will be performed on the RGS and EPIC MOS instruments in order to lower the nominal operating temperatures of their detectors, and subsequently re-establish the calibration of the instruments. The purpose of the cooling is to move important detector characteristics back towards the values which they had earlier in the mission.

Radiation effects on detector performance

Since the start of the XMM-Newton mission the RGS CCDs have been operated at a temperature of -80 degrees C, and the MOS detectors at a temperature of -100 degrees C. Over these first three years of the orbital mission the daily passage of the spacecraft through the Earth's radiation belts, combined with irregularly occurring Solar Flares, have subjected the CCDs to high (but predicted and expected) levels of ionizing radiation. Resulting changes in the operating characteristics of the detectors were anticipated before launch, and instrument design and ground testing provided for the eventual implementation of corrective measures in-flight.

At this point in the mission, ongoing calibration monitoring has demonstrated that the Charge Transfer Efficiency (CTE) of the EPIC MOS CCDs has degraded somewhat relative to its value immediately after launch. As a result the energy resolution achievable has fallen and, for example, the measured energy width of the Mn and Al lines has increased 15 to 20% above its original value. For RGS, the main effect of the ionizing radiation dose has been a significant increase in the number of "hot pixels" and of resulting bad columns affecting RGS spectra.

Restoration of performance

At the second XMM-Newton Users Group meeting, held in September of this year, it was agreed that the time had come to implement the long-envisaged mitigating strategy. That is to reduce the operating temperature of the detectors. Such a reduction in operating temperature is expected to lead to the recovery of most of the original performance levels of the RGS and MOS detectors. (A test, made in June of this year demonstrated that cooling the MOS2 instrument to an operating temperature of -120 degrees C led to recovery of most of its original CTE.)

The Users Group therefore urged the XMM-Newton SOC team to explore approaches to cooling the RGS and EPIC MOS instruments and re-establishing their calibration while causing minimum delays in distribution of reduced data products to observers.

Instrument cooling and re-calibration

As a result, in a campaign starting on November 4 just after midnight at ESAC, and extending through November and into early December, the instruments will be cooled and recalibrated according to the schedule below;

Dates Revolution Comments
Nov 4 - 5 532 Cool RGS2 from -80 to -115 degrees and recalibrate
Nov 6 - 9 533/534 Cool MOS1 and 2 from -100 to -120 degrees and recalibrate
Nov 14 - 15 537 Cool RGS1 from -80 to -115 degrees and recalibrate
Dec 2 - 3 546 Check RGS 1&2 calibration after some weeks cold.

For most of the intervening revolutions normal scientific observations will continue, although as time progresses these will increasingly relate to the new, reduced, operating temperatures and new values of certain calibration parameters.

Throughout the above period, interleaved (re-)calibration and contamination monitoring observations will be fed to immediate interactive data analysis, and updated calibration files (CCFs) will be derived for the Science Analysis System (SAS). In this way the overall interruption in pipeline data product distribution is expected to be confined to a very limited period from mid-November to mid-December, 2002. Updated CCFs will be made available to SAS users in the December/January time frame.

Monitoring for contamination

Specific contamination monitoring observations will address the remote possibility that, upon cooling the detectors, any low level ambient contaminants within the telescope/focal-plane subsystem might start to freeze out onto the cold detector surfaces, although these will still NOT be the coldest points in the system by a significant margin. In the event of any contamination appearing counter-measures will be taken immediately.

The end result

The full series of operations, calibrations and calibration file derivations, along with a comprehensive schedule of tracking meetings, will give the months of November and December at VilSpa something with the complex character of operations like an early orbit phase. At the end of these activities it is hoped that the RGS and EPIC MOS instruments will see key performance characteristics restored to values which they had early in the mission, with immediate benefit for science data quality and minimal ongoing impact on the service provided to the observer community.

XMM-Newton Source Designations

As reported in XMM-Newton News #6, a naming convention for the XMM-Newton sources detected by individual researchers was approved by the IAU "Clearing House" of Commission 5 Working Group on Designations. Details on this naming convention have been summarized now at;
    http://xmm.esac.esa.int/external/xmm_science/src_naming.shtml

XMM-Newton On Board Time Wrap-Around Events

Maintaining accurate time stamps for all events on-board XMM-Newton is a very complex procedure. The XMM-Newton SOC is in the process of generating an accurate, full, end-to-end description of all elements relevant to deriving accurate timing. One of the elements crucial in the entire chain is the so-called "Central Data Management Unit" (CDMU) which distributes and generates the on-board master clock. The CDMU master clock has a finite number of bits to represent time, and as such every so often this clock wraps around. Although these wraparound (and other) events should be transparent to the end-user, when using the SAS or the PPS products to look at your data, we will make all information available on the WWW, if only as a reference. The first in this series is the CDMU.

After a CDMU reset or a wrap-around, the XMM-Newton instruments' clocks and the CDMU have to re-synchronized. This is either a manual procedure or it automatically occurs at instrument switch-on.

SOC has published now a table indicating when the clocks re-synchronization took place for each event (and the potential impact associated with the delay between on-board clock re-start and instruments' clocks re-synchronization).

Information on all past CDMU Clock Events as well as the next predicted wrap-around have been made available at the following URL;
    http://xmm.esac.esa.int/external/xmm_user_support/documentation/clock_events.shtml


Yours sincerely,
XMM-Newton SOC