ROSAT Status Report #23
April 8th 1992
Satellite and Instrument Status:
Spacecraft and instruments are currently working well.
Processing Problems:
A bug has been discovered in the timing of individual PSPC events and is due to a time synchronization problem in the preprocessing software. The origin of the problem is the spacecraft clock reset that followed the disastrous spacecraft tumbling incident on January 25th 1991. ALL PSPC data after that time will be affected (in fact, this problem recurs after each clock reset, something that must be done annually to accommodate "roll-over" of the counter).
This problem leads in many cases to relative shifts of 1 second between adjacent PSPC events. Although the problem is not cumulative, throughout the data there may be relative time errors of the order of a second. Thus, the effect of the problem is to invalidate some timing analyses on seconds or subseconds time scales. Although it is not possible to derive a quantitative measure of how severely a given data set is effected, it is possible to make the determination of whether or not at least some events have had their times perturbed to the extent that they appear out of time order. (The EXSAS command "check/flips" serves this purpose.)
The bug will have negligible impact on spectral and spatial analyses, and since it is is now understood, event times can be allocated correctly in a forthcoming version of SASS. Unfortunately, there is no algorithm for correcting bad data: SASS will have to be rerun to be certain that all PSPC event times are correct. The policy on reprocessing is not yet established but will be an agenda item for the forthcoming IUC meeting (April 27-28th).
HRI times can also be affected, but the effect is less severe and is limited to those analyses which must combine HRI data across spacecraft-clock reset events. The clock reset introduces a constant, fractional-second offset into the time base at the reset event. Hence the relative timing of all photons arriving between reset events will be correct, but comparisons of photon arrivals on opposite sides of reset events will be subject to the offsets. Absolute times will also be in error by the amount of the offset(s), but this is not expected to have any practical consequence for observers.
Now Available On-line: the ROSAT WFC Bright Source Catalogue
The ROSAT Wide Field Camera Bright Source Catalogue is now available to users who log in to the High Energy Astrophysics Science Archive Research Center (HEASARC) On-line Service. The catalogue, contained in a database called WFCBSC, lists information on 384 bright EUV sources found during the ROSAT all- sky survey of July 1990 to January 1991. The information in the WFCBSC database is based on 'The ROSAT All-Sky Survey of Extreme Ultraviolet Sources: I The Bright Source Catalogue' by K.A. Pounds et al., 1992, to be submitted to MNRAS. The WFCBSC database contains source names, coordinates, count rates and errors, and spectral classifications.
To access WFCBSC, or any of the other ROSAT databases, users should log in to the HEASARC On-line Service's captive account, called XRAY. The XRAY account resides on a VAX at GSFC and is accessible 24 hours a day; users can log in via these methods:
DECnet - node name NDADSA, address 15761
NSI-TCP/IP - address 128.183.36.17
Direct Dial - dial (301)286-9000, type "CALL SISC" and "CONNECT NDADSA"
When the NDADSA computer asks for a USERNAME, enter XRAY; no password is
required.
To access the WFCBSC database, type "BROWSE WFCBSC"; this will load the database into the BROWSE program. BROWSE is a command-driven interface which allows users to display, search, sort, and plot the database entries. From here, typing the BROWSE command "HELP" will produce a summary of all BROWSE commands, while typing "DBHELP" will display detailed information about the contents of the WFCBSC database.
Calibration
PSPC Point Spread Function (on-axis)
The point spread function (psf) of the ROSAT X-ray mirror assembly (XMA) + PSPC is a convolution of several components:
- The XMA scattering profile
- The off-axis Blur of the XMA
- The Intrinsic Resolution of the Counter
- Focus and photon penetration effects
- Ghost images below ~ 0.15 keV
- Ellipsoidal Blur due to residual attitude error
Ground calibration data were taken for PSPC A and PSPC C. These data comprised four monochromatic X-ray energies and were fit to determine the exact parameters of the functional forms above. Five in-flight datasets were then used to test this predicted performance. These test datasets included both PV (PSPC C) and AO1 (PSPC B) and two datasets were from the "long pointing" phase of ROSAT.
Comparison of the in-flight data with the MPE model showed that the psf can be well modeled as described above between energies 0.15 and 2 keV. Below 0.15 keV the ghost imaging effect prohibits a precise prediction of the psf and we recommend that users extract data with large enough region to include most counts below 0.15 keV (at least ~ 3 arcminutes) as it is not possible to calculate a correction factor for the scattering of those events.
No systematic effects are evident in the test datasets used to date. Future brighter source datasets may provide a more stringent test of the model.
An Office For Guest Investigator Programs (OGIP) calibration memo (CAL/ROS/92-001) has been written describing the on-axis point spread function in detail, and copies will shortly be sent out on this distribution list. A more detailed summary of this memo will also appear in the next ROSAT newsletter, and the full memo will appear in the HEASARC journal (Legacy #2). Additional copies are available on request to Pat Tyler, code 668, OGIP, NASA/Goddard Space Flight Center, Greenbelt MD 20771.
PSPC Spectral Response Matrix
A "new" PSPC spectral response matrix available from MPE will soon be distributed, both with PI data to observers and with the upcoming PROS 2.0 release. There are now three different "official" versions of this matrix, known in PROS by the following names:
xspectraldata$dtmat_6.ieee - 2nd post-launch matrix <-- DEFAULT PROS 2.0 xspectraldata$dtmat_5.ieee - 1st post-launch matrix xspectraldata$dtmat.ieee - pre-launch matrix (default PROS 1)
In PROS (xray.xspectral) the pkgpars default (dtmat_6.ieee) can be changed manually by the user.
An ascii XSPEC format version also exists on the anonymous ftp account at Goddard as PSPC_mar11.ascii.
To reach the account
- ftp rosserv.gsfc.nasa.gov
- cd pub
- get pspc_mar11.ascii
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