Glenn E. Allen
2002-07-10
(external links fixed by H. M. Guenther in Nov 2020)
This document describes potential problems or concerns about ACIS event data processed using the standard pipeline. The issues are categorized according to the nature of the caveat.
TIME:
The values recorded in the column TIME of the ACIS event data files for continuous-clocking mode observations are associated with the times the events are read out, not the times of arrival of the events. As a result, searches for pulsed signals may yield inaccurate results unless the effects of the motion of the telescope (dither) and the SIM are is removed from the event TIMEs. The results of absolute timing or absolute phase analyses will be inaccurate unless the length of time required to read out an event is subtracted from the TIME of the event. The tool acis_process_events will be modified to compute the times of arrival of events. For the time being, users should contact the helpdesk for specific instructions about how to compute the times of arrival of events.
The values in the column TIME of the continuous-clocking Level 0 event files were computed incorrectly before October 5, 1999. The contribution to the time associated with the value of TROW was double counted. This problem was propagated to the Level 1 and Level 2 event files. The problem was corrected as of October 5, 1999. The earlier data sets are being reprocessed.
The values of TIME associated with data collected using very-long exposures (e.g. the RAW, HISTO, and GRADED_HISTO DATAMODEs) are not reliable because the counter associated with the front-end processor rolls over every 328 s and this roll over affects the time assigned to the events. These data sets are generally analyzed by only the instrument scientists.
COORDINATES:
PULSE HEIGHT:
An algorithm is being developed to compensate for some of the effects of the loss of charge due to charge-transfer inefficiency (CTI). Analyses show that the algorithm can significantly improve the energy resolution of the front-side illuminated CCDs, especially for events that occur far from the read out (i.e. at large CHIPY values). The algorithm is being implemented in acis_process_events.
Users who are analyzing ACIS data from continuous-clocking mode observations should be aware that the method used to compute the values of the bias map for continuous-clocking mode fails if a cosmic ray deposits a lot of charge along most of a column of a CCD. In this case, the bias values will be systematically overestimated and the values of PHA, ENERGY, and PI will be systematically underestimated for the pixels in this column. Since the tools used to generate ARFs and exposure maps do not compensate for this problem, a user may wish to exclude events that occur in the affected columns from their analyses and compute ARFs and exposure maps appropriately. The problem applies to the data for all existing continuous-clocking mode observations. Alternative techniques of computing the bias that are not affected by this problem are being studied. However, until such a technique is implemented, all new continuous-clocking mode observations will also be affected. Only the front-side illuminated CCDs are sensitive to this problem. The back-side illuminated CCDs are not. It is possible for a fairly large fraction of the columns of a front-side illuminated CCD to be adversely affected. This problem is unique to continuous-clocking mode observations. It does not affect the bias maps of timed exposure mode observations. If a user is concerned that the spectral data for the source they are studying is adversely affected by a bad bias, please contact the Chandra helpdesk.
The energy-scale of the ACIS CCDs is calibrated only over the range 0.277-9.886 keV. Users should be cautious about analyzing data outside this range.
The files used to determine the gain and spectral response of the ACIS CCDs have changed several times over the life of the Chandra mission. These changes reflect reductions in the temperature of the focal plane, increases in the charge-transfer inefficiency, and improvements in the understanding of the performance of the detectors. Users interested in the PIs or ENERGYs of events should verify that their data were processed using the latest gain file appropriate for the focal-plane temperature of their observation. If necessary, an ACIS event data file can be reprocessed using acis_process_events to update these columns. Users analyzing PI spectra should ensure that the FEF file used to produce a PI response-matrix file (RMF) for their data matches the gain file used to process the data. This condition should be satisfied if the latest appropriate versions of the FEF and gain files are used. If users are concerned about a possible mismatch, they should contact the helpdesk.
The values in the columns ENERGY and PI in ACIS event data files are quantized. The difference between one energy and the next is about 4 eV. (The exact value is energy dependent and varies from region to region on a CCD). The difference between one PI and the next is (by default) always 14.6 eV. Since the value of the PI of an event is a quantized representation of the value of the ENERGY of the event and since the width of a PI bin is not an integer multiple of the difference between neighboring values of ENERGY, PI spectra for bright sources may exhibit periodic spikes. For example, some PI bins may contain 3 energy points and some may contain four. PHA spectra do not suffer from this problem. The algorithm used to compute the value of the ENERGY of an event has been modified to include a small random component uniformly distributed between -0.5 and +0.5 adu (i.e. about -2 eV and +2 eV). Observations performed after July 2, 2001 do not suffer from this problem. The older data is being reprocessed, but until it has been, it is possible for users to rerun acis_process_events to remove the problem in the older data.
The meaningful values of the column PI in an ACIS event data file are n <= PI <= 1023, where n is approximately 6 for the two back-side-illuminated CCDs (ACIS-S1 and ACIS-S3) and approximately 10 for the eight front-side-illuminated CCDs. (The exact value of the lower limit varies from region to region on a CCD.) PI = 1024 is an overflow bin. PI = 0 corresponds to the special case where PHA <= 0. This value should not occur normally.
The values in the PHA, ENERGY, PI, GRADE, and FLTGRADE columns of ACIS event data files may not be meaningful if the observed source is a bright optical source (e.g. Jupiter). Users should contact the helpdesk for assistance with the analysis of the ACIS data for such observations.
EVENT FILTERING:
Users analyzing ACIS event data should ensure that the data have been filtered to exclude events that may not be "good" X-ray events. The data filtering may include the STATUS or GRADE of an event, events occurring during time intervals where the cosmic-ray background rate is substantially larger than the nominal rate, events associated with a cosmic-ray "afterglow", events occurring on flaring pixels, events found in horizontal "streaks", and so forth.
The column STATUS in ACIS event data files contains a bit-encoded description of possible problems that may be associated with an event. One or more of the bits in the STATUS column is set to one if an event is suspicious. While all telemetered events are included in ACIS "evt1" data files, only those with no STATUS bits set to one are included in ACIS "evt2" data files.
The column GRADE (and FLTGRADE) in ACIS event data files contains a numeric description of the distribution of charge in a 3 pixel x 3 pixel event detection region. Some GRADEs are dominated by events associated with cosmic rays instead of X rays. These GRADEs should be excluded from event data analyses. Since the ACIS detectors are calibrated using data that have GRADE = 0, 2, 3, 4, or 6, events with GRADE = 0, 2, 3, 4, or 6 should, in general, be included when performing analyses of the data and events with GRADE = 1, 5, or 7 should be excluded. Interested users should refer to the Proposers' Observatory Guide for additional information about GRADE and FLTGRADE.
At times, the Chandra satellite passes through regions of relatively high particle fluxes. These occasions may be associated with "flares" in the ACIS background rate. Users may want to exclude the data obtained during the time intervals associated with these flares. The flares can be identified by examining light curves of source-free regions of the back-side illuminated CCDs ACIS-S1 and ACIS-S3.
An "afterglow" is produced on a front-side illuminated CCD when a cosmic ray deposits a large amount of charge on a CCD and some of the charge is removed relatively slowly. As a result, events are reported for the same pixel in two or more consecutive frames. In general, the PHA values of these events steadily decrease from one frame to the next and the values of the FLTGRADE (and GRADE) of the events remain the same. This problem can produce false sources or confuse the spatial or spectral analyses of real sources. The tool acis_detect_afterglow can be used to identify events that may be associated with afterglows and to set appropriate STATUS bits to one for these events. The tool is part of the standard pipeline processing and the CIAO data-analysis suite. Be aware that acis_detect_afterglow may have the undesired effect of identifying real X-ray events as afterglows, especially for bright X-ray sources. (See this link for more information.)
Horizontal streaks (i.e. streaks at a constant value of CHIPY) are observed on the ACIS-S4 CCD. A streak is the occurrence of several events that have the same value of CHIPY on the same CCD in the same frame of data. Streak events are spatially correlated with the read-out nodes and have small pulse-height amplitudes. These streaks are produced by read-out noise. The tool destreak can be used to identify potential streak events and set a STATUS bit for them. The horizontal streaks should not to be confused with the vertical "frame-transfer" streaks that are evident for bright sources. The vertical streaks are produced when events from a source are detected as the accumulated charge in a frame is being transfered row-by-row into the image-store region for read out.
For observations that are performed using TIMED VFAINT mode, the outer 16 pixels of a 5 pixel x 5 pixel event "island" may be used to help reject potential cosmic ray events. An algorithm to perform this kind of event filtering has been incorporated into the tool acis_process_events. Interested users should use ahelp to obtain more information about parameters associated with this algorithm: check_vf_pha and trail. Users should be very cautious about using a value other than the default value for the parameter trail. Be aware that the algorithm can reject up to several tens of percent of good X-ray events for bright sources.
A relatively large number of events are reported for the node boundaries at CHIPX = 256, 257, 512, 513, 768, and 769. Most of the events are associated with cosmic-ray events that would not normally have been telemetered if the event islands had not been split across the node boundary. Events in these columns should, in general, be excluded from data used for analyses and are now excluded by default from ACIS "evt2" data files.
MISCELLANEOUS:
During the time interval from August 14, 1999 to September 17, 1999 the ACIS CCDs experienced episodes of relatively large doses of ionizing radiation. As a result, the efficiency with which charge is transferred from pixel to pixel during the read out process is diminished, especially for the front-side illuminated CCDs. The increased charge-transfer inefficiency (CTI) means that (1) the total amount of charge associated with an X-ray of a given energy is larger near the read out (i.e. at low values of CHIPY) than far from it, (2) the energy resolution and quantum efficiency are better near the read out, and (3) the distribution of event GRADEs varies as a function of position on a CCD. Since the energy scale and energy resolution are functions of the position of the source on a CCD, analyses of spectra from different regions of a CCD should use RMFs that are appropriate for each region.
If more than one X-ray photon interacts in the same event-detection region of a CCD (a 3 pixel x 3 pixel "island") during the same frame, the charge clouds produced by these photon events are said to be "piled." The event-processing software can not distinguish between an event produced by one photon and an event produced by more than one photon. Therefore, the X-ray spectrum of a source that is significantly piled may not resemble the spectrum of the photons that are incident on the detector. A technique has been developed to analyze piled spectra. Pileup models are available in the spectral-fitting packages ISIS, Sherpa, and XSPEC. If pileup is severe, piled events may be lost because the events do not satisfy the telemetry criteria or the events have GRADE = 1, 5, or 7. For example, Chandra images of the Crab pulsar and nebula exhibit a "hole" where the pulsar is located because the X-ray flux is large enough that many X-ray events are detected in a single frame. The resulting, single, piled event does not satisfy the telemetry criteria because it saturates the analog to digital converters or the FLTGRADE of the event is one of the few FLTGRADEs that is dominated by cosmic-ray events and not telemetered to the ground.
Cosmic rays that interact in the detectors can produce particle cascades that deposit charge in many adjacent pixels. Typically, it is not possible to detect a good X-ray event if the event interacts in one of the affected pixels. Therefore, these charge cascades (or "blooms") reduce the detection efficiency of the detectors. A preliminary analysis indicates that the detection efficiency is reduced by roughly 5% for the front-side illuminated CCDs and is not significantly affected for the back-side illuminated CCDs. For these reasons, the fluxes of X-ray sources observed using a front-side illuminated CCD are underestimated by roughly 5%. The cosmic-ray flux that produces the blooms varies in time and the spatial distribution of the blooms adversely affect some regions of a CCD more than others. The effects of the blooms are being investigated.
The count rates of parts of two CCDs are relatively low. These regions are NODE_ID = 3 of CCD_ID = 2 (ACIS-I2) and NODE_ID = 1 of CCD_ID = 6 (ACIS-S2). These two nodes are functioning properly. Fewer events are telemetered for these two nodes because they have relatively small overclock values. As a result, fewer of the events that saturate the analog-to-digital converters are telemetered. Since these events are generally background events, this feature should not impair the scientific utility of the two nodes.
The front-end processor designated FEP0 had a problem. Half of the bias-map memory for the processor was corrupted. This problem resulted in the loss of some useful X-ray data and the telemetry of many invalid events. To the best of our knowledge, this problem has only affected twelve observations. The effects of the problem should be visually obvious in images of the data: half of the CCD associated with FEP0 has many events and appears bright in the image. A patch to the flight software has been prepared to avoid this problem. This patch became active in February 2000. Now, after ten bias parity errors have occurred during a science run (an extremely rare event under normal conditions), the patch causes the affected half of the CCD to be disabled until a new bias-map is placed in memory. Since the patch was implemented, the problem has not recurred.