Caveats about ACIS Pipeline-Processed Event Data

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Last Updated: 26 June 2008

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. Users may find the ACIS Data Preparation analysis guide to be a useful companion to this information.

TIME

1. As of DS 7.6.0 (15 June 2005), the values of TIME associated with events for continuous-clocking mode observations are the times of arrival at the detector, instead of the read-out times. Associated with this change is a change to the good-time intervals (GTIs). The START and STOP times in continuous-clocking mode GTIs are now appropriate for the times of arrival instead of the read-out times. The values of TIME for observations processed using earlier versions of acis_process_events are still the read-out times. The older data will be reprocessed. Users may rerun the CIAO tool acis_process_events with calc_cc_times=yes to compute the times of arrival for their data. It is also possible to reset the values of TIME to the read-out times by rerunning acis_process_events with calc_cc_times=no. Note that the times of arrival are at the detector, not the barycenter of the solar system. The tool axbary can be used to compute the times of arrival at the solar system barycenter from the times of arrival at the detector before performing timing analyses.

2. The value of the keyword TIMEDEL in some ACIS continuous-clocking mode event-data files is incorrectly set to 1.4592 s instead of 0.00285 s. While this does not represent a problem if the pipeline-produced files are used for timing analyses, it is a problem if an affected event-data file is reprocessed using acis_process_events with the parameter calc_cc_times=yes because the values of the TIMEs in the output file will be corrupted. The problem affects data processed with software versions DS7.6.0 (June 2005) through DS 7.6.7 (May 2006); the correction was made in DS 7.6.8 (June 2006).

   To see if your data file is affected, check the ASCDSVER (software version) and/or TIMEDEL header keywords in the evt1.fits file:

   unix% dmkeypar acis_evt1.fits ASCDSVER echo+
   7.6.2
   
   unix% dmkeypar acis_evt1.fits TIMEDEL echo+
   1.4592
   

   This file was processed between DS 7.6.0 and DS 7.6.7 and has the incorrect TIMEDEL value, so we use dmhedit to update it:

   unix% dmhedit infile=acis_evt1.fits filelist="" op=add key=TIMEDEL value=0.00285 unit=s
   
   unix% dmkeypar acis_evt1.fits TIMEDEL echo+
   0.00285
   

   If you do not make this change before reprocessing the data with acis_process_events, the TIME values in the output file will be incorrect.

3. As of DS7.6.0 (15 June 2005), the START and STOP times of continuous-clocking mode good-time intervals (GTIs) are aligned with the beginning and ending times of frames. The START and STOP times for GTIs processed using earlier versions of acis_build_chip_gti could occur at arbitrary times during a frame. As a result, the keyword EXPOSURE may be inaccurate for older data. This problem affects the computation of the flux of a source, but should be negligibly small for most users (i.e. the uncertainty in the affective area of the detector produces a larger error in the flux than the inaccuracy of the EXPOSURE time). Concerned users should contact the Helpdesk for assistance. The older data will be reprocessed.

4. 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 have been reprocessed.

5. Before version DS 6.9 of the processing software (10 August 2002), the values of the keywords EXPOSURE, LIVETIME, DTCOR, ONTIME and a few others were computed incorrectly in some observations, especially those where the frame time was chosen to be less than the optimal frame time; see also the Time Keywords in Interleaved Data caveat. (For descriptions of the meanings of these keywords, see the ACIS exposure time keywords page.) As a result, analyses of the spectral distribution or total count rate from a source may have produced inaccurate values. This problem has been fixed. Data processed with DS 6.9 or higher should have accurate EXPOSURE values. The older data will be reprocessed. Users concerned about the accuracy of the EXPOSURE for their data should contact the Helpdesk.

6. 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 behavior affects the time assigned to the events. These data sets are generally analyzed by only the instrument scientists.

PULSE HEIGHT

1. The charge-transfer inefficiency (CTI) of the front-illuminated (FI) and back-illuminated (BI) CCDs results in a degradation of the energy resolution and an apparent gain shift. These effects are worse for events that occur far from the readout of a CCD (e.g. near the ACIS-I and ACIS-S aim points). The tool acis_process_events can be used to compensate for some of the effects of CTI. Applying the CTI adjustment significantly improves the energy resolution of the CCDs and removes most of the effects of the apparent gain shift from the ENERGY and PI data (but not the PHA data). The improvements are most noticeable near the tops of the FI CCDs and near the one-quarter and three-quarter chip node boundaries of the BI CCDs.

2. Since it is practical to calibrate the effects of CTI only every few years, more or less quarterly updates to the calibration of the apparent gain shift are used to apply a time-dependent gain adjustment to the event pulse heights. This adjustment is applied using the tool acis_process_events, which interpolates between epochs. A bug in the time-dependent gain algorithm resulted in a mean -0.5 adu (about -2 eV) shift to the adjusted pulse heights. This problem applies to all ACIS event data processed using the time-dependent gain adjustment in software versions DS 7.3 to DS 7.6.9. Users can reprocess their data using the CIAO 3.4 (or later) version of acis_process_events by following the Create a New Level=2 Event File thread to fix the problem.

3. The values in the columns PHAS_ADJ, PHA, ENERGY and PI of an ACIS event data file may not be accurate for continuous-clocking mode observations. The computation of the CTI and time-dependent gain adjustments (i.e. of PHAS_ADJ and, hence, PHA) and the computation of ENERGY from PHA (and, hence, PI) require knowledge of the CHIPY location of an event. In continuous-clocking mode observations, the CHIPY coordinate is used for timing information instead of spatial information. However, if the right ascension and declination coordinates of the source (i.e. RA_TARG and DEC_TARG) are accurate, then these coordinates and the aspect-solution data can be used to determine the CHIPY location of the source. As of DS 6.5 (12 December 2001), the tool acis_process_events uses the inferred CHIPY location of the source when the values in the columns PHAS_ADJ, PHA, ENERGY and PI are computed. The older data will be reprocessed. Users can reprocess their data using the CIAO 3.3 (or later) version of acis_process_events by following the Create a New Level=2 Event File thread.

4. 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.

5. The algorithm used to compute the one-dimensional bias maps for continuous-clocking mode observations is susceptible to the deposition of a lot of charge by a cosmic ray if the charge is deposited vertically in a long streak along the length of one or more adjacent columns. In this case, the bias value for the column is anomalously high by up to several tens of adu. As a result, events with small pulse heights are not telemetered because they do not satisfy the minimum pulse height threshold. The reported pulse height amplitudes (PHAs) for events with larger pulse heights are systematically underestimated. This problem occurs often for continuous-clocking mode observations and can affect the bias values of many columns of a front-illuminated CCD. (Back-illuminated CCDs do not suffer from this problem.)

   The problem may be diagnosed by plotting a histogram of the bias for a CCD. If the bias exhibits "spikes" in some columns, the bias in the affected columns is inaccurate. The problem of a systematic offset in the reported PHA values for events in the affected columns can be remedied by reprocessing the data using an appropriately adjusted bias. Users should contact the Helpdesk if they think that their data is affected. A new algorithm to compute the bias for continuous-clocking mode observations is being implemented.

6. 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 (as discussed in the ACIS data preparation guide). Users analyzing PI spectra should ensure that the P2_RESP (or 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.

7. If an ACIS event data file is processed with the acis_process_events parameter rand_pha=0 (the default value is rand_pha=0.5), then the values in the column ENERGY 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). Since the value of PI is a quantized representation of the value of ENERGY (as described in the CIAO dictionary) and since the width of a PI bin is not an integer multiple of the spacing between adjacent ENERGY values, the quantization of ENERGY can cause periodic spikes in the PI spectrum of a bright source. That is, the width of a PI bin is 14.6 eV by default, which is not an integer multiple of 4 eV. Therefore some PI bins contain three quantized values of ENERGY and other bins contain four.

   This problem is avoided by running acis_process_events with the parameter rand_pha=0.5 (the default value). In this case, a random component uniformly distributed between -0.5 and +0.5 adu (i.e. between about -2 eV and +2 eV) is added to the value of PHA when the value of ENERGY is computed. (The value of PHA in the output file is not changed.) Observations performed before July 3, 2001 do not have this randomization enabled. These data are being reprocessed to add the randomization. Users can also rerun acis_process_events to perform the randomization themselves or to remove the randomization if it has been applied. Note that this problem only affects PI spectra. PHA spectra are unaffected (even if rand_pha=0).

8. 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-illuminated CCDs (ACIS-S1 and ACIS-S3) and approximately 10 for the eight front-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.

9. The values in the columns PHA, ENERGY, PI, GRADE, and FLTGRADE of an ACIS event data file 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.

COORDINATES

1. Prior to DS 7.6.4 (09 November 2005), the data for the SIM rotation angle (DTHETA) in the aspect solution files was used incorrectly when computing the values of DETX and DETY (and, hence, X and Y) from the values of CCD_ID, CHIPX and CHIPY. Since the values of DTHETA are typically quite small (0.0001 rad), the change in the coordinates should generally be less than one ACIS pixel. The problem has been fixed as of DS 7.6.4. Older data in the archive will be reprocessed. Users can reprocess their data using the CIAO 3.3 (or later) version of acis_process_events by following the Create a New Level=2 Event File thread.

EVENT FILTERING

In addition to the onboard 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 pixels that are hot or that have bad bias values, events found in horizontal "streaks," and so forth.

1. 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.

2. The column GRADE (and FLTGRADE) in ACIS event data files contains a numeric description of the distribution of charge in a 3 x 3 pixel event detection region. (Interested users can refer to the Proposers' Observatory Guide for additional information about GRADE and FLTGRADE.) 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 one of these five "good" GRADEs should (in general) be included when performing analyses. Events with GRADE = 1, 5, or 7 (the "bad" GRADEs) should be excluded.

3. 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-illuminated CCDs ACIS-S1 and ACIS-S3.

4. An "afterglow" is produced on a front-illuminated CCD when a cosmic ray deposits a large amount of charge on the CCD and some of the charge is removed relatively slowly. As a result, events are reported for the same pixel in two or more frames. The frames may be consecutive or may be separated by gaps of up to a few frames. The pulse-height amplitudes (PHAs) of afterglow events typically decrease from one frame to the next, but this trend is not necessarily monotonic. Afterglows can appear to be false sources.

   The tool acis_run_hotpix can be used to identify events that may be associated with afterglows. The tool is part of the standard pipeline processing as of DS 7.4.0 (29 November 2004). It replaced the tool acis_detect_afterglow, which occasionally had the undesired effect of identifying real X-ray events as afterglows, especially for bright X-ray sources. (See the Remove the acis_detect_afterglow correction thread for more information.) The tool acis_run_hotpix also finds pixels that are hot or that have bad bias values, but misses many afterglows that have fewer than about 10 counts. (See the About the ACIS Hot Pixel Tools section of the New ACIS Bad Pixel File: Identify ACIS Hot Pixels and Cosmic Ray Afterglows thread for details.)

5. 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 and remove potential streak events.

   The horizontal streaks should not be confused with the vertical "frame-transfer" streaks that are evident for bright sources (see the Remove the ACIS Readout Streak thread). Events in the frame transfer streak are also called "out-of-time" events because the TIMEs recorded in the event file for these events are inaccurate by as much as the time between one frame and the next.

6. For observations that are performed using TIMED VFAINT mode, the outer 16 pixels of a 5 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.) Be aware that the algorithm can reject up to several tens of percent of good X-ray events for bright sources.

7. 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 in these columns are associated with cosmic-ray events that would not normally have been telemetered if the event islands had not been split across a node boundary. Events in these columns are excluded from ACIS "evt2" data files by default.

MISCELLANEOUS

1. Since launch, the ACIS CCDs have experienced fluxes of ionizing radiation. During the time interval from August 14, 1999 to September 17, 1999 and on October 18, 2003, the fluxes were relatively large. 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-illuminated CCDs. The increased charge-transfer inefficiency (CTI) means that:

   • the total amount of charge (i.e. the PHA) associated with an X-ray of a given energy is smaller far from the read out (i.e. at high values of CHIPY) than near it,

   • the energy resolution and detection efficiency are better near the read out,

   • the distribution of event GRADEs for a source depends on the location of the source on a detector.

   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.

2. If more than one X-ray photon interacts in the same event-detection region of a CCD (a 3 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 (PS file, 23 pp) has been developed to analyze piled spectra. Pileup models are available in the spectral-fitting packages ISIS, Sherpa, and XSPEC (see the comparison, for low pileup fractions, of the pileup models document).

   If pileup is severe, piled events may be lost because the events do not satisfy the telemetry criteria. Furthermore, telemetered events are excluded from Level 2 event files if 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.

3. 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. An analysis indicates that the detection efficiency is reduced by roughly 4% for the front-illuminated CCDs and is not significantly affected for the back-illuminated CCDs. For these reasons, the fluxes of X-ray sources observed using a front-illuminated CCD are underestimated by roughly 5%. The spatial distribution of the blooms adversely affects some regions of a CCD more than others. The cosmic-ray flux that produces the blooms varies in time. The effects of the blooms are being investigated.

4. For X-rays passing through the mirrors, the very bottom of each CCD is obscured by the frame store. As a result, some of the events in rows with CHIPY <= 8 are not detected. (The set of rows affected varies from CCD to CCD.) Since the CIAO tools do not compensate for this effect, the ARFs and exposure maps for sources in these regions may be inaccurate.

5. 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 typically produced by cosmic rays, this feature should not impair the scientific utility of the two nodes.

   Similarly, the count rate of part of one CCD is relatively high. This region is NODE_ID = 3 of CCD_ID = 1 (ACIS-I1). This node is functioning properly. More events are telemetered for this node because it has a relatively large overclock value. As a result, more of the events that saturate the analog-to-digitial converter are telemetered. Since these events are typically produced by cosmic rays, this feature should not impair the scientific utility of the node.

6. 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 to address this problem 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.