Description

Background

Images of the ACIS-S4 chip show a variable pattern of linear streaks which fall along rows of pixels with constant CHIPY. It appears that these streaks are caused by a flaw in the serial readout which randomly deposits significant amounts of charge along the pixel row as charge is read out. This excess charge appears as a string of spurious X-ray events which are not removed by standard grade filtering. Since the streak locations vary between readouts, the streak events cannot be removed using standard methods appropriate for bad columns and pixels.

Instead, streak events can be identified - and hence removed - from the fact that they occur within a single frame and deposit events along a single CHIPY row of a single node. For low count-rate observations, it is relatively rare for multiple events to mimic this behavior. A more detailed description of the theory can be obtained from The CIAO web site contains a document which describes the theory in more detail. See also the thread "Destreak the ACIS-S4 Chip"

Algorithm

To run destreak, the user must, at a minimum, enter the names of the input and output event files. The input file will typically be an ACIS level 2 FITS event file. Although destreak can filter a level 1 event file, it is important to set the mask option appropriately when doing so. The output event file will be a copy of the input file modified to flag or (if filter=yes) remove the streak events. destreak is designed to remove, as accurately as possible, streak events from affected ACIS chips (only S4 in current practice), with minimal impact on actual source events. However, in questionable cases (e.g. observations which have bright sources on ACIS-S4), the careful observer would be well advised to examine the spatial distribution of identified streak events to verify that the streak filter has not misidentified a significant number of valid source events as streaks.

The user is given a number of options. CCDs can be treated as single node devices or four-node devices. destreak will filter streaks from each node of the data on a frame by frame basis. destreak filters out streak events based on a threshold value either given by the user or determined by the program; these values may be provided on a per-node basis.

Observations will typically have a low event count in each row of a node during a single frame interval. If a chipy row rarely records more than two events per row in a single frame, then three or more events might be considered a streak. In this case the threshold value could be 2 and all rows with a count higher than 2 would be flagged or discarded as streak events.

The user may specify the threshold for either 1 node, 4 nodes or none at all. The one and four node thresholds will be applied to all filtered chips. If no max value is given by the user, destreak will determine the optimal threshold for each node of each chip using a histogram of the observed events.

Streak events may be filtered from the data or propagated to the output and flagged with status bit 15. This behavior is determined by the filter parameter.

The mask parameter may be used to have destreak remove certain events from consideration as candidate streak events. In general, events already flagged as "bad" for other reasons should not be considered as candidate streak events. For example, the default value

 
mask="[status=0,grade=0,2:4,6]"

ensures that destreak will consider only events with good grades and zero status as potential streak events. This ensures that the rate of candidate streak events is as low as possible, minimizing the rate of spurious event coincidences. Such a mask setting is particularly important when the input file is a level 1 event file. Alternatively, the user may avoid filtering certain spatial regions of an observation by setting the mask parameter to specify a spatial region which will be ignored by destreak. For example, the user may wish to avoid losing photons associated with a bright source, and provide a mask region that includes the source. The mask string uses the data-model syntax for filtering regions (see "ahelp dmsyntax", "ahelp dmfiltering", and/or "ahelp dmregions" for details), and can be the name of a region file (e.g. mask=region(filename)).

Normally users apply destreak with ccd_id = 8 and default values for the threshold (max) parameter, but the program is capable of fine control for expert users. destreak can filter a single chip, some chips, or all of the chips used in an observation. The chip identifier can be specified in the ccd_id parameter. Multiple chips use the format 'a:b:c:d:e', where a,b,c,d, and e are integers between 0 and 9, and any number of chips (up to 6) may be specified. Any chips that are not in the event list will simply be ignored by the destreak filtering routine. If no chip is specified, destreak will filter all of the chips used in the observation. Additionally, If no DETNAM keyword is present in the infile header, destreak will attempt to filter all 10 ACIS chips.

The user may specify the names of the chip and node columns used in the input file, allowing destreak to handle files that are not standard CXC event files. If invalid column names are specified, destreak will default to ccd_col="ccd_id" and node_col="node_id" or "ccdnode". destreak will produce an error if these columns are not found. If node_col has the special value "none" then the chip will be treated as a single node device.

Three statistics files can, optionally, be generated as additional output. The first of the three files, defined by the countfile parameter, holds the observed and predicted data for the number of counts per row and the number of occurrences. The countfile also holds the equation and parameters that were used to fit the observed event data.

The second file, defined by the fracfile parameter, contains the cumulative event contamination based on the observed and predicted counts.

The third file, defined by the timefile parameter, holds the amount of lost observation time due to streak events for each row of each node. The user should note that generating statistics files can significantly increase program execution time.

The user also has the ability to define the length of time that will constitute a frame for the lost-time statistics file. The exptime parameter will default to the value of the EXPTIME keyword if no positive value is given (in particular, the default value of -1 tells destreak to use the header keyword).

Notes

• Destreak should always be used with caution because it removes counts from any source sufficiently bright to generate, within a single frame-time, multiple events within a single row of one CCD node. Bright point sources are certain to be affected.
• No EXPTIME keyword is present in the header of Chandra level 2 event files of ACIS-HETG or ACIS-LETG observations processed prior to March 2003. So, if the user wants to generate the lost-time statistical file, a value for exptime must be manually supplied in these cases. For these observations, the value of the EXPTIME keyword can be found in the header of the associated level-1 event files.