Stacked Observation Detections Event List (evt3.fits)
The stacked observation detections event file consists of a single FITS format event file which contains all the events from the observations that form this stack. That is, it can be thought of as the merged per-observation evt3 files for the stack with slightly-different header keywords.
The event files used for the stacked detections event file have been reprocessed through acis_process_events or hrc_process_events, as appropriate, to apply the latest instrument calibrations and the standard event status and event grade (ACIS only) filters. In addition, the destreak algorithm has been applied to the ACIS-S4 CCD. Good time intervals have been revised to eliminate periods of background flares. The observations have been aligned and reprojected so that they have a consistent coordinate system (both sky and equitorial).
The good time intervals (GTIs) associated with stacked observation detections event data are included as additional FITS HDUs (CXC Data Model blocks) in the stacked observation detections FITS event file. For an event file containing ACIS data, there is a GTI for each ACIS CCD included in the stacked observation detections observation; there is a single GTI for an event file containing HRC data.
The stacked observation detections event FITS file is named: 〈i〉〈s〉〈stkpos〉_〈stkver〉N〈v〉_evt3.fits
where 〈i〉 is the instrument designation; 〈s〉 is the data source; 〈stkpos〉 is the position component of the stack name, formatted as "Jhhmmsss{p|m}ddmmss"; 〈stkver〉 is the 3-digit version component of the stack name, formatted with leading zeros; and 〈v〉 is the data product version number.
![[WARNING]](../../imgs/warning.png)
The stack evt3 file must not be used with CIAO tools to create "response" products - such as an exposure map, ARF, or RMF - as the CIAO tools can not support the multiple configurations of the observation (such as different aim points or roll angles).
If the CSC data products are not sufficient, then the per-observation products should be used instead, and then analysed sepatately or combined, as appropriate for your analysis.
Example
An example stack evt3 file from release 2.0 is:
unix% dmlist acisfJ0519345m690156_001N020_evt3.fits blocks
--------------------------------------------------------------------------------
Dataset: acisfJ0519345m690156_001N020_evt3.fits
--------------------------------------------------------------------------------
Block Name Type Dimensions
--------------------------------------------------------------------------------
Block 1: PRIMARY Null
Block 2: EVENTS Table 15 cols x 657136 rows
Block 3: GTI7 Table 2 cols x 4 rows
Block 4: GTI1 Table 2 cols x 5 rows
Block 5: GTI6 Table 2 cols x 4 rows
Block 6: GTI3 Table 2 cols x 3 rows
Block 7: GTI8 Table 2 cols x 8 rows
Block 8: GTI2 Table 2 cols x 5 rows
The observation, or observations, that form this stack are available via the OBS_ID keyword. In this case, there are multiple so the value returned is LIST, and the values are in the OBSISxxx keywords:
unix% dmkeypar acisfJ0519345m690156_001N020_evt3.fits OBS_ID echo+ LIST unix% dmkeypar acisfJ0519345m690156_001N020_evt3.fits OBSID001 echo+ 11241 unix% dmkeypar acisfJ0519345m690156_001N020_evt3.fits OBSID002 echo+ 12062 unix% dmkeypar acisfJ0519345m690156_001N020_evt3.fits OBSID003 echo+ 12063 unix% dmkeypar acisfJ0519345m690156_001N020_evt3.fits OBSID004 echo+ # dmkeypar (CIAO 4.12): ERROR: Keyword 'OBSID004' was not found in file 'acisfJ0519345m690156_001N020_evt3.fits'.
This particular event file can be viewed with ds9 using a command similar to the following, which creates the following figure:
unix% ds9 acisfJ0519345m690156_001N020_evt3.fits \
-bin filter 'energy=500:7000' -bin factor 8 \
-scale log -scale limits 0 500
Stack event file
![[The ds9 image viewer shows the image data, where there are multiple ACIS observations (visible by the different rotations for the sets of chips).]](imgs/acisfJ0519345m690156_001-evt3.png)
Stack event file
The three observations are centered on the same object - in this case SNR E0519-69.0 - but have different roll angles (there are at least two separate roll angles). The read-out streak from the supernova remnant is visible as the band of enhanced emission covering the whole chip - parallel to the chip edge - one for each roll angle.