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XIS data affected by the erroneous dark frame

2007-09-20 by the XIS team

Summary

It was found that the dark frame data used for the on-board processing became erroneous in some occasions. The error was found some time between April 29 and October 10, 2006, and only the event data in the region of RAWY < 119 were affected. Thus the impact on point source observations was limited, but extended sources and background observations were affected. We describe below the nature of this error and how it affects the X-ray data.

Description of the Phenomenon

Characteristics of this error are summarized as follows:
(1) It is found in some of the data obtained between April 29 and October 10, 2006.
(2) It affects only the data in the region of RAWY < 119. Note that the region is defined in the RAW coordinate, i.e. in the memory map of the raw CCD data, not in the physical CCD coordinate.
(3) X-ray events (G02346 events) become very few in RAWY < 119 and shows artificial structure in the image, when this error occurred.
(4) The error starts just after the SAA and also ends just after the SAA.
(5) The error is often observed in XIS2 and XIS3 simultaneously, but can happen independently for the 4 sensors.

We show an example of the image in the figure below when this error occurred. The data were obtained in Sept 2-4, 2006, when the Perseus cluster was observed in the normal mode (without options) as a part of the XIS calibration. The figure shows an image created with the X-ray events (i.e. G02346 events). Lack of events is clear in the lower part of the image, which corresponds to RAWY < 119. Two (or more) rectangles seen in the region are characteristics of the data affected by the erroneous dark frame. jpg

This error can be easily recognized in the image, because little X-ray events (G02346 events) are output in the region of RAWY < 119. This means that even the background events, both CXB and NXB, are scarce in this region.

Cause of the error

Small but non-negligible amount of dark current is produced in an X-ray CCD. In addition, the 'zero signal' or bias level produced by the XIS analog electronics varies slightly from pixel to pixel. It is necessary to subtract this dark current and bias levels before extracting the event data. XIS keeps an image of dark current plus bias levels on board, which is called a dark frame, and subtract it from the raw frame data before extracting the event data. Because the dark current may change during the SAA passage, the dark frame is usually updated just after the SAA. However, the update is skipped when the XIS is looking at the day earth, because the optical light leaked through the OBF (Optical Blocking Filter) may mimic the dark current.

Exposure of XIS restarts after the SAA passage. AE/TCE of XIS was first restarted, then DE was restarted. However, if the time interval between the AE/TCE and DE restarts was short, it was found that DE sometimes failed to recognize the frame boundary of the 1st exposure. If the frame boundary is not recognized, frame data overwrite a part of the dark frame in the XIS memory, which corresponds to RAWY < 119. This does not cause a significant problem in most cases, because the dark frame is updated anyway immediately after the restart of DE. However, the dark frame is not updated when the XIS field of view is blocked by the day earth. In this case, incorrect dark frame is used in the on-board processing until the next update of the dark frame, which is done after the next SAA passage. This is the cause of the error described above.

We revised the command sequence used to restart AE/TCE and DE on April 29, 2006. It was realized later that this update made the interval between AE/TCE and DE restarts too short and introduced above-mentioned error. This error was removed on October 10, 2006, by modifying the command sequence once again.

Countermeasure

Because the event data became incorrect on board, there is no method to correct the error on ground. We would like to ask the users not to use the region of RAWY < 119 when this error occurred.

List of largely affected data

Here we list up the data largely affected by this error. Please note that data affected only for a few minutes are not included in the list. Start and end times are approximate values.

Start (yy/mm/dd hh:mm) End (yy/mm/dd hh:mm) Affected sensors
06/05/18 11:2006/05/18 13:00XIS2, XIS3
06/06/01 09:2006/06/01 14:30XIS2, XIS3
06/06/02 12:4006/06/02 14:30XIS2, XIS3
06/06/04 12:4006/06/04 14:20XIS2, XIS3
06/06/11 09:1006/06/12 00:50XIS2, XIS3
06/06/12 07:4006/06/12 23:00XIS3
06/06/29 14:1006/06/29 16:00XIS2, XIS3
06/07/23 16:5006/07/24 03:40XIS2, XIS3
06/07/25 08:3006/07/25 13:30XIS3
06/07/25 08:3006/07/25 15:20XIS2
06/08/05 03:2006/08/05 06:50XIS2
06/08/12 06:4006/08/12 18:40XIS2, XIS3
06/08/20 15:1006/08/20 17:00XIS2, XIS3
06/09/02 09:4006/09/04 11:40XIS1
06/09/02 13:1006/09/04 06:30XIS2, XIS3
06/09/09 06:1006/09/09 08:10XIS1, XIS2, XIS3
06/09/20 07:4006/09/20 20:20XIS1
06/09/20 07:4006/09/20 23:20XIS2, XIS3
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