XMM-Newton Science Analysis System


epframes (epframes-8.104) [xmmsas_20141104_1833-14.0.0]

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Time jumps

In EPIC PN data the full-second counter can show glitches. These are usually additional increments of the FTCOARSE values in PNAUX1 extension by about +1s, and also negative values of the form $-2^{n}-1$s (back-jumps) can occur. All other time jumps are much less likely and should make you become suspicious. For SAS_VERBOSITY of 5 (or higher) the library oal issues relevant information about time jumps in the data.

The EPIC-pn time information is stored in the PNAUX1 extension in the FTCOARSE (unit 1s] and FTFINE (unit $20.48\,\mu$s) columns. The latter unit is 512 EPIC-pn oscillator units (nominal frequency 25MHz). As the frames times of the EPIC-pn sub-modes are not integer multiples of 512 oscillator clocks, there is a jitter of a fine time unit in the time stamps
(see http://xmm2.esac.esa.int/docs/documents/CAL-TN-0081.pdf).

Time jumps are detected by dividing the time difference between two events by the nominal frame time and looking for significant deviations from integer values. To account for jitter effects some small deviations are allowed, and this is controlled by the environment variable SAS_JUMP_TOLERANCE. The larger the value is, the larger deviations of time differences from integer multiples of the nominal frame time are still accepted.

The environment variable SAS_JUMP_TOLERANCE is read and used by the oal library to detect and correct these time jumps. The task epframes only uses it for reference: write the value to screen and to the event file as keyword JUMP_TOL. Internally, the allowed relative time tolerance (deviation from integer number of frame times) is then the ratio of SAS_JUMP_TOLERANCE and the frame time (in units $20.48\,\mu$s).

If SAS_JUMP_TOLERANCE is set to a too small value (`` too picky''), then the oal introduces false time jumps, i.e., reduces the length of real gaps and thus shifting event times to earlier times. If SAS_JUMP_TOLERANCE is set to a too large value (`` too relaxed''), then the oal does not recognize all real time jumps anymore.

The recommended value ranges from 3 for early (2000) observations to about 45 for current (early 2012) observations. This time dependence is due to temperature effects on the PN oscillator. More details can be found in the calibration report and calibration presentations:
http://xmm2.esac.esa.int/docs/documents/CAL-TN-0081.pdf
ftp://ftp.xray.mpe.mpg.de/people/mjf/public/epic_boc_leicester_20120306.pdf
ftp://ftp.xray.mpe.mpg.de/people/mjf/public/epic_calws_esac_20130423.pdf