XRT Team Meeting

1. XRT Team Meeting Location: ISAC/ASDC September 17-18, 2003 XRT Team Meeting – ASDC – September17-18, 2003 (Page 1)

2. XRT Bias Subtraction Scheme • Image Mode • We have no built-in bias information in the telemetry with the current flight software. • Image mode uses low gain, so bias is not expected to change much at all. • Image mode is only used to get centroids, so exact bias value is not important. • We can get image mode bias information by taking occasional raw images in low gain mode on side B. • We could modify the flight software to always telemeter some pixels from a fixed part of the CCD. This could be used to calculate bias levels on the ground. We think that this is unnecessary effort on both the FSW side and the ground processing side. XRT Team Meeting – ASDC – September17-18, 2003 (Page 2)

3. XRT Bias Subtraction Scheme • Photodiode Mode • PuPD mode data can be used on the ground to calculate bias, since it sends down every pixel. • Technique is to make a histogram of the raw PuPD data and find the peak of the distribution. • Mean or median of noise distribution can be used, but subject to bias by low energy photons • Gaussian fit of noise distribution is more reliable. • LrPD mode has been modified to include some data that can be used on the ground to estimate the bias level • LrPD frame header now includes a bias level estimate calculated on-board by calculating the mean and sigma of all the pixels below threshold. • Frame header actually contains the sum and sum of squares. Mean and standard deviation must be calculated by ground software. • This works well for distributions with no low energy photons (like the Fe-55 door source), but does not work well for distributions with low energy photons (like the Panter data). It probably will not be very accurate in flight. • LrPD frame header also includes the values of the last N (=20) pixels in each frame. These can be used on the ground to build up a noise histogram and measure the bias level accurately. We need a software tool to do this. XRT Team Meeting – ASDC – September17-18, 2003 (Page 3)

4. XRT Bias Subtraction Scheme • WT mode and PC mode bias levels are calculated during slew as follows: • Previous snapshot is terminated with either the Target ID changes or the IS_SETTLED ACS flag falls, indicated the beginning of a slew. • When either of the above conditions occurs, XRT: • Terminates the current snapshot observation • Sends the telemetry report for the current frame to the SSR • Writes a Snapshot Trailer record to the SSR • Switches to Slew mode • Calculates a new bias row • Calculates and telemeters a new bias map • This takes 12.5 seconds to complete • Telemeters a Raw Image mode frame if flag is set • Switches to LrPD mode and begins new snapshot observation. • Bias values are calculated according to the following equation: BPi = (RML - 1 / RML)*BPi-1 + (1 / RML)*Pi (This implicitly assumes a slowly changing bias) XRT Team Meeting – ASDC – September17-18, 2003 (Page 4)

5. XRT Bias Subtraction Scheme • Windowed Timing mode • Windowed Timing mode calculates bias rows on board during slews using 602 rows of data, and telemeters the bias row in the Snapshot Trailer. • WT mode bias row is recalculated on every snapshot (instead of running mean of previous bias row) • A bias row calculated in this way could be corrupted if we happen to slew across a bright X-ray source at the time we calculate it. • Bias row is calculated very quickly at the beginning of the slew. Such corruption seems unlikely. • Q: how far have we moved on the sky before bias row is calculated? Could it be corrupted by the previous target being in the field of view? • Only “correct” way to calculate bias is to point at blank sky or close sun shutter. • If necessary, we can calculate the bias rows at some other point in the observations, such as when we first switch into WT mode. However, • This will require changes to the Mode Sequence Table and possibly to the DCC code in the FSW. • This calculate could easily be biased by photons from the new target, which is expected to be very bright. XRT Team Meeting – ASDC – September17-18, 2003 (Page 5)

6. XRT Bias Subtraction Scheme • Photon Counting mode • Photon Counting mode calculates bias map on board during slews, and telemeters the bias map in a Bias Map record for each snapshot. • Bias map takes about 720 kB plus header • Recent re-estimate of the XRT data shows shows: For no bias maps, data rate is NOTE: average bit rate does not depend significantly on PuPD frame at beginning of AT slew. • Adding bias maps increases data rate by 1 kbps per PPT target. For 4 PPTs: • Data rate is completely dominated by bias maps XRT Team Meeting – ASDC – September17-18, 2003 (Page 6)

7. XRT Bias Subtraction Scheme • Photon Counting mode (cont) • So the question now is: What will we do about the bias maps? • Stop telemetering them? • Requires only trivial change to MST • Compromises data analysis • Telemeter only central part of the bias map? • Compromises serendipitous source analysis • Requires FSW mod • Request higher data rate from Project? • We might need up to 8 kbps (our original allocation) • Needs no change to FSW • Impacts total telemetry margins to ground • Other??? • Take bias maps less often? • Use constant bias map? (simple change to MST) • . • . • . XRT Team Meeting – ASDC – September17-18, 2003 (Page 7)

8. XRT Lightcurves • Suggestion from Ralph Wijers • Prompt X-ray lightcurves would be useful for ground-based observers • We currently provide the following: • Flux from first source image (Image Mode) @ 5s • Cumulative spectrum (counts per channel) @ ~600s • We calculate count rate for each image frame • Could store these count rates and generate new telemetry report for TDRSS • Could be sent down when we transition between modes, or at fixed time after start of snapshot (?) • Needs new FSW to implement (no $$ available) XRT Team Meeting – ASDC – September17-18, 2003 (Page 8)