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[1007.5307] On the suspected timing-offset-induced calibration error in the Wilkinson microwave anisotropy probe time-ordered data
Authors:Boudewijn F. Roukema (Torun Centre for Astronomy)
Abstract:In the time-ordered data (TOD) files of the WMAP CMB observations, there is an undocumented timing offset of -25.6 ms between the spacecraft attitude and radio flux density timestamps. If the offset induced an error during calibration of the raw TOD, then this would add variance per pixel. This variance would be present in the calibrated TOD. Low-resolution map-making as a function of timing offset should show a minimum variance for the correct timing offset. Three years of the calibrated, filtered WMAP 3-year TOD are compiled into sky maps at HEALPix resolution N_side=8, individually for each of the K, Ka, Q, V and W band differencing assemblies (DA's), as a function of timing offset. The median per map of the temperature fluctuation variance per pixel is calculated and minimised against timing offset, over a range of +- 5 exposure times. Minima are clearly present. The timing offsets that minimise the median variance are -38 +- 9 ms (K, Ka), -30 +- 4 ms (Q), -27 +- 10 ms (V), and -29 +- 550 ms (W), i.e. an average of -31 +- 3 ms, where the WMAP collaboration's preferred offset is 0 +- 1.7 ms. Hence, the latter is rejected at a significance of 8.5 sigma. The hypothesis of a -25.6 ms offset, suggested by Liu, Xiong & Li from the TOD file timing offset, is consistent with these minima at 1.4 sigma. It is difficult to avoid the conclusion that the WMAP calibrated TOD and inferred maps are wrongly calibrated. CMB quadrupole estimates (3/pi)C_2 based on the incorrectly calibrated TOD are overestimated by roughly (17 +- 2) % (KQ85 mask) to (58 +- 5) % (KQ75 mask). Ideally, the WMAP map-making pipelines should be redone starting from the uncalibrated TOD and using the -25.6 ms timing offset correction.
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Tom Shanks

Joined: 30 Jul 2010
Posts: 1
Affiliation: Durham University

PostPosted: July 30 2010  Reply with quote

This looks to be an interesting paper by Boud Roukema, following up on earlier discussions in this forum. He suggests that the 25.6ms timing offset issue in the WMAP time-ordered data raised by Liu & Li (2010, 0907.2731) could still apply to the step of calibrating the WMAP data and produces evidence based on minimising large pixel variance as a function of timing offset. He argues that his method makes it possible to pick out the timing offset that was used in the calibration step and that his result favours the Liu and Li offset rather than the WMAP team offset. Previously Boud (1004.4506) had shown in a different analysis using source profile sharpness that the WMAP team offset was likely to have been properly applied at the map-making stage. The question now is whether an improperly applied offset at the calibration stage could still explain the anomalies associated with the low-l multipoles in the WMAP data and also those at small scales associated with the wider than expected WMAP radio source profiles found by Sawangwit & Shanks (2010, 0912.0524). Comments?
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