On the suspected timing error in Wilkinson microwave anisotropy probe map-making
Context. It has recently been suggested that the compilation of the calibrated time-ordered-data (TOD) of the Wilkinson Microwave Anisotropy Probe (WMAP) observations of the cosmic microwave background (CMB) into full-year or multi-year maps may have been carried out with a small timing interpolation error. A large fraction of the previously estimated WMAP CMB quadrupole signal would be an artefact of incorrect Doppler dipole subtraction if this hypothesis were correct.
Aims: Since observations of bright foreground objects constitute part of the TOD, these can be used to test the hypothesis.
Methods: Scans of an object in different directions should be shifted by the would-be timing error, causing a blurring effect. For each of several different timing offsets, three half-years of the calibrated, filtered WMAP TOD are compiled individually for the four W band differencing assemblies (DA's), with no masking of bright objects, giving 12 maps for each timing offset. Percentiles of the temperature-fluctuation distribution in each map at HEALPix resolution Nside = 2048 are used to determine the dependence of all-sky image sharpness on the timing offset. The Q and V bands are also considered.
Results: In the W band, which is the band with the shortest exposure times, the 99.999% percentile, i.e. the temperature fluctuation in the ≈503-rd brightest pixel, is the least noisy percentile as a function of timing offset. Using this statistic, the hypothesis that a -25.6 ms offset relative to the timing adopted by the WMAP collaboration gives a focus at least as sharp as the uncorrected timing is rejected at 4.6σ significance, assuming Gaussian errors and statistical independence between the maps of the 12 DA/observing period combinations. The Q and V band maps also reject the -25.6 ms offset hypothesis at high statistical significance.
Conclusions: The requirement that the correct choice of timing offset must maximise image sharpness implies that the hypothesis of a timing error in the WMAP collaboration's compilation of the WMAP calibrated, filtered TOD is rejected at high statistical significance in each of the Q, V and W wavebands. However, the hypothesis that a timing error was applied during calibration of the raw TOD, leading to a dipole-induced difference signal, is not excluded by this method.
Astronomy and Astrophysics
- Pub Date:
- July 2010
- cosmology: observations;
- cosmic background radiation;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- 8 pages, 6 figures, accepted in Astronomy &